All things around us are made of atoms. The clothes we wear, the buildings we live in, the air around us, even our own bodies. Atoms are the microscopic building blocks of every solid, liquid or gas. If all the atoms in a substance are the same type - it is called an element. This means it cannot be broken down into any "ingredients". Oxygen (chemical symbol O) is a pure element. Oxygen is oxygen - it is not made up of anything else. If a substance contains more than one element, it is either a compound or a mixture. We will discuss these later, for now let's stick to elements.

We said before that elements have no "ingredients" - but actually they do. Even a tiny atom is built of smaller building blocks. All atoms are built of 3 basic things : Protons, Neutrons, and Electrons.

The simplest atom is Hydrogen. A hydrogen atom is built of one proton and one electron. The proton sits in the centre of the atom and forms the nucleus of hydrogen. The electron spins around the nucleus - a bit like a moon orbiting a planet. Hydrogen, as you can see in the diagram below, does not have any neutrons. Larger atoms have many protons and neutrons in their nucleus, and have many orbiting electrons.

So why doesn't the electron just spin off ? Why does it stay orbiting the nucleus?

The answer is that the electron is attracted to the nucleus because of its 'charge'. Electrons have a negative charge and protons have a positive charge (neutrons do not have a charge). Opposite charges attract each other.

Need help to understand charge attraction? This is why clothing taken from the drying machine 'clings'. A static charge is formed when the clothing rubs together in the machine, and this builds up a negative charge. The negative clothing is then attracted to more positively charged things around it.

The force caused by the spinning, which should cause the electron to spin off away from the nucleus, is balanced by the charge force attracting the electron towards the nucleus. So it doesn't fly off OR cling to the nucleus, it spins around being pulled equally in both directions.

Although the proton has a positive charge, and the electron has a negative charge, the atom itself has no charge - it is neutral. All atoms have the same number of negative orbiting electrons as they have positive protons in their nucleus (each '+' is cancelled out by a '-'). If you have trouble grasping this - just do the maths. Every proton is +1, every electron is -1, and the total of all protons and electrons has to be zero.

know that opposite charges attract each other, but the converse is also true - the same charges repel each other. When there is just one proton in the nucleus (like the Hydrogen we just looked at), there isn't a problem, but if we had two protons in the nucleus, lying side by side, they would repel each other. They need something to keep them apart. Enter the neutron. If we draw another element, with two protons in the nucleus, we have a different element - Helium.

It is the number of protons in an atom that determines which element it is.

Because Helium has 2 protons, it needs 2 electrons to be charge neutral (2 positives need 2 negatives). Also notice that Helium has 2 neutrons in the nucleus to keep the positive protons from repelling each other and breaking the atom apart. Because neutrons do not have a charge, they do not affect the overall charge of the atom.

We could carry on drawing bigger atoms with increasing numbers of protons, neutrons, and electrons. If we did this, we would draw the elements described in the table below :

Protons Neutrons Electrons Element Name

1 0 1 Hydrogen

2 2 2 Helium

3 4 3 Lithium

4 5 4 Beryllium

5 6 5 Boron

6 6 6 Carbon

7 7 7 Nitrogen

8 8 8 Oxygen

9 10 9 Flourine

As we increase the number of protons, the number of electrons also increases.

As we start to get more and more negative electrons around the nucleus, they will start to repel each other (Electrons have the same charge, and remember that same charges repel). For this reason, the electrons that orbit the nucleus cannot be all bunched together. Like the protons in the nucleus they need to be kept apart. So electrons orbit the nucleus at different distances, forming layers or 'shells' of electrons. This is a little bit like the planets orbiting the sun.

In lithium, the 3 electrons are in shells: two in the first, and one in the second shell.

The first shell can hold 2 electrons before it is full, the bigger second shell can hold up to 8, the third shell up to 18. Because we know the maximum number of electrons in each shell, we can accurately draw the atomic structure of Oxygen (lets think of the nucleus as one blob in this diagram rather than drawing all the protons and neutrons).

Protons Neutrons Electrons Element Name

8 8 8 Oxygen

Oxygen has 8 protons, therefore it must have 8 electrons. We know that 2 of these electrons can fit into the first shell, and the remaining 6 go into the second shell (which can hold 8).

When scientists write down a description of an element, they try to show all the information needed to draw the diagrams, and there is a method for doing this. Let's look at the standard way of writing Hydrogen, Oxygen, and Fluorine :

The letter is the CHEMICAL SYMBOL for the element.The top number is the MASS NUMBER: the sum of the protons and neutrons.The bottom number is the ATOMIC NUMBER: the number of protons or the number of electrons (as there are the same number of protons and electrons in an atom).

Using these descriptions we can work everything out for the third example, fluorine, even though we have not talked about flourine yet.

Flourine has the chemical symbol F. The ATOMIC number tells us that It has 9 protons and 9 electrons. The MASS number, representing protons and neutrons, is 19. We already know that there are 9 protons, so the other 10 are neutrons. Try to work out the same answers for Hydrogen (H) and Oxygen (O).

Stability.Every atom wants to be stable, and stability for an atom is achieved by having the right number of electrons in its outermost shell. As we have

seen, each shell has a maximum number of electrons that it can hold, but shells also have an ideal number of electrons. If the outermost shell of an atom contains this ideal number, it will be a stable atom.

If the outermost shell does not contain this ideal number, the atom will strive to achieve stability by gaining or losing electrons in their outer shell. If this cannot be achieved by shuffling their own electrons around, atoms interact or react with other atoms to take or share electrons to achieve their goal.

Atoms that already have the ideal number of electrons in their outer shell have no desire for more electrons, and don't want to lose any either. These atoms tend to be unreactive or inert. Atoms that that do not have the ideal number tend to be reactive, striving to get their hands on more, or get rid of some electrons.

The table below lists shells one to five, and shows the maximum number of electrons each shell can hold, but also the ideal number fot the atom to be stable if that shell is the outermost shell..

Shell Can hold is stable with an outer shell

holding

1 2 2

2 8 8

3 18 8

4 32 8

5 50 8

Lets think about two atoms that we are familiar with: Helium and Oxygen.

Helium has 2 electrons in its outer shell. This is the ideal number for the first shell, so as we would expect, Helium

is unreactive and inert.

Oxygen has 2 electrons in the first shell, but only 6 in its second outer shell. This is not the ideal of 8 electrons in the second shell. We would expect oxygen to do something about this, and try to get 2 more electrons from somewhere by reacting with another atom. This is indeed the case, and oxygen gets involved in many reactions with other atoms.

However, rather than seek out something to react with, there is an easier way for an oxygen atom to achieve stability. If we took two Oxygen atoms, each with 6 electrons in their outer shell, there are a number of options :

1) One oxygen atom could give 2 electrons to the other oxygen atom, producing one stable atom with 8 electrons in its outer shell (ideal), but one unstable atom with an outer shell of 4 (not ideal) - so that is not the answer.

2) The two oxygen atoms pair up and share electrons with each other.

In the diagram above, we have coloured the electrons differently for our explanation, but the two oxygen atoms are identical.

By pairing up and sharing, each oxygen atom effectively has the ideal 8 electrons in its outer shell. The atom on the right has 6 orange electrons of its own, and two blue electrons shared with the atom on the right. The atom on the left has 6 blue electrons of its own, and two orange

electrons shared with the right atom. As long as they stay together, the atoms are stable.

This is called COVALENT bonding, which means that electrons are shared between 2 atoms. Oxygen atoms are so reactive, that the element does not exist naturally as atoms. Oxygen atoms always pair up and exist as units of two atoms. This is an oxygen MOLECULE (as shown above) which you will see written as O2 (pronounced "oh two").

Many other gases also exist as molecules, not atoms. Hydrogen has only one electron in its outer shell, and gets the ideal two electrons by existing as H2 and sharing its electron with another hydrogen atom. Nitrogen, with 5 electrons in its outer shell, needs to get another three, so it shares 3 electrons with another nitrogen atom to get its ideal outer shell of 8.

When these molecular gases get involved in reactions, the bond in the molecule must first be broken apart so that the individual atoms can react. The more electrons that are shared in the bond, the stronger and more difficult to break the bond is. Molecular gases with stonger bonds are less reactive than those with weaker molecular bonds.

So we know that the bond in hydrogen gas (H2 - with one shared electron) will be easy to break, so hydrogen will be very reactive. Oxygen gas (O2 - a stronger bond with two shared electrons) will be quite reactive, and Nitrogen (N2 a strong bond with three shared electrons) will be faily unreactive.

Although you can split O2, H2, and N2 molecules into two atoms, they are still a elements. N2 is pure nitrogen. However when two different atoms react together and form a bond, the result is called a compound. The most common compound, using atoms that we are already familiar with,

is water (H2O).

In a water molecule, the two hydrogen atoms each share one electron with the oxygen. This gives each hydrogen atom two electrons in their outer shell (the ideal number for shell 1), and the oxygen atom eight in its outer shell (the ideal number for shell 2). Each atom has an ideal outer shell, so we would expect water to be a stable and unreactive compound (and indeed it is).

Lets take a different example. The reaction between Sodium (Chemical Symbol: Na) and Chlorine (Chemical Symbol: Cl).

Sodium (Na) has 11 protons, thus 11 electrons (we know from the Atomic Number). The electrons will be arranged in shells: 2 in the first shell, 8 in the second shell, and 1 in the outer shell. We write this electron arrangement in this format: 2,8,1.

Chlorine (Cl) has 17 electrons: arranged 2,8,7.

Chlorine (2,8,7) needs one electron to fill its outer third shell to the ideal of 8, Sodium (2,8,1) would be better to lose the electron in its outer third shell, and allow the full second shell of 8 underneath to become its outer shell.

Sodium needs to lose an electron, and chlorine needs to gain one - these two atoms are a very compatible pair. When a reaction takes place between them, there is no sharing of electrons, sodium gives away one electron to chlorine.

However, this introduces something new. Sodium now has 11 protons, but only 10 electrons (because it has given one away). Chlorine has 17 protons, but now has 18 electrons (as it has taken an extra one from Sodium). The effect of this is easy to calculate - it leaves the atoms with a charge - they are no longer neutral.

The Na (Sodium) with one more positive proton than negative electrons is positively charged, and the Cl (Chlorine) with one extra negative electron is negatively charged. However as a pair, they are still neutral (the sodium is "+1" and the chlorine is "-1").

Any atom that carries a charge is called an ION (pronounced "eye on"), and ions can be positive or negative. The ions in this example would be written as Na+ and Cl-. If sodium lost another electron, it would become Na2+ and so on.

Because our Na+ and Cl- ions carry opposite charges, they are attracted together as a pair. They are bonded by ionic attraction. This type of bonding, where atoms give away or accept electrons to form a pair of charged ions, is called IONIC bonding. The compound formed when Na and Cl react is written NaCl. This is Sodium Chloride, which is more commonly known as salt. The next time you sprinkle some on your dinner, you will know exactly how it is formed.

When atoms have an ideal number of electrons in their outer shell, we know that they are stable and unreactive. But the closer that they are to being ideal, the more reactive they are. Atoms that are just one electron away from ideal are very reactive. It's almost as if they know that the ideal shell is within their reach, so they are very keen to achieve it.

The sodium in our last reaction needed to lose just one electron, so we can predict that sodium is very reactive. The chlorine needed to gain just one electron, so again we can predict it is very reactive.

Because these atoms are both very reactive, and are also perfect partners (one needs to lose an electron, and one needs to find an electron), we would expect the reaction between them to be very fierce - and it most certainly is. The reaction between sodium and chlorine is so violent, that if you simply mixed the two together there would be a huge explosionWith our new knowledge, we could now go through the range of atoms one by one and draw their atomic structure, and try to determine how reactive each one might be by looking at the number of electrons in its outer shell.

Remember: the first shell wants 2 electrons, the second, third and fourth want 8 electrons.

Protons ElectronsElement

NameSymbol

Electrons in shells

Reactive ?

1 1 Hydrogen H 1 Very

2 2 Helium He 2Not at all (ideal !)

3 3 Lithium Li 2,1 Very

4 4 Beryllium Be 2,2 Fairly

5 5 Boron B 2,3 Some

6 6 Carbon C 2,4 Some

7 7 Nitrogen N 2,5 Some

8 8 Oxygen O 2,6 Fairly

9 9 Flourine F 2,7 Very

10 10 Neon Ne 2,8Not at all (ideal !)

11 11 Sodium Na 2,8,1 Very

12 12 Magnesium Mg 2,8,2 Fairly

13 13 Aluminium Al 2,8,3 Some

14 14 Silicon Si 2,8,4 Some

15 15 Phosphorus P 2,8,5 Some

16 16 Sulphur S 2,8,6 Fairly

17 17 Chlorine Cl 2,8,7 Very

18 18 Argon Ar 2,8,8Not at all (ideal !)

19 19 Potassium K 2,8,8,1 Very

Can you see a pattern? The numbers of electrons in the outer shell give different elements similar properties. Those with ideal outer shells are unreactive. The elements either side of this, that are nearly ideal are very reactive. Those further away from being ideal are not so reactive.

Lets reorder the elements in the table above, grouping them by level of reactivity. It is easy to see in the table below that elements with 8 electrons in the outer shell are all stable, those with 1 or 7 electrons in the outer shell are very reactive :

Outer shell

electrons1 2 3 4 5 6 7 8

Reactive ?

Very Fairly Some Some Some Fairly Very Not

Shell 1 H He*

Shell 2 Li Be Bo C N O Fl Ne

Shell 3 Na Mg Al Si P S Cl Ar

Shell 4 K ?? ??

*although Helium has only 2 outer shell electrons, it is a full (ideal) number for shell 1, so we put it in with the other unreactive elements with ideal shells in column 8, not in column 2.

Once we have the elements in this format, we can actually go one step further and start guessing elements.

For example, we can predict that an element exists in the yellow cell in column 2 (next to Potassium [K]). It will have 20 protons and electrons (one more than the 19 protons and electrons that K has). This mystery element's electrons will be arranged 2,8,8,2 - thus with 2 in its outer shell it will be fairly reactive.

Bravo - you have just "discovered" an element. This element is Calcium

[Ca] (2,8,8,2).

Further along the row, there is a mystery red cell in column 8 (under Argon [Ar]). We know that there will be an element with 26 electrons (six more than the Calcium that we just discovered) in the configuration 2,8,8,8. As this element has an "ideal" outer shell, it will be unreactive. This element is Krypton [Kr] (2,8,8,8).

This method is how early scientists began to discover the elements. They loooked for gaps in this table, and realised that there must be an element to fill it.

We have just covered some fairly advanced stuff - constructing the foundation of chemistry. The table we have just produced is a basic version of the Periodic Table of the Elements.

The columns represent the number of electrons in the outer shell (from 1 to 8) and are called GROUPS. Elements in the same GROUP will have similar properties.Each row represents a new shell of electrons, (notice: each new row is the start of a new shell once the previous shell is full). Rows are called PERIODS.

So using this convention we can say that Lithium (Li) is in Group One, Period Two.

We can guess the characteristics that an element might have by its group and period, and also pair elements together that are likely to produce strong reactions.

Remember Sodium (Na: Group 1) and Chlorine (Cl: Group 7) reacting violently when Na gave one electron to Cl to form an ionic bond?

We can predict that the other members of Group 1 - Lithium (Li), Hydrogen (H), and Potassium (K) will also react well with Chlorine (Cl) as they too want to get rid of one electron from their outer shell. For the same reason other members of Group 7, that want to find an extra electon for their outer shell will react well with Sodium (Na) - so Fluorine (Fl) will react well.

Group 1 elements with one electron to spare will always react well with group 7 elements that need an additional electron.

We can also go one step further, and predict how violent a reaction might be.

We know that it is the attraction between the negative electrons and the positive protons in the nucleus that keep the atom held together. So it makes sense that the further away the electrons are from the protons in the nucleus, the weaker the attractive forces will be between them.

Electrons very close to the nucleus in shell 1 (period 1 elements) will be held strongly, electrons far away in shell 3 or 4 (periods 3 and 4) will be held quite weakly.

Lets think about the reactions between group 1 and group 7 again. The element in group one needs to give away one electron. The element in group 7 needs to attract one electron.

It makes sense that Hydrogen (H) with its electron close to the nucleus in shell 1 will find it hard to give that electron away, as the attraction between the negative electron and the positive nucleus will be strong.

Lithium (Li) holds its single electron further away in shell two - further from the positive nucleus. The attraction will be weaker, and Lithium will find it easier to give away this electron, and should be more reactive.

Sodium (Na) with its single electron even further away in shell 3 should find it easier still and be more reactive again. Potassium (K) with the electron in shell 4 will have significantly weaker forces to overcome to give away its electron, and be very reactive.

From this we can deduct that the attractive forces in shell 1 are greater than in shell 2, those in shell 2 are greater than in shell 3, and so on.

So - it also makes sense that Fluorine will attract an electron into shell 2 more strongly than chlorine attracting the electron into shell 3, as the attraction in shell 2 is stronger than shell 3.

Group one elements will be more reactive as we move down the periods (from row to row, shell to shell), because the electron attraction gets weaker and it is easier to give electrons away. Group seven elements will be more reactive as we move up the periods, as the electron attraction is stronger in shells closer to the nucleus, and they can pull electrons with more force.

So to test this theory we can predict that a reaction between Potassium (K) and Fluorine (F) would be particularly violent, as it is very easy for Potassium to lose its electron from shell 4 where the attraction is weak, and Flourine will attract that electron rapidly into shell 2 where the attraction is strong.

This is indeed true. Potassium (K - Group 1, Period 4) finds it easy to lose this outer electron from shell 4, and Fluorine (F -Group 7 Period 2) attracts the electron strongly into shell 2. This reaction is very, very violent.

Finally (at last - phew !!) - it is worth making clear that we cannot create or change elements. We don't have the ability to add and subtract protons from the nucleus (as you might have thought when we were drawing them here). Elements occur naturally, and while we can react them together to form compounds, we don't change them. If we could add protons to the nucleus we would be able to change Lead (82 protons) into Gold (79 protons) by taking three protons from its nucleus. If anyone finds out how to do this - please let me know.