Why do atomic masses increase down a group?

Atomic masses increase down a group because there are more protons and neutrons in the nucleus of the atoms.

In the periodic table, elements are arranged in groups and periods. A group is a vertical column, and as you move down a group, the atomic number of the elements increases. This means that there are more protons in the nucleus of the atom, which increases the atomic mass.

But it's not just the protons that contribute to the atomic mass. Neutrons also play a significant role. Neutrons are particles in the nucleus of an atom that, along with protons, make up the majority of an atom's mass. As you move down a group, the number of neutrons in the atoms also increases, further contributing to the increase in atomic mass.

It's important to note that the increase in atomic mass doesn't necessarily mean that the atom is 'bigger'. The size of an atom is determined by the area occupied by the electrons orbiting the nucleus, known as the electron cloud. While the number of electron shells does increase as you move down a group, the increase in positive charge in the nucleus pulls the electron cloud closer, often resulting in a smaller atomic radius.

In summary, atomic masses increase down a group due to the increase in the number of protons and neutrons in the nucleus of the atoms. This is a fundamental concept in understanding the structure of the periodic table and the properties of elements.