Define atomic size. Give its unit of measurement. In the modern periodic table what trend is observed in the atomic radius in a group and a period and why it is so?

Atomic size, also known as atomic radius, refers to the distance between the nucleus of an atom and the outermost electrons. It represents the size of an atom and determines the region in which the electrons are most likely to be found. Atomic size is an important property that influences various chemical and physical behaviors of elements.

The unit of measurement for atomic size or atomic radius is angstrom (Å) or picometers (pm). One angstrom is equal to 0.1 nanometers, and one picometer is 0.01 angstroms.

In the modern periodic table, certain trends are observed in the atomic radius within both groups and periods:

Group Trend (Vertical Columns):
As you move down a group (from top to bottom) in the periodic table, the atomic radius increases. This occurs because as you go down a group, each successive element has one more electron shell or energy level than the element above it. The outermost electrons are farther away from the nucleus, resulting in a larger atomic radius. Additionally, the increased electron shielding from inner electron shells reduces the effective nuclear charge experienced by the outermost electrons, further contributing to the increase in atomic size.

Period Trend (Horizontal Rows):
As you move across a period (from left to right) in the periodic table, the atomic radius decreases. This happens because, within a period, the number of protons (and electrons) in the nucleus increases as you move from left to right. The additional protons in the nucleus result in a greater positive charge, pulling the electrons closer to the nucleus, thereby reducing the atomic size. The effect of electron shielding remains relatively constant across a period, so it does not offset the increase in nuclear charge significantly.

It's important to note that there can be some irregularities or exceptions in the trend when transitioning between different blocks of the periodic table (s-block, p-block, d-block, f-block), but the general trend of increasing atomic radius down a group and decreasing atomic radius across a period holds true for most of the elements in the periodic table.