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Chemistry
The structure of matter
The matter on earth consists of atoms and molecules. Chemistry deals with atoms and molecules and with the orgin
of new substances. The Bohr atom model says that atoms consist of a positive charged atomic nucleus and of a negative
charged atomic shell with electrons. This model says that the atomic nucleus consists of positive charged protons and
neutral neutrons. Throught that, that the atomic nucleus is charged positive and the atomic shell is charged negative the
whole atom is neutral. In an atom the number of the electrons must be equal to the number of the protons. The number of the
protons decides what a element an atom is. The number of the neutrons is not important for this. Further information about
the structure of the atom are the area of nuclear physics or nuclear chemistry. This website does not deal with nuclear
chemistry, because the main chemical aspects take place in the atomic shell. On the
homepage of nuclear physics you can find more information about the atom. The next
point is that there are different elements. More than 110 elements are known. Elements consist of an atom. Famous and
important elements are for example oxagen, nitrogen, hydrogen, carbon and iron. Water for example is a molecule. Molecules
consist of at least two atoms of the same element or of different elements. A water molecule consists of two hydrogen atoms
and one oxygen atom. Later more about it.
Chemical bonds
In the first chapter we talked about molecules. Molecules consist of at least two atoms of the same or of a
different element. One of the most simple and famous molecules is water. A water molecule consists of two hydrogen atoms
and one oxygen atom. In the first chapter we saw that elements have got symbols and molecules have got formulas. Water
has got the formula H2O. Both elements hydrogen (H) and oxygen (O) are in the formula and the proportion, too.
There are also bonds between the same atoms. Oxygen is for example a molecule O2. O3 for
example is ozone. A molecule is a substance with other properties than its components. At room temperature hydrogen and
oxygen are gases, but water is liquid. It is a very good and simple example. Many gases like hydrogen are often molecules
and not elemental. Hydrogen has got the formula H2. This shows us that there must be bonds between atoms.
The three main bonds: atomic bond (pair of electrons), ionic bond and metallic bond will be presented. The connections
between the particles of the different bonds will be presented, too. These connections are crystal lattices and other
strengths.
Atomic bond (pair of electrons): How the name says it already it is a bond of a pair of electrons. This
bond is very often among nonmetals. Atomic bonds do not lead current. This may surprise because water has got an atomic
bond, but it is dangerous to life to put a lamp into water. The reaons is that in normal water there are some water ions
and they lead the current. These few ions are enough to killed somebody. Later we will see what ions are. In atomic bonds
the electron of one atom forms a bond with the electron of another atom. it is a pair of electrons. Both atoms share there
electrons. The reason for bonds is to reach the noble gas configuration. The outer shell of an atom is full when she
contains 8 electrons. The noble gases have got 8 electrons on their last atomic shell. Helium has got only two electrons on
his last shell, but it is enough to fill his outer atomic shell. Generally 8 electrons are the target of every atom.
Hydrogen is also a problem like helium. Hydrogen has got only one shell and one electron. His shell can only contain 2
electrons like the shell of helium. When a hydrogen atom forms a bond with another hydrogen atom then both hydrogen atoms
has got a full electron shell, because both together has got two electrons. So a H3 molecule is not possible.
Because of the full last shell of noble gases they do not react with other elements. They are in a very stable state.
Elements which react only a little bit or not are called noble. Gold is noble, because it only reacts which a few materials.
There is another style to write chemical formulas. We have used the sum formulas, but there is another style to show the
number of the electrons on the last shell and there are also structural formulas.
This style shows the electrons on the outer shell. A line stands for two electrons and a pont stand for one electron.
The lower style is the shortend style which is normally used for chemical reactions. These all are reaction equations,
so on the left and on the right side there must be the same number of particles. In the first case two oxygen atoms react to
an oxygen molecule. In the second reaction two nitrogen atoms react to a nitrogen molecule and in the third case two
hydrogen molecules react with one oxygen molecule to two water molecules. In all three cases the noble gas configuration is
reached. In the first reaction both oxygen atoms, in the oxygen molecule, have access to 8 electrons, because 4 electrons
forms two bonds and 4 electrons has got every oxygen atom for himself. In the second case every nitrogen atom, in the
nitrogen molecule, has got 2 electrons and 6 electrons forms 3 bonds together every nitrogen atom has access to 8 electrons.
In the last case the oxygen atom has got 4 electrons and 4 electrons froms two bonds with the hydrogen atoms. The hydrogen
atoms have access to the electrons of the bonds and so they have got 2 electrons, too. These bonds are relative stable.
There are not only single bonds like in the water molecule, but there are also double bonds like in the oxygen molecule
or even triple bonds like in the nitrogen molecule.
There exist polar and nonpolar atomic bonds. Different elements pull the electrons from the bonds different strong.
this is called electronegativity. When two oxygen atoms form an oxygen molecule or when two hydrogen atoms form a
hydrogen molecule these bonds are nonpolar. They are nonpolar, because the same atoms have got the same
electronegativities. The strengths which act between two nonpolar moelcules are called van der Waals-strengths. In the
water molecule the electronegativity of oxygen is much bigger than the electronegativity of hydrogen and so
oxygen is charged a little bit negative and hydrogen is charged a little bit positive. The oxygen atom of one water molecule
and the hydrogen atom of another water molecule pull each other. This is called hydrogen bond. A hydrogen bond is much
weaker than an atomic bond. This polarity causes that water has got a much higher melting and boiling point than oxygen
and hydrogen, because the water molecules pull each other. Water is a dipole. In the solid state molecules form a
molecule lattice, but it is weak because of the little power of attraction. These materials are soft.
Ionic bond: This bond is very often, too. Here we have got ions. Ions are atoms which have less electrons
than protons or which have more electrons than protons. Ions are not neutral. They are charged positive or neagtive.
In nuclear physics sometimes we have only even atomic nucleus without any electron, but not in chemistry. For those
processes we need more energy. Ionic bonds originate when nonmetals form bonds with metals. Nonmetals have got a strong
electronegativity and they pull electrons very strong. The outer electrons of metals are not connected very strong with
the atomic nucleus. It happens that a nonmetal take one or even more electrons from a metal. The most known example is
common salt. Common salt consists of the metal sodium (Na) and of chlorine (Cl). Chlorine take one electron from the sodium
and it is charged negative. The formula of common salt is Na+Cl-. The positive charged ion is called
cation and the negative charged ion is called anion. The whole substance is neutral, because we have got the same number
of cations and anions. In ionic bonds we have not got molecules, but the formula is the same as formulas of molecules. These
ions forms crystal lattices. They are called ion lattices, too. There are many more salts, not only common salt, and they
consist also of ions. In the solid state ionic bonds do not lead current. But when we dissolve a salt in water then the
crystal structure do not exist any longer and the ions are free to move. A solution like that lead current.
The solubility of salts depends on the lattice energy which has to be reached to dissolve the crystal lattice. Common salt (sodium chloride) dissolves good in water, calcium carbonate (marble, corals) nearly not.
Because of the
chargements there are stronger strenghths betweens ions than between molecules. This is the reason why the melting points
and the boling points of ionic bonds are higher than of molecules. Here is am example for an ionic reaction.
The upper styles shows the chargement and the lower styles do not show the chargement. In the first case we can see
common salt (Na+Cl-) and in the second case we can see
Fluorit (Ca2+F-2)
which is important for mineral collecters. You can find more information about the Fluorit on the
homepage of minerals. The fluorit consists of calcium (Ca) and fluorine (F)
and Fluorin even takes two electrons from Calcium. Fluorine can only take one electron and so we need two negative charged
fluorine ions. Generally it is difficult to dissolve many electrons from the atomic shell. It is important to know that atoms
have got a bigger radius than their cations and that atoms have got a smaller radius than their anions, which is logical.
When an atom has got a big radius it is more simple to take one of the outer electrons, because it is far from the atomic
nucleus and so the power of attraction is weak. The radius of the atoms in a group of the "Periodic table of the elements"
growns from the top to the bottom. The reason is that we have got one more shell in every period, which is further from
the atomic nucleus. In a period the radius of the atoms shrinks from left to right, because on the right there more protons
ans so the power of attraction is stronger. Noble gases have got a small radius and it is another proof for there
chemical resistance against bonds. Caesium has got a very big radius and so it is very reactively and not noble.
Transition between atomic bonds and ionic bonds: There are substances which are a transition between
atomic bonds and ionic bonds. These substances for example consist of a metal and a nonmetal or of a metalloid and a
nonmetal. The nonmetal is very polar and tries to pull the electron of the other atom to his side. But for the nonmetal it
is not possible to take the whole electron and we have not got ions. As well it is an atomic bond which is very near to an
ionic bond, because of the strong polarity. Those bonds froms very hard crystals and they have very high melting points.
Silicon dioxide (SiO2) for example has got a melting point of 1710 degrees. Silicon (Si) is a metalloid. Because
of these properties those substances are important materials. Glasses of quartz (Silicon dioxide) are used for materials
which are very hot. Silicon dioxide is also resistant to many acids and so it is used to keep acids. You can find more
information about quartz on the homepage of minerals. Here we have not got
molecule lattices, but atomic-ionic-lattices which are a transition. These materials are very similiar to the diamond.
Diamond consists of carbon (C) and do not froms molecules, but a very stable atomic lattice. This atomic lattice and the
diamond differs from these transition bonds, but their properties are very similiar. These materials cannot be dissolved in
water. The diamond is the hardest known substance and it is another similiarity.
Metallic bond: There is also the metallic bond which we can find between metals. The outer electrons are in
the whole metal free to move. These free electrons are also connections between the metal atoms. So here we have also a
lattice which is called metal lattice. The strengths between the electrons and the atoms are very strong, because we can
bend metals and they do not break. In the whole metal the number of the electrons and the number of the protons is equal,
so a metal is neutral, too. Metals cannot make atomic bonds with other metals. In comparison to the atomic bond the electrons
do not belong to two atoms, but they belong to all atoms. These free electrons are the reason why metals in the solid state
can lead current. In the ionic bond the ions in the solid state are not free to move, because of the lattice. The electrons
of the metals can move in every time. Metals are very good leaders of current. Silver is the best leader at all. Often we
compare these electrons with a gas, which is called electron gas. This electron gas is the reason for the metallic gloss
and the missing transparency of metals. The mettalic bond is very important, because most of the elements are metals.
Chemical reactions
When we talked about the bonds we saw that there are chemical reactions. The whole chemistry is about reactions
and that different substances react together in different ways.
The reasons for all reactions is that everything in the universe trys to reach the lowest level of energy. In those reactions energy is emited as heat.
These reactions are presented by reaction equations. On the
left side there are the educts and on the right side there are the products. There are reactions with backward reactions.
Those reactions are called equilibrium reactions, because after a certain time there is an equilibrium between the educts
and the products. Then there would not be more products. there are different reactions. In some reactions heat is delivered.
These reactions are called exotherm. Other reactions need energy and those reactions are going cool during the reaction.
They are called endotherm. Temperature is an important factor for reactions, because when the temperature is higher
reactions are faster, because when the temperature is higher the particles are moving faster. When the particles are moving
faster the probability is higher that they can collide and react together. When we have got an exotherm reaction the
temperature can be a problem, because the energy from the reaction cannot be delivered. There are many factors which have
an influence on the reaction. The concentration and the pressure are very important, too. Reactions can also happen,
because of the growing of the disorder in the universe. The second main sentence of thermodynamics says that everything in
a closed system is going to reach the biggest disorder. It is not sure if the universe is a closed system. There are two big
types of reactions: redox reactions, acid base reactions.
Redox reactions: The word redox is a mixture of the two words oxidation and reduction. In former times
reactions with oxygen were called oxidations. The inversion was the reduction. In a reduction the oxygen is taken from
an oxide. For example we can take ferric oxide (rust). In a reduction we would get iron and oxygen from the ferric oxide.
Later it was discovered that not only reactions with oxygen flows like that, but also many other reactions. So the words
oxidation and reduction were widespreaded. Oxidation is generally the delivery of electrons and reduction is the
reception of electrons. These both processes are also together so they are called redox reaction.
We can see the whole reaction of magnesium (Mg) and oxygen to magensium oxide. In the next line we can see how
magnesium is oxidized. In the third line oxygen is reduced. The oxygen is called oxidizing agent, because it oxidizes the
magnesium. The magnesium is called reducing agent, because it reduces the oxygen. The burning of oxygen and hydrogen to
water is a redox reaction, too.
In redox reactions it is often not easy to see what is the oxidation and what it is the reducation. The oxidation numbers can help. Every atom in a molecule or in an ionic bond has got an oxidation number. These are the rules.
- A single atom has got always the oxidation number 0.
- In an ion the oxidation number is the same than the charge of the ion.
- Hydrogen has got always the oxidation number +1 (in metal hydrides: -1).
- Oxygen has got always the oxidation number –2 (in peroxides -1).
- The sum of the oxidation numbers in a molecule is 0.
- In a complex ion the sum of the oxidation numbers is equal to the charge of the complex ion.
- Metals have got always positive oxidation numbers.
The oxidation numbers of the other atoms can find out with these rules. When we have got for example CH4 then every hydrogen atom get the oxidation number +1. There are four hydrogen atoms and this compound is neutral, so carbon has got the oxidation number -4. It does not mean that carbon has got always the oxidation number -4. When an atom is oxidized the oxidation number increases and when it is reduced the oxidation number decreases. With these oxidation numbers it can be found out what is an oxidation and what is a reduction.
You can find more information about redox reactions and their application in the chapter
of electrochemistry.
Acid base reactions: This is the seond type of reactions. The definiton of acid and base is not easy. For
example we say hydrochloric acid when we mean HCl. H is hydrogen and Cl is chlorine. HCl is not hydrochloric acid. For a
chemist and offcially it is enough to say HCl for hydrochloric acid. When we dissolve HCl in water than it is
hydrochloric acid. When we mean HCl we had to say hydrogen chloride and not hydrochloric acid. At room temperature
hydrogen chloride is a gas. In water HCl dissociates to the ions H+ and Cl-. The atomic nucleus of
hydrogen is only one proton and in the atomic shell there is only electron, so that H+ is a single proton.
Cl- is called acid residue. The definiton of an acid from the chemist Arrhenius says that acids are bonds which
dissociate in aqueous solutions to positive charged hydrogen ions (protons: H+) and negative charged
acid residue ions. His definition for bases was that they dissociate in aqueous solutions to positive charged metal ions
negative charged hydroxide ions (OH-). The chemist Bronsted had widespreaded these definitions. He said that
acids deliver protons and so they are called proton donors. Bases take protos and so they are called proton acceptors. The
hydroxid ion (OH-) is an important base, because it has got one electron more than protons. After this definition
and after Arrhenius definition NaOH is an important base, because it has got a hydroxide group. Other alkali metals which
froms bonds with hydroxide ions are important bases, too. In aqueous solutions bases dissociate and so alkalinely solutions
originate which are called caustic solutions. So it is the same like with acids that NaOH is not a caustic solution, but a
base. Water is an interesting substance, because water can be sometimes an acid an sometimes a base. There is also the
auto-dissociation of water which is responsible that water can lead current. Because of this dissociation from two
water molecules two different ions originate. One of them is H3O+ (hydronium ion) and the other is
OH- (hydroxide ion). The hydronium ion is responsible that acids can dissolve different materials. This
process of disbandment of a substance by an acid is a redox process.
These reactions are equilibrium reactions so we can write them with an equilibrium arrow. In the first reaction we can
see the auto-dissociation of water. In the second reaction hydrogen chloride (HCl) dissociates in water and in the third
reaction H2SO4 dissociates in water to sulfuric acid. In this reaction there are two water molecules,
because H2SO4 has got two hydrogen atoms and so it is a double proton acid. Multi proton acids exist,
too. Sulfuric acid can dissociates twice, but the second dissociations are rarer than the first dissociations. Here we come
to the point that acids do not dissociate completely. When we says that the second dissociation is rarer then it means that
only a part of the molecules dissociate. When the degree of dissociation is high than many molecules dissociate to ions.
Sulfuric acid and hydrochloric acid for example are very strong acids, because they have got a high degree of dissociation.
A stronger acid for example is iodine hydrogen (HI). We have seen that an acid always needs a base which can take the proton.
So we always talk about acid base pairs. When we have got a very strong acid then the corresponding base is very weak.
When the base is very strong then the acid is very weak. In comparison to acids caustic solutions are always completely
dissociated. A good example for a caustic solution is caustic soda solution (NaOH) or caustic potash solution (KOH). Acids
and bases can neutralize each other. When we mix an acid and a base in the same proportion then a salt and water originate.
In the reaction of hydrochloric acid (HCl) and caustic soda solution (NaOH), common salt (NaCl) and water originate.
When we take other acids and bases then other salts originate. Often we talk about the salts of a certain acid, because
a reaction of an acid and a metal always forms a salt and hydrogen. When hydrochloric acid (HCl) reacts with sodium (Na)
then common salt (NaCl) and hydrogen originate. Salts are completely dissociated in aqueous solutions. We can also take
other substances for example substances from the lead-acid battery (Chapter Electrochemistry). In the lead-acid battery
lead (Pb) reacts with sulfuric acid (H2SO4) to lead sulfate (PbSO4) which is a salt of
sulfuric acid. Another important and interesting thing is the pH factor. Water is neutral, so it is not sourly or alkalinely.
Water has got a pH factor of 7. Solutions with a pH factor under 7 are acids, solution with a pH factor over 7 are caustic
solutions. If an acid is stronger then the pH factor is smaller. When a caustic solution is stronger then the pH factor is
higher. One number stands for a ten times. So a substance with a pH factor of 4 is ten times so sourly than a substance
with a pH factor of 5. Substances with the pH factors of 0 and 14 are very corrosive and very dangerous. The pH factor do not
only shows the degree of dissociation. Otherwise it would not be clever to use the pH factor for caustic solutions, because
they are completely dissociated. The second factor is how much of a substance can be dissolved in water or was dissolved. So
there are also caustic solutions with different strengths. When we want to know the pH factor of an acid we have to look
at the degree of dissociation and the dissolved quantity.
Electrochemistry
The electrochemistry is based on the redox reactions. Now we will see how we can use
different substances electrochemical. We know that there are substances which deliver electrons
very easy (reducing agent) and they are not noble. There are also substances which take
electrons (oxidizing agent) and they are noble. Lithium for example delivers an electron,
because then it reachs noble gas configuration. Oxygen reachs the noble gas configuration by
taking two electrons and so it is much more noble than lithium. Because of this facts there is
the electromotive series. In this electromotive series every substance has got a certain place.
On the one side there are the strong reducing agents and on the other side there are the
strong oxidizing agents. Noble gases are not in this electromotive series, because they have
already reached the noble gas configuration. The least noble substance in this
electromotive series is lithium. The strongest oxidizing agent in the electromotive series is
fluorine and so it is the nobliest substance in the electromotive series. This
electromotive series says that there is a voltage gradient or a difference of potential
between two different substances. This electromotive series bases on hydrogen. The definition
says that hydrogen has got a voltage of 0 Volt.
Galvanic elements: Now we will look at the experiments which give us the values
of the electromotive series. These experiments are the galvanic elements. In the galvanic
elements one metal is put in a salt solution and another metal is put in another salt
solution. These both metals are connected by a wire. Both metals with the salt solutions are
in different glasses so they must be connected. For example we can use a pipe. Now we have got
a closed electric circuit. Everyone of the glasses with the metals and the solutions is called
half cell. This is the generall structure of a galvanic element. We will explain this process
with the help of another galvanic element. It is called daniell element. The difference is that
here we have got one glass with a partition wall. This wall can be only passed by ions of the salt
solutions. The whole solutions cannot be mixed, but the ions can move. To make a daniell elment
it is also important to use the right substances. We have to take copper (Cu) and to put it in a
copper sulfate solution (CuSO4) and as the second substance we have to take
zinc (Zn) and put it into a zinc sulfate solution (ZnSO4). These both metals must be
connected by a wire and now the daniell element is ready. Now we will explain why current flows
here.
We can see that at the zinc electrode there to much electrons, because zinc is less noble
than copper. Zinc is the negative pole. Here the oxidation takes place. These electrons go
on the wire to the copper electrode. This is the positive pole. Here the reduction takes place.
At the zinc electrode cations originate which go into the solution. At the positive pole
copper cations become copper atoms and they stay at the surface of the copper electrode. The
sulfate anions (SO42-) from the copper sulfate solution do not find here
enough copper cations, because they have became atoms. So they move throught the partition wall
and on the other side they find zinc cations. The copper electrode becomes bigger and bigger and
the zinc electrode smaller and smaller. The copper sulfate solution becomes less and the
zinc sulfate solution grows. We can see that there is a voltmeter in the electric circuit to
measure the voltage which is between copper and zinc always 1.1 volt. Batteries and accumulators
are nothing else then galvanic elements.
This experiment can be made with many different substances. When the difference of
potential is bigger then the voltage is bigger, too. Now we will come to the definition with
the help of hydrogen. The definition says that hydrogen has got a voltage of 0 volt. All the
other substances in the electromotive series are put in relation to the normal hydrogen
electrode. To measure the voltages for all substances we have to build a galvanic element
which consists of the substance with the unknown voltage an the normal hydrogen electrode.
Because hydrogen is a gas it is nessecary to take platinum which is surrounded by hydrogen. This
is a hydrogen electrode.
The Lead-acid battery: There are many types of batteries, but we will speak
about an advanced type of batteries. It is the accumulator. An very important example is the
lead-acid battery. The lead-acid battery is used often used, because it is used as the
battery for cars. The lead-acid battery consists of serveral plates. These plates consist of a
lattice of lead. To make electrodes of the plates some plates had to be filled with lead and
other plates had to be filled with lead dioxide. The lead electrode is the negative pole and the
lead dioxide electrode is the positive pole. These plates have to be so big to make a very high
current (over 400 A), when the car engine starts. Often there are 6 plates used and so the
voltage is 12 V. As the electrolyte there are not used two solutions as in the daniell element,
but only one, sulfuric acid (H2SO4). At the negative pole lead (Pb) is
oxidized to lead sulfate (PbSO4) and at the positive pole lead dioxide (PbO2)
is reduced to lead sulfate (PbSO4). The oxidation is very simple here, but it may
wondern why we speak about a reduction. This can only be explained with the oxidation numbers.
The lead in the compound lead dioxide (PbO2) has got an oxidation number of 4 and
in the compound lead sulfate (PbSO4) only an oxidation number of 2. When we reduce
the oxidation number then it is a reduction. When the oxidation number grows then it is an
oxidation. At the negative pole we can make it in the same way. Lead has got an oxidation number
of 0, but in the compound lead sulfate (PbSO4) it has got an oxidation number of 2,
so this is an oxidation.
We can see here the oxidation, the reduction and the whole reaction. All these reactions
are not complete, but it is often made so, because it is easier to see every substance. Only
the substances which are important for the reaction are in the reaction equation. The whole
reaction is written here with an equilibrium arrow. This shows us that this reaction functions
in both directions. The other direction needs special conditions. When the lead-acid battery
produces current it is unloading. This is the reaction from the left to right. Loading the
lead-acid battery is the backward reaction. The backward reaction happens throught an external
voltage. The direction of the current changes and the electrons flows from the positive pole
to the negative pole. This process reproduces the educts lead (Pb), lead dioxide (PbO2)
and sulfuric acid (H2SO4). In this process the oxidation takes place at the
positive pole and the reduction takes place at the negative pole. Because of some factor the
lead-acid battery has not got an unlimited life time.
The Electrolysis: In an electrolysis reactions happen with the help of current. We
will show it with the help of the zinc bromide electrolysis. We have to put two graphite
electrodes (carbon electrodes) into a zinc bromide solution (ZnBr2).
We can see that the Zn2+ ions go to the negative pole, when there is a big
voltage, because they are charged positive and the Br- ions go to the positive pole.
The bromide ions deliver one electron at the positive pole. At the negative pole the zinc
ions take two electrons. At the negative pole zinc atoms are formed and at the positive pole
bromine atoms are formed. In the electrolysis the less noble substance, here zinc, is reduced,
because it takes two electrons. The more noble bromine is oxidized, because it delivers one
electron. When we cut down the voltage source and when we connect both electrodes with a wire,
then we can measure current which flows in the other direction than the current during the
electrolysis. From the negative pole to the positive pole. Now it is a galvanic element. The
electrolysis is the reversal of a galvanic element. Now the zinc atoms deliver two electrons
and are oxidized and the bromine atoms take one electron and are reduced. Zinc bromide
(ZnBr2) originates. The difference of voltage between zinc and bromine is 1,83 Volt
and we can measure this voltage.
Another important examples for electrolysis are the chlorine alkali electrolysis.
With this process we can win sodium (Na) from common salt (NaCl), because it is so reactively
that it cannot be find in nature. The fusion electrolysis of bauxite is very important,
because the final product is aluminium.
The Periodic table of the elements
In the "Periodic table of the elements" you can find every known element. They are
sorted after the number of the protons. Hydrogen is the first element, because its atomic nucleus consists of only one
proton. The atomic nucleus of an oxygen atom consists of 8 protons. In the "Periodic table of the elements" there are also
main groups and other groups. The number of the group tells us about the number of the electrons on the last atomic shell.
The electrons are on different shells or better energy levels. The outer shell can contain maximal 8 electrons and then she
is full. Materials with a full last shell are nearly not reactively. Helium is a special problem in the
"Periodic table of the elements", because it is in the 8th main group. The reason is that it has got a full
last shell, but this shell can only take two electrons. The lower shells are not always full in the main groups, but
nevertheless a new shell originates. These empty places in the lower atomis shells are filled in the other groups. The number
of the electrons grows in a period of the "Periodic table of the elements" (horizontal in the "Periodic table of the
elements") from left to right. In every period there is one more atomic shell. This putting-up of the elements in the
"Periodic table of the elements" show also the electron configuration. Every chemical element has got a symbol which
consists of one or two letters (sometimes even three letters). The first letter is always a capital letter and the second
letter is always a small letter. Hydrogen has got the symbol H, nitrogen N, oxygen O and carbon C. The elements of one group
are chemical very similiar. I have not mentioned these groups earlier, because it is important to understand why some
groups are more reactively than other groups. So it was important to explain chemical bonds
and chemical reactions.
Author and Webmaster: Lukas Czarnecki
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Last Update: 15.12.2004