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Atomic Structure

Atoms | Atomic Number and Mass Number | Electron Arrangement

Atoms

All substances are made of tiny particles called atoms.

There are approximately 100 different types of atoms from which everything is made.

A substance which contains only one type of atom is called an element.

Atoms are made up of three tiny particles called protons, neutrons and electrons.

Particle	Position in atom	Charge	Mass
Proton	Nucleus	Positive	1
Neutron	Nucleus	No charge	1
Electron	In energy levels (shells) outside the nucleus	Negative	Negligible

Atoms have no overall electrical charge because the number of positive protons and negative electrons are equal, so they cancel each other out.

At the centre of an atom is the nucleus, which is very small.

It has a positive charge because it contains protons. It is very dense because it contains protons and neutrons which make up most of the mass of an atom.

Atomic Number and Mass Number

All the elements are listed in the Periodic Table.

Each element has a symbol and two numbers. The numbers tell us how many protons, neutrons and electrons there are in the atom.

This is the information for sodium:

Mass number	23
	Na
Atomic number	11

The atomic number shows how many protons there are in the atom. It is sometimes called the proton number.

It is the number of protons in an atom that determines which element it is. All atoms of a particular element have the same number of protons.

Because there is no overall charge on an atom the number of electrons is the same as the number of protons.

The mass number tells us how many protons and neutrons there are altogether in the atom, because these are the particles which have the most mass.

So sodium has 11 protons and 11 electrons.

The number of neutrons =	mass number	–	atomic number
=	23	–	11
=	12

Electron Arrangement

The electrons are arranged around the nucleus in orbits called energy levels or shells.

The electrons fill the lowest energy levels, which are the innermost shells, first.

The innermost shell can only hold 2 electrons, the next shells are bigger and can hold 8 electrons.

So in sodium the electron arrangement is 2, 8, 1

Some atoms of the same element can exist with different numbers of neutrons in their nucleus. These are called isotopes.

For example most carbon exists with a mass number of 12, but some carbon atoms have a mass number of 14.

	12 C 6	14 C 6
Name	Carbon-12	Carbon-14
Protons	6	6
Neutrons	6	8
Electrons	6	6

Both are atoms of carbon because they both have 6 protons in their nucleus, but Carbon-14 has two extra neutrons.