Structure and Properties of Matter
Inside the atom
Count protons, neutrons and electrons from an atomic number and a mass number, fill the shells and read off a valency, and say what makes two atoms isotopes.
A hydrogen atom has one proton and a helium atom has two, so helium ought to weigh twice what hydrogen does. It weighs about four times as much. Something in the nucleus was adding mass without adding charge, and in 1932 James Chadwick found it: the neutron, about as heavy as a proton and carrying no charge.
Two numbers describe an atom
The atomic number Z counts the protons, and it is what decides which element you have. The mass number A counts the protons and neutrons together — the nucleons — because the electrons are too light to matter to the mass. Since an atom is electrically neutral, the electron count matches the proton count. Subtract Z from A and what is left is the number of neutrons.
Have a play
The parts of an atom, each one named.
Tap any part of the picture.
Worked example
An atom is written with mass number 23 above and atomic number 11 below its symbol. How many protons, electrons and neutrons has it?
Read the lower number: the atomic number is 11, so there are 11 protons.
The atomic number is defined as the proton count, and it is the number that says which element this is.
Try it together
Now do the same for a chlorine atom, and go one step further.
A chlorine atom has atomic number 17 and mass number 35. Answer with a bare number each time.
1.A chlorine atom has 17 protons and a mass number of 35. How many neutrons has it?
Have a go
Have a go on your own. An atom has atomic number 12 and mass number 24. How many neutrons has it? Answer with a number.
Ready to practice?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.