What's a group? What do elements in the same group have in common?

The vertical column of periodic table are called group.

The elements in same group having same properties.

Explanation:

The vertical column of periodic table are called group.

There are eighteen groups in periodic table.

As we move from top to bottom atomic radius increases, electronegativity decreases and ionization energy also decreases.

All elements in same group have same number of valance electrons.

The elements in same group having same properties.

Consider the first group:

All members share common properties.

They react with halogens and form salts.

2Na + Cl₂ → 2NaCl

2K + Cl₂ → 2KCl

2Li + Cl₂ → 2LiCl

They react with water and produce hydrogen gas and hydroxide of metals.

2Li + 2H₂O → 2LiOH + H₂

2Na + 2H₂O → 2NaOH + H₂

2K + 2H₂O → 2KOH + H₂

They react with oxygen and form oxides.

4Li + O₂ → 2Li₂O

4Na + O₂ → 2Na₂O

4K + O₂ → 2K₂O

Explanation: Each column in a periodic table of elements is called a group. So every element that's in the same group has the same number of valence electrons so all the elements in group 1 have one valence electron and all the elements in group 2 have two valence electrons and so on. However helium is the exception since it's in group 18 or 8A but helium doesn't have 8 valence electrons so that's the exception. Like I said before, elements in the same group have the same number of valence electrons and valence electrons are the electrons in the outermost shell.