Inside the atom

model 1

The periodic table shows the 109 elements.

There are 109 different types of atom - one for each of the 109 different elements. Differences between the atoms give the elements their different chemical properties.

An atom is tiny - about a ten millionth of a millimetre across (10^-10 m). You might think that there could be nothing smaller than an atom and that it is a fundamental particle. However, this is not the case. Atoms have structure - they are made up of even smaller particles

The first fundamental
	One of these particles is the negative electron, which belongs to the family of leptons. An electron cannot be split into anything simpler so it is a fundamental particle. There are a varying number of electrons in an atom - as few as one (in hydrogen) and up to 109.

H 1

The electrons are whizzing about outside the nucleus. They are not necessarily going in orbits.

Electrons have very little mass (less than a thousandth of the mass of the smallest atom). Most of the mass of the atom is in a central nucleus. Despite carrying most of the atom's mass, the diameter of the nucleus is 100,000 times smaller than the diameter of the atom. This is like a pinhead in the middle of an athletics stadium: the pinhead is the nucleus; spectators are the electrons.

H 2

What's in the nucleus?
	There are two particles in a nucleus: the proton and the neutron. They are both members of the hadron family of particles. As we will see, hadrons are not fundamental: they can be split into simpler particles.

Picturing sub-atomic particles
In diagrams, all these particles are made to look a bit like small coloured snooker balls. This is because we have to draw them somehow. However, it is important to understand that they are not like tiny coloured snooker balls - solid with well-defined edges.

•	They do not have any colour because they are smaller than the wavelength of visible light. So light waves pass straight over them without changing direction.
•	We cannot talk about their being solid because a solid is a collection of atoms. These particles are smaller than atoms.
•	They do not have well-defined edges but can influence other particles over a long distance

Protons and neutrons

The proton has a positive charge and a large mass (1800 times more than an electron) (table 1). The number of protons in a nucleus is the same for all the atoms of a particular element. It corresponds to the atomic number of that element. If a nucleus contains three protons, then it is part of an atom of lithium (atomic number 3).

The neutron has a similar mass to the proton but has no charge. The neutrons help keep the nucleus together.

Particle	Representation	Relative charge	Relative mass
electron		- 1	1 1800
proton		+ 1	1
neutron		o	1

Table 1

The story so far

•	Atoms have a nucleus with the electrons outside it
•	The nucleus contains protons and neutrons
•	Protons and neutrons are members of the family of hadrons, which are not fundamental
•	Electrons are members of the lepton family - they are fundamental

Forces
	The negative electron is held in the atom by the positive charge of the nucleus. This electrostatic attraction is part of one of the basic forces of nature - electromagnetism. Another fundamental force that you will know about is gravity. Gravity has a more noticeable effect over long distances between neutral objects with large masses.

The story so far
Matter is made of particles called atoms Each element is made from its own unique atoms