What is the atomic radius? Why does the atomic radius decrease across a period?

The atomic radius is the distance from the nucleus of an atom to the outermost electron shell. It's essentially a measure of the size of an atom.

The atomic radius generally decreases across a period (horizontal row in the periodic table) due to the increasing effective nuclear charge. As you move from left to right across a period, the number of protons in the nucleus increases, which means there is a greater positive charge in the nucleus. This increased positive charge attracts the electrons more strongly towards the nucleus, pulling them closer and reducing the atomic radius.

Additionally, across a period, electrons are added to the same principal energy level (shell), but the effective nuclear charge increases. This means that even though electrons are being added to the same energy level, they are pulled closer to the nucleus due to the increasing positive charge, leading to a decrease in atomic radius across the period.