This is a way of working it out:
- How many electrons are on the outer shell?
- How many shells does it have?
- To fill up its outer shell how many electrons will it take?
- Now see weather it will take more transferring to loose electrons (to go down an shell,) or gain electrons (to fill the shell)
- Which ever one takes the least transferring will be the route that was taken
- If it lost, it will have a positive charge of the number of electrons lost to empty the shell
- If it gained, it will have a negative charge of the number of electrons it gained to fill the shell
Most of the time the atoms will be on the second orbital in which case its simpler to think of it like this:
Similarly atoms with more than four electrons will gain electrons to fill their outer shell. This means they will make negative ions: as an example a group 7 electron has to gain one electron to fill its outer shell and so will become -1.