In groups on the periodic table, how do you know the charges of the elements in that group?
The number of valence electrons gained or lost will help you to know the charge of an ion.
Group 1 elements (alkali metals such as Li, Na, K...) all have one valence electron that they donate when forming ionic bonds. This will cause them to have a charge of +1.T he alkaline earth metals (Ca, Mg) have two valence electrons they donate which means their charge when forming ions is +2. The members of the aluminum family lose three electrons giving them a charge of +3.
The halogens (Cl, F, Br) all have 7 valence electrons and will gain one electron. This means they have a -1 charge. The oxygen family elements have 6 valence electrons. They gain two electrons and have a -2 charge. The nitrogen family elements gain 3 electrons, so their charge is -3.
Members of the carbon family sometimes gain 4 electrons (-4 charge) but in other situations may lose 4 electrons (+4 charge).
Transition metals (d block of the periodic table) often have more than one oxidation state.
These videos provide further discussion of this topic.