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We know that matter is made up of particles called atoms and that an element is a substance that is made up of only one kind of atom. But what are atoms made up of?

A very important part of chemistry is understanding the structure of atoms and the differences between them.

Around 400 B.C., there was a brilliant philosopher named Democritus, and he proposed that all matter was made of small particles. The Greeks called these particles atomos, meaning indivisible, and the modern word “atom” is derived from this term.

Although Democritus had the right idea, we now know that the atom is not indivisible. Atoms are in fact made up of three smaller particles called protons, electrons and neutrons.

The protons and neutrons are contained in the nucleus, the tiny centre of an atom, with smaller electrons orbiting outside the nucleus.

Each particle has its own charge and its own mass.

The mass of an electron is very small compared to a proton or a neutron. 1 840 electrons have the same mass as 1 proton or neutron.

Since the nucleus only contains protons and neutrons, most of the mass of an atom is concentrated in its nucleus.

Protons and electrons have electrical charges that are equal and opposite. An atom is neutral because it always contains the same number of protons and electrons.

Remember: Proton Positive Neutron Neutral

Oxygen and carbon are both elements. Both are made up of atoms which in turn are made up of protons, neutrons and electrons. But what makes oxygen different from carbon?

It is the number of protons in the nucleus of an atom that is the defining feature of the atom. It's what makes one element different from another. The number of protons in an atom is called its atomic number. The atoms of each element in the periodic table have different number of protons, and hence different atomic number. In fact, the periodic table is arranged in order of increasing atomic number.

The atomic number of an atom is the number of protons in its nucleus

• all the atoms of a given element have the same atomic number
• the atomic number of each element is unique - no two elements have the same atomic number

For example,

the atomic number of carbon is 6

This means that:

• there are 6 protons in the nucleus of every carbon atom
• no other element has atomic number 6

the atomic number of oxygen is 8

This means that:

• there are 8 protons in the nucleus of every oxygen atom
• no other element has atomic number 8

A second number is needed to fully describe an atom. This number is called the mass number. Since the mass of an atom comes almost entirely from the protons and neutrons, it is fairly obvious that the mass number is the total number of protons and neutrons

The mass number of an atom is the total number of protons and neutrons in its nucleus

mass number = number of protons + number of neutrons

If you know the mass number (number of protons + number of neutrons) and the atomic number (number of protons), then you can calculate the number of neutrons:

number of neutrons = mass number – atomic number

Summary

atomic number = number of protons in nucleus

• mass number = number of protons + neutrons in nucleus
• number of neutrons in nucleus = mass number - atomic number

Since an atom is neutral there are the same number of protons and electrons. So, the atomic number also equals the number of electrons orbiting the nucleus. But remember, there are no electrons inside the nucleus.

When we write the symbol for an atom, we can place its mass number at the top left and its atomic number at the bottom left.One way to remember the order of these numbers is the word ma: mass number then atomic number

This is the chemical symbol for sodium

• Mass number = 23• Atomic number = 11

Number of protons in a sodium nucleus = 11 (the atomic number)Number of electrons orbiting a sodium nucleus = 11. (the atom is neutral)Number of neutrons in a sodium nucleus = 23 – 11 = 12. (mass number - atomic number)

Write down the number of particles in each of the following atoms:

Be careful of a question like the one below!

Always check: is the question asking about the number of particles in the atom (in which case you include the number of electrons), or is it asking about the number of particles in the nucleus (in which case there are no electrons)?

Isotopes are atoms of an element with the same atomic number but a different mass number, indicating a different number of neutrons.

Electrons orbit the nucleus in shells. There can be several shells and:

• each shell is at a different distance from the nucleus
• the first shell is nearest to the nucleus
• different shells hold a different maximum number of electrons
• electrons completely fill up the lowest available shell first – i.e., the one nearest the nucleus
• when this shell is full, the electrons begin to fill the next shell, and so on

The table below shows the maximum number of electrons for each of the first three shells

The electronic configuration or structure of an atom is how the electrons are arranged as they orbit the nucleus. Electronic structure can be shown as a list of numbers or as a diagram.

Take lithium for example. The diagram above shows each shell as a circle around the nucleus, with each of the three electrons represented by a dot.

The electronic structure for lithium is written as a list of numbers separated by commas is 2,1. This shows that lithium atoms have three electrons, two in the first shell and one in the second shell.

Sodium has 11 electrons. These will be arranged as:

• 2 electrons in the first shell
• 8 electrons in the second shell
• 1 electron in the third shellYou can write sodium’s electronic structure with as: 2, 8, 1

The electronic structure of sodium can also be shown in a diagram. Each dot represents an electron.

There are some very interesting links between electronic structure and the periodic table.You will remember from the Periodic Table study guide that the vertical columns in the periodic table are called groups. The groups are numbered from left to right. Elements in the same group have similar chemical properties.

• The number of electrons in the outer shell of all the elements in a group is the same as the group number.
• The number of electron shells of all the elements in a period is the same as the period number.

Examples

All the Group 1 elements - have one electron in the outer shell.

The Group 7 elements - have seven electrons in the outer shell.Group 0 elements have full outer shells.The atoms of all group 1 elements have similar chemical properties and reactions because they all have one electron in their outer shell.

Similarly, the atoms of all group 7 elements have similar chemical properties and reactions to each other because all of them have seven electrons in their outer shell.

Remember that we use chemical symbols to stand for the elements. For example, C stands for carbon, O stands for oxygen, S stands for sulfur and Na stands for sodium.

For a molecule, we use the chemical symbols of the atoms it contains to write down its formula. For example, the formula for carbon monoxide is CO. It tells you that each molecule of carbon monoxide consists of one carbon atom joined to one oxygen atom.

Take care when writing your symbols and formulae. Be careful about when to use capital letters. For example, CO means a molecule of carbon monoxide but Co is the symbol for the element cobalt.

Formula and formulae

The word ‘formulae’ is the plural of ‘formula’. If we have more than one formula, we don’t say formulas, we say formulae.

The formula for a substance is always the same

The table below shows some common compounds. It’s useful to know that mono means one, and di means two.

• carbon monoxide has one atom of oxygen
• carbon dioxide has two atoms of oxygen

Chemical formulae for simple metal compounds - valency

So, how are the numbers in simple formulae worked out?

When writing formulae for simple metal + non-metal compounds, the valency of the elements must be considered.

Valency is the ability of an atom to gain or lose electrons. The valency of an element is linked to how many electrons an atom must gain or lose to achieve the same electronic structure as a noble gas i.e., have a full outer shell.

For example

• an element in group 2 has two electrons in its outer shell. The simplest way to have a full outer shell is to lose two electrons, and so the valency is 2

• an element in group 5 has five electrons in the outer shell. The simplest way to have a full outer shell is to gain three electrons, and so the valency is 3

A quick way of calculating the valence of an atom is:

• For Groups 1 – 4 the valency is the same as the group number
• For Groups 5 – 7 the valency is 8 minus the group number

Sometimes a Roman numeral is used to tell you the valencyFe(III) means that iron has a valency of 3

Use the periodic table and the rules above to check the valency of each of the following elements

To write the chemical formula for a compound you can use the S.V.S.D.F system.

1. S - write down the symbols of both the elements involved.
2. V - beneath each symbol, write it’s valency. Memorise the rules above.
3. S - swap the valences over.
4. D - if the valences can be simplified, divide them both by the smaller of the two numbers.
5. F - write the formula.

(A mnemonic to remember this is: Study Valency to Simply Draw-up Formula)

Example

What is the formula for sodium chloride?

The formula for sodium chloride is NaCl

Example

What is the formula for water if the elements involved are hydrogen and oxygen?

Example

What is the formula for carbon dioxide?

Example

Some elements, particularly the transition metals, do not always have the same valency in their different compounds. The valency of these elements is usually given in roman numerals inside brackets.

Example

What is the formula of copper(I) oxide?

Try this one for yourself:

It is worth noting that that valency will be given on the back of the Periodic table for transition metals.