Atmos consist of a Nucles plus Orbiting Electrons
The nucleus of an atom contains protons and neutrons.
It makes up most of the mass of the atom, but takes up virtually no space - it's tiny.
The electrons are really really small.
They whizz around the outside of the atom.
Their paths take up a lot of space giving the atom an overall size (though it's mostly empty space).
The number of neutrons in an element isnt fixed
1) Every atom of a particular element has the same number of protons in it's nucleus, e.g. every carbon atom has 6 protons in it's nucleus, every nitrogen proton has 7 protons in it's nucleus, etc.
2) The number of neutrons isn't fixed though. Many elements have a few different isotopes - atoms with the same number of protons but different number of neutrons.
3) E.g. There are two different isotopes of carbon - carbon-14 has two more neutrons than more carbon (carbon-12).
4) Usually each element only has one or two stable isotopes - like carbon-12.
5) The other isotopes tend to be radioactive - the nucleus is unstable so it decays (breaks down) and emits radiation. Carbon-14 is an unstable isotope of carbon.
Radioactive elements emit ionising radiation
1) Some elements emit ionsing radiation all the time - these elements are radiocative.
2) Radiocative atoms are unstable - they break up (decay) to make themselves more stable.
3) Unstable atoms decay at random and you can't predict when it will happen - it's completely unaffected by physical conditions (like temperature) or physical processes (e.g. bonding)
4) When an atom does decay it spits out one or more of three types or ionising radiation - alpha beta and gamma.
5) In the process the atom often changes into a new element.
6) Ionising radiation can transfer enough energy to break an atom or molecule into bits called ions - this is called ionisation.
7) These ions can then go on to take part in other chemical reactions