Similarity in chemical properties of atoms in a group of the periodic table is most closely related to the number of valence electrons.

Valence electrons are the electrons present in the outermost energy level or shell of an atom. These electrons are responsible for the chemical behavior of an atom, including its ability to form bonds with other atoms.

Here is a detailed explanation of why the number of valence electrons is the most important factor in determining the similarity in chemical properties within a group of elements:

1. Definition of a group:
- A group in the periodic table is a vertical column that consists of elements with similar chemical properties.
- Elements in the same group have the same number of valence electrons.

2. Valence electrons and chemical properties:
- Valence electrons determine the reactivity and bonding behavior of an atom.
- Atoms with the same number of valence electrons have similar chemical properties because they have the same outermost electron configuration.

3. Similarity in chemical properties:
- Elements in the same group have similar chemical properties because they have the same number of valence electrons.
- They tend to have similar patterns of chemical reactivity, form similar types of compounds, and show similar oxidation states.
- For example, Group 1 elements, such as lithium, sodium, and potassium, all have one valence electron and show similar reactivity, readily forming +1 cations.

4. Atomic numbers and atomic masses:
- Atomic number is the number of protons in an atom's nucleus, and atomic mass is the average mass of an atom.
- While atomic numbers and atomic masses increase as you move across a period or down a group, they do not directly determine the chemical properties of an element.
- Elements with different atomic numbers and atomic masses can still have the same number of valence electrons and exhibit similar chemical properties.

In summary, the similarity in chemical properties of elements in a group of the periodic table is most closely related to the number of valence electrons. Elements with the same number of valence electrons exhibit similar chemical behavior and form similar types of compounds.