The Atom

Atoms are made up of Protons, Neutrons and Electrons.  These are sub-atomic particles (things inside an atom, just like a submarine is a thing inside the sea).

Protons

Here's the chemical symbol for Aluminium, from the periodic table:

The number at the bottom is the Atomic number or Proton number.  This tells us how many protons there are in one atom of the element.  This is very important.  Take a look at your periodic table and you'll notice that every element has a different atomic number.  This means that the atomic number can't change - otherwise the element would be different.  And if we could do that then we'd all be turning everything into gold, right?

So we know that the atomic number can't change.  This is the number of protons in the atom.

Electrons

When an atom has no charge, and for now we'll assume they all have no charge, it has an equal number of electrons to the number of protons.  That's really useful, because it means that when we look at the chemical symbol for Aluminium, we know it has 13 protons and 13 electrons as well.

Neutrons

So what about neutrons?  To work that out we need to know what the subatomic particles weigh.

Sub-atomic Particle	Relative mass
Proton	1
Neutron	1
Electron	1/2000 (so small it’s practically zero)

Wondering about that 1/2000?  That means we'd need two thousand electrons to weigh as much as one proton or neutron, so we can ignore them when we're working out the mass of the atom.

The top number in the chemical symbol tells us the relative atomic mass.  This is how much the atom weighs.  Using this we can work out how many neutrons there are in the atom.  Let's look at Aluminium.  It has a relative atomic mass of 27.  We know it has 13 protons (which weigh 1), so taking those away from the total weight we're left with 14.  That's how many neutrons there are.

Appearance

What does all this look like?  The real answer is nobody actually knows - the atom is too small to see what it looks like.  Instead we use a model to represent it.  We'll look at the different parts again in a later lesson, but for now, here's a rough idea:

Ignore the word "orbital" for now, but here we can see 4 sub-atomic particles in the nucleus - 2 protons and 2 neutrons - and 2 electrons as well.  Using what we've learnt, can you identify the element?