module 01:

module 02:
Periodic Table / Elements

module 03:
Chemical Bonds

module 04:

module 05:
Structures of Matter

module 06:
Substances of the environment

module 07:
Chemical Reactions

module 08:
Reactions in Equilibrium

module 09:

module 10:
Redox reactions

module 11:
Carbon chemistry

module 12:

module 13:
Qualitative analysis

module 14:
Quantitative analysis

module 15:
Chemical Industry

module 16:
Reactions in the environment -->



This is a module about atoms, where you can learn how atoms are constructed.
It is on a basic level, but yet you wil understand more about atoms, their discovery history, how they are composed, about their important properties.
Besides that, concepts like 'ion', 'atom mass', 'electron formula' and radio activity' wil come up.


1. Short history of the atom concept

2. The nucleus

2.1 Protons & neutrons

2.2 Radio activity & radiation

3. Electrons

3.1 'orbitals'

3.2 Valency-electrons

3.3 Electron formulas

4. Ions

4.1 Ionization energy

5. Atom mass

6. Relation between atom structure and light

1. Short history of the concept 'atom'

  • The word 'atom' appeared for the first time in the Old Greek Period, 400 befor Christ, when the phylosopher Democritos applied the concept.
    "a-tomos = not cutable" (cannot be cut into smaller pieces)
    He invented in his mind the smallest particles ever of matter.
    Imagine: one cristal of sugar that you try to cut with a razor blade into the finest pieces. At a certain moment this is not longer possible. The sugar part can no longer been cut. It cannot longer be divided.
    That this particle would turn out to be so small as what we now call 'atomd', that he certainly did not imagine.
    The properties of a substance, according to Democritos, depend directly on the shape of that smallist particle: itching substances must have atoms with a needle shape, because they prick. And sugar atoms must have a nice round shape; they feel/taste well on the tongue.
    Every substance, in the view of Democritos, has its own atoms. Well, there are enormously many substances, so there must be many different atoms. We have now other thoughts about that.

  • John Dalton (1803) went back to that concept of 'atom' as the elementary particles, building stones of all substances. But according to Dalton the number of different atoms is limited. Different substances are constructed by composing with different atoms (making molecules).

  • Thomson (1897) discovered the electron in the atom.

  • Rutherford (1871) discovered the nucleus and the composition of it.

  • Today we know lots and lots more about particles of atoms, like quarqs and the neutrino.
There are many websites about atoms.

Question 1
What is the fundamental difference between the atom of Democritos and the atom of Dalton?

2. The nucleus of the atom

Atoms have a nucleus, which is built up of nucleons (nuclear particles, the most important ones: protons and neutrons.
The nucleus has its position in the middle of the atom.

The nucleon (one nuclear particle) has a mass of 1 a.m.u. (more about this later). All nucleons, nuclear particles toghether determine the atomic mass.
The presence of protons provides the nucleus with a positive charge. Where neutrons are neutral, every atom nucleus must have a postive charge. The (positive) nucleus attracts the (negative) electrons.

Changes in the nucleus are called: 'nuclear reactions'.

2.1 Protons & neutrons

Protons & neutrons (the nucleons) are the biggest particles in the nucleus. In the nuclear physics have been discovered many more particles. f.e. the nuclear particles are built up of quarks. More information can be found on the internet (link). Those small particles stay out of this course.
Protons and neutrons have about the same mass, but different charge: protons are positive and neutrons are neutral. The positive protons attract the negative electronshells around the atom. Ehe neutrons take care for the stability in the nucleus. That is needed because equally charged particles to repell (the positive protons). The very short distance forces of masses appear toe be stronger thant the repelling electric forces. The proportion between the number of protons and neutrons in the nucleus is extremely important for the needed stability.

Question 2
  1. What kind of repulsive forces are found in the nucleus?
  2. Explain why Hydrogen atoms do not have neutrons.

Question 3
Atoms with a very high atom number (>92) cannot longer be kept stable. What could be the reason for that?
Goto answer 01-03

2.2 Radioactivity & radiation

A wrong proportion between the number of protons and neutrons will cause an unstable nucleus. Such an unstabel nucleus has the tendency to look for more stability.
That can be found, only if in the nucleus the proportion between protons and neutrons will change. There must be added neutrons, or the number of protons must change.
In unstable nuclei such a process is natural, spontaneous. It just goes on. You can't do anything against it.
Often the nucleus will reach stability via a series of this kind of changes. they can take milliseconds, but sometimes also millions of years. This type of changes (that the nucleus is managing alone) we call that: 'natural radioactivity'. They are natural nuclear reactions.
It can go differently:There are artificial ways for nuclear reactions, for example if an atom nucleus is being shot with certain very energy rich particles. The nuclei then are made unstable in an artificial way. You've got 'artificial radioactivity'.
This whole phenomena is called radioactivity.

The steps that an atom can take to find more stability, thus to change itself, are limited to the following two options:
  1. One neutron can change ointo a proton. An electron is made, and the atom will lose that electron and send it away with lots of energy (=β-radiation).
    This will happen when an atom has too many neutrons.
  2. The second option for an unstable atom is to send away from the nucleus packets of four nucleons, out of the nucleus, out of the atom even. Such a packet is composed of 2 protons and 2 neutrons; and it has a positive charge. We call this packet a α-particle or α-radiation.
    This process has the preference if there are not enough neutrons in the nucleus.

When something changes in the nucleus, you call that a nuclear reaction. You can write such a reaction in a chemical way, with a so called nuclear reaction equation.

This is a natural nuclear reaction, whereby the atom nucleus of an Uranium aton sends away a packet of 4 nuclear particles. A new atom is born: Thorium.

This is a natural nuclear reaction, whereby a radioactive Carbon atom has too many neutrons, and therefor starts to change one of them into a proton. En electron is made and powerfully sent away.

Borium wordt beschoten met neutronen en stoot dan alfa-deeltjes af
This is an artifical nuclear reaction, whereby a neutron with great power was shot inside the nucleus of a Borion atom. That's how the atom becomes unstable and suffers a nuclear reaction: a packet of four nuclear particles is immediately removed, as a reaction to the attack.

Question 4
Francium(221) is an alpha radioactive isotope. Give the nuclear reaction equation.

Fuel for nuclear plants can be Uranium(235) of which the nuclei catch a neutron, and then start a nuclear reaction that is extremely exothermic (creates lots of energy).
The other isotope, Uranium(238) can also catch a neutron and form Plutonium(239). In its turn, this Plutonium can serve as basic material in the production of atom bombs.

Question 5
Give the reaction equation of the nuclear reaction of the production of Pu.

3. Electrons

Electrons are the atom particles with negative charge, practically without any mass (2000 x lighter than a nucleon) and they are not found in the nucleus, but around it, at distance. They belong to the atom, but can occur loose, for example in cathode radiation, or in certain forms of electricity.

Electrons have a particle character, but you can consider the electron in different ways, also as electromagnetic vibrations. You can see them als pieces of radiation. A word for that is a 'photon'. This module is limited to the particle character of the electron, with a bit of extension.
An electron is just as negative as a proton is positive; so they neutralize each other 100 %.
Thats why a neutral atom must have the same number of protons in the nucleus as electrons around.

3.1 The electron shells, levels

The electrons of an atom, each with a negative charge, need a well organized space around the nucleus. They all are negative, so they will not stick toghether. On the contrary. Their behaviour and their position must be carefully arranged.
The positive nucleus attrackt them, keeps the electons toghether in the atom. But there is not only attraction, there is also repulsion between the electrons themselves. You could say: they all have to compete with each other and fight for a place. Here it is also very important to know that every electron possesses more or less amounts of energy.
Consider electrons as charges in movement, and in fact, that is the definition of electrical current. Moving charges cause an electromagnetic field and that field, in its turn, will influence the position of all electrons present in the atom.
All this contributes to the complicated arrangement of electrons in an atom, for example, the division of the electrns over the different levels. Electron levels.

We give now some simplified rules for the electron division inside the atom, in accordance to the atom theory.

Electrons have energy, and the amount of energy depends on various factors, like the distance of an electron to the nucleus.

The further from the nucleus, the more energy has an electron.

The own energy of an electron
that is under the influence of the powers of an atom nucleus,
determines in what energy level that electron may stay.

Better to speak about energy level, main level and sublevel of an electron, than about electron shells.

Below three basic rules for the main levels: (note: do not automatically fit all atoms!!)
  1. The nucleus attracts electrons; dependent on their eneregy and on the available space, the electrons stay on a certain distance from the nucleus. The inner shell, main level, most close tot the nucleus, is called number 1; the next is number 2, etcetera.
    We use for that number the letter n = main quantum number.
  2. The maximum number of electrons in level n equals 2n2;
    This number has another impact; this number is also the preference number of level n
  3. The most outside main level has a maximum of 8 electrons.

Question 6
That last rule knows a couple of exeptions in elements with a very low atom number. Explain that.

An atom of Sodium has a total of 11 electrons to divide. In level 1 can be put 2 electrons, level 2 is full with 8 electrons. Remains one electron for lever 3. There is nothing more to divide.
That third level is then the outer shell of the Sodium atom and has only one electron. This electron gets the name of "valency electron" and is very very important for the properties of Sodium. (more about this later)

Now you come to look at it a bit more precisely. Now you get some rules that cover all atoms, without exeptions:
The main levels are subdivised in sublevels.
Main level 1 has only one sublevel, of the type s with a maximum electron number of: 2
Main level 2 has 2 sublevels of the type s and p with a maximum electron number of: 2 and 6
Hoofschil 3 has 3 sublevels of the type s, p and d with a maximum electron number of: 2, 6 and 10
Main level 4 has 4 sublevels of the type s, p, d and f with a maximum electron number of: 2, 6 , 10 and 14
The main levels 5 to 7 - theoretically - could be divised in 5, 6 and 7 sublevels.
But no such big atoms exist; nature did not realise them. so:
Main level 5 divides itself in 4 sublevels of the type s, p, d and f with a maximum electron number of: 2, 6, 10 and 14
Main level 6 divides itself in 3 sublevels of the type s, p and d with a maximum electron number of: 2, 6 and 10
Main level 7 divides itself in two sublevels ofthe type s and p with a maximum electron number of: 2 and 6

When creating a sublevel, you must imagine that the filling of it always starts as close as possible to the nucleus (+ and - do attract each other). The closer to the nucleus, the less energy the electron posssesses.
The complication starts from sublevel 3d/4s on, where the sublevels start to overlap.
And always: after filling one of the sublevels with electrons, according to the rules, a small rest remains (exept at noble gas atoms). That rest of electrons takes position in the outer sublevel. That outer sublevel is mostly important for the properties of the atom of the substance. This sublevel determines also the position of the element in the periodic table. (see next module)

An example:
The Sodium atom has the following electronic division in sublevels:
1s2 2s2 2p6 3s1.

So, the atom has 3 main levels and in the outer main level 1 (valency) electron.
In this case we come to the same result with the simple 2n2 -rule.

Question 7
Every main level has at least one sublevel.
What type of sublevel must be present in every main level?

Question 8
  1. Make a table that must contain the following data: the number of valency electrons (vertical) and the total number of main levels (horizontal), for the following atom numbers: 32   54   83   56   22   73   44   68   94, and according to the simple 2n2 -rule.
  2. Make just the same table for the same atom numbers, but noiw according to the full scheme sublevels.
  3. Put the date next to each other and compare, investigate the possible differences; try to draw conclusions.

NB: Remember those data: number of valency electrons and number of main levels; they characterize the atom. So theyt are important.

3.2 Valency electrons

The electrons of the various sublevels do not differ from each other, exept in the amount of energy. Also the electrons in the outer shell (the valency electrons) are not of a different type than the other electrons in the atom. Yet we gave them this special name 'valency electrons' because of their special function. They define in an important way the properties of the element.
Normally the main shells have a preference number of electrons. When they are satisfied, the electron level is stable. The outer main level always (!) had a preference for 8 electrons, but in reality, only the noble gases have this preference numberr in their outer shell.
All the other atoms do not have such stable outer shell, and so they will always try to stablilize their situation. They do so by cooperating with other atoms. Toghether they can have succes.
Such a process to work toghether and arrive at a more stable situation is calles a 'chemical reaction'. (see also module 07)
So: The valency electrons play a main role in the creation of chemical bonds (see also module 03) during a chemical reaction (see module 07).

Question 9
The electronic structures of the noble gases are, in a certain way, similar.
Helium has not 8 valency electrons, but still is a noble gas.
Explain that.
Goto answer 01-09

3.3 Electronic formulas

We know about hundred different atoms, and every atom has an own symbol. Electronic formulas show the symbol of the atom with all its present valency electrons. You can dot that for single atoms as well as for atoms in bonds.
So the electronic formula of a Sodium aton: Na·
The dot represents one valency electron. Two dots (i.e. two valency electrons) make an electron pair that also, and often, can be indicated with a dash.

A bonding between two atoms, like in the molecule of Chlorine (Cl2) may thus be given as:

_    _
|Cl - Cl|

4. Ions

Ions are charged particles, negative or positive. Anion can be a simple atom (but with charge), or a group of atoms with charge.
They have too many or too few electrons.
So you have simple ions and complex ions. An example of a complex ion: SO42-
This particleis built up of four Oxygen atoms and one Sulphur atom, toghether five atoms. Apart from the (valency) electrons that every atom of these five has itself, there must be two electrons extra.

Question 10
Chose the right answer: A. An ion Cu2+ is:
  1. a Copper atom that lost 2 electrons
  2. a Copper atom that gained 2 electrons
  3. a particle with more electrons than protons
  4. a particle with more protons than neutrons
B. Giv en the ion Hg22+
What does it mean:
  1. Mercury atoms lose one electron, become Mercury ion and then the two ions unite, combine
  2. Every Mercury atom loses 2 electrons
  3. Two Mercury atoms catch 2 protons
  4. Every Mercury atom gains a proton and then the two ions combine.

Question 11
How to resolve the following problem:
We know that Fluorin is good for teeths, but at the same time we also know that Fluorin is very poisonous.

4.1 Ionization energy

To make positive ions, one or more electrons (negative charges) must be removed.
There is a general rule (see moldule 07 and others) that removing particles from each other always costs energy.
Some atoms only need a littlebit of energy, others a lot to get rid of an electron.
An electron on larger distance to the nucleus escapes more easy than an electron close to the nucleus.
Of the two effects (distance and charge), the distance dominates: distance is more important than charge, talking about need of energy.

Ionisation energy is the energy needed to remove completely an electron from an atom.

Question 12
Explain why the nuclear charge and the distance to the nucleus are so important when defining the ionisation energy.

Mass of atoms

Particle Mass (g) Mass (a.m.u) Charge
Electron 9,1*10-28 0 -1
Neutron 1,67495*10-24 1 0
Proton 1,67254*10-24 1 +1

The mass number of an atom equals the total number of nuclear particles (nucleons).
The atom number of an atom equals the number of protons is the nucleus.

The mass of electrons can be neclected, compared to the mass of nucleons (protons and neutrons). To determine the mass of an atom, you only need to count the number of nucleons: than you get the mass number. That's why an atom will/must have the same mass as its fellow ion. That ion only has one or some electron extra or short, and they weigh about nothing.

An element can have isotopes. Atoms of that element have an equal atom number, but a different mass number. In nature, many elements are composed of some isotopes, and they occur is a fixed proportion.
The average atom mass of that element is then determined by the proportion of the various isotopes. The values normally can be found in tables.

Every atom has its own mass number: the sum of the nuclear particles. The Clorine atom has 17 protons, and can have 18 neutrons. Or: the mass number of this isotope is 35.

But Chlorine can have 20 neutrons; then the Chlorine isotope has a mass number of 37.
The average atom mass of the element of Chlorine is based on the mass number of the different isotopes of the element.
Easy to believe that the average is 36, but that is wrong thinking. In nature we find about three times mor 35Cl compared to 37Cl.
Finally you can calculate that the average atom mass of Chlorine is about 35,5.

Question 17
Natrual Carbon is composed of 998,89% Carbon isotope 13C
The relative atom masses of the isotopes is 12,000 for 12C and 13,003 voor 13C
Calculate the average atom mass of this element.
Goto answer 01-17

6. atoms and the light spectrum

In a hot flame, atoms can acquire so much energy that some electrons (sometimes but not always the valency electrons) become enormous moving, and they can not longer remain at their fixed position: they become in an 'exited state' in stead of staying in their 'ground state'.
For a moment they leave their own energy level and remove themselves a bit further from the atom nucleus.

Then you can have two options:
  • Such an electron (mostly a valency electron)has got enough eneregy to remove completely from the atom. Thus an ion is formed, and we talk about 'ionisation' of the atom. The needed energy for that is called the ionisation energy. There are table showing ionisation energy of atoms.
  • Such an electron (in this case mostly not a valency electron) does not get enough energy to remove completely. It just goes to a bit higher energy level. No ion is formed but yes, an exited state. And an exited state is not stable. (as everyone knows). This has to go back to its original state, ground state.
    Going back means: the before gained energy must get lost. The electron cannot go back to the old energy level and keep its newly gained energy. This loss of the energy often occurs in the form of electromagnetic radiation (can be visible light). In that case you can observe a color effect.
    Every type of atoms causes in this way its own light spectrum.

    Sodium yellow   Copper green
    Potassium weak violet   Stannium blue
    Calcium red   Lead weak blue
Question 18
Bring with a Platinum wire a couple of cristals of NaCL (kitchen salt) in a hot flame. What do you observe?