When moving from one period to another or from one group to another in the periodic table, the size of an atom generally changes in predictable ways.
Periods: As you move from left to right across a period (horizontal row), the size of atoms generally decreases. This is because the number of protons and electrons increases, resulting in a greater positive charge in the nucleus and a stronger attraction for the electrons in the outermost energy level. This increased attraction pulls the electrons closer to the nucleus, reducing the atomic radius.
Groups: When moving down a group (vertical column) in the periodic table, the size of atoms generally increases. This is due to the addition of new energy levels as you go down the group. Each new energy level adds to the overall size of the atom, making it larger. Additionally, the increasing number of electron shells also increases the shielding effect, where the outer electrons are screened from the full force of the positive charge in the nucleus. This reduced effective nuclear charge allows the outermost electrons to be further away from the nucleus, resulting in an increase in atomic size.
It's important to note that there are exceptions and variations in these trends due to factors such as electron-electron repulsion and specific orbital structures. However, the general trend of decreasing atomic size across periods and increasing atomic size down groups provides a useful framework for understanding the size variations of atoms in the periodic table.