Atoms, Molecules, Elements and Compounds

The Atom

An atom is the smallest particle of a substance which can have its own characteristic properties.

Atoms are built up of even more fundamental sub-atomic particles. These are electrons, protons and neutrons.


The protons and neutrons are in the nucleus (centre) of the atom and the electrons orbit round the outside in shells (energy levels or layers).

So you will often see pictures of atoms that look a little like this:


How many protons, neutrons and electrons does an atom have?

You can work this out using the periodic table. Every element in the periodic table has two numbers with it: the atomic number and the mass number.


For example for lithium, the numbers are:

The atomic number is the number of protons that the atom has. It is also the number of electrons that the atom has. So lithium has 3 protons and 3 electrons.

The mass number is the number of protons and neutrons added together. So, for lithium there are 7 protons and neutrons combined, and we know that 3 of them are protons so there must be 4 neutrons.

The atomic number (Z) is also known as the proton number of the nucleus of a particular element. It is the proton number that determines the specific identity of a particular element and its electron structure. The mass number (A) is also known as the nucleon number that is the sum of neutrons and protons in the nucleus of an atom.

The neutron number (N) = mass number (A) – proton/atomic number (Z)

Protons and neutrons are the nucleons present in the positive nucleus and the negative electrons are held by the positive nucleus in ‘orbits’ called energy levels or shells. In a neutral atom the number of protons equals the number of electrons.



How many electrons, protons and neutrons are present in an atom of sodium?

(a) Sodium has mass number 23 and atomic number 11

Number of electrons = atomic number = 11

Number of protons = atomic number = 11

Number of neutrons = mass number – atomic number

= 23 – 11

= 12




  • Are made up of protons, neutrons and electrons
  • Are the smallest units or building blocks of elements
  • Take part in chemical reactions
  • Of the same element are the same
  • Of different elements are different due to different numbers of protons, neutrons and electrons
  • Have equal number of electrons and protons


Elements and Symbols

An element is a pure substance made up of only one type of atoms. About 92 in the Periodic Table naturally occur from hydrogen H to uranium U. Note that each element has symbol which is a single capital letter like H or U or a capital letter + small letter e.g. cobalt Co, calcium Ca or sodium Na.

Each element has its own unique set of properties but the Periodic Table is a means of grouping similar elements together. They may exist as atoms like the Noble Gases e.g. helium (He) or as molecules e.g. hydrogen (H2) or sulphur S8. All the atoms of the same element have the same atomic or proton number.

This number determines how many electrons the atom has, and so ultimately its chemistry.


Common Elements

You should know the name and symbol for the following elements. If you see the name, you should know the symbol.

If you see the symbol, you should know the name. For the elements, there are other names for the element, sometimes Latin, from which the element symbol was derived or some other name that makes the element more recognizable. You do not need to know the names in parentheses.


See also

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15 Places to WIN $10,000 Cash






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