Because they have shells!!!! Well, electron shells, or energy levels as they are sometimes called. You need to know where to place electrons within a shell, it's easy once you know how! Make sure you've know how to work out the number of electrons in an atom - there's an explanation here.
Some are smaller than others! The first shell is tiny and can only hold 2 electrons - all other shells hold 8 electrons (you'll learn that this isn't quite true at A-level). Let's start by drawing lithium together step-by-step:
Grab a piece of paper and draw beryllium - click on this link to see if you're correct! (PS - the link opens in a new page)
If you're not sure what groups and periods are click here (link opens in a new page).
Look at the atoms drawn below and see if you can work out the link between an atom's electronic structure, its group and its period:
The number of electron shells an element possess is the same as its period. Have a look at the diagram above - hydrogen and helium are in period 1 and they have one electron shell. Every element in period 2 has two electron shells. Simples!
The no. of electrons in an atom's outermost shell is the same as its group. If you look at the diagram above - every element in group 1 has 1 electron in its outermost shell. Every element in group 2 has 2 electrons in its outer shell and so on. How clever is the periodic table? VERY!!!!!!
The answer is yes and no. At GCSE you can put them anywhere in the shell and you'll still get the marks provided you have the right number of electrons in each shell. But knowing where to place electrons will help at A-level and with something called covalent bonding.
In the first shell the two electrons are placed at the top and bottom of the shell. In every other shell the first four electrons are placed at the four compass points (North, South, East, West) and then you repeat the process (i.e. the fifth, sixth, seventh and eighth electrons are paired). Look at the diagram above and double check you understand where to place electrons.