They have similar properties as well as levels of reactivity
They are the same
number of valence electrons
The atoms of the elements in the same group have the same number of valence electrons. This means that they react with other elements in a similar way. For example group 1. All the atoms in group 1 react with water and oxygen, so they have to be kept in paraffin-oil. Another example is group 18. They don't react with anything. The atoms of the same row have the same electron shells.
protons
Elements.
If the group of atoms are all the same, it is called a molecule. If the group of atoms are different, it is called a compound, assuming they are bonded in some way. Otherwise, it is just called a group of atoms.
valence electrons
shells
same group
same no of protons
Atoms of the elements in a group have the same number of electrons in their outer energy shells. This gives the elements similar Chemical Propetries.
a group of the same attoms create an element and a group composed of different atoms forms a compound