The atomic number of an element refers to the number of protons in the nucleus of an atom of that element. Since protons have a positive charge, the atomic number determines the element's identity and its position on the periodic table. As we move from left to right across a period on the periodic table, the atomic number increases because each subsequent element has one more proton in its nucleus compared to the previous element.
However, when we move down a group (also known as a column) on the periodic table, the mass of an atom generally increases. This is because as we move down a group, we are adding more energy levels or electron shells to the atom. The additional energy levels can accommodate more electrons, which increases the total number of particles in the atom (protons, neutrons, and electrons).
While the number of protons (atomic number) increases down a group, the increase in the number of neutrons and electrons is usually greater. Neutrons, unlike protons, do not directly affect the atom's chemical properties, and the number of neutrons can vary within a specific element, leading to different isotopes of that element. Electrons, on the other hand, occupy different energy levels or shells around the nucleus. As we move down a group, the electrons are located in higher energy levels, farther away from the nucleus. The increased distance between the outermost electrons and the positively charged nucleus reduces the attractive force between them, resulting in a larger atomic radius.
Since the mass of an atom is mainly determined by the sum of its protons and neutrons (located in the nucleus), the addition of more neutrons compensates for the increase in protons and contributes to an increase in the atom's mass. Therefore, as we move down a group on the periodic table, although the atomic number (number of protons) increases, the mass of the atom generally increases due to the addition of more neutrons and electrons in higher energy levels.