Explanation:
As you move left to right, the atomic radius gets SMALLER, so atoms hold on to their electrons more tightly and are more reactive. As you move down a group, the atomic radius gets LARGER, so atoms have a weaker hold on their electrons and are less reactive.
Taking into account the definition of atomic radius, the radius of an atom will increase as a group goes down on the periodic table.
The atomic radius represents the distance between the nucleus and the valence shell (the outermost).
But the size of the atom or atomic radius is not an easy parameter to determine, because the electrons are not located in defined orbits, at fixed distances from the nucleus.
However, since atoms are not usually isolated, the value assigned to the atomic radius is half the distance between the nuclei of two identical atoms bonded together.
By means of the atomic radius it is possible to determine the size of the atom.
On the other side, the effective nuclear charge is the charge that the nucleus should have so that, in the absence of other electrons, the attraction of the nucleus on the considered electron would be the same as the net attraction that the electron experiences in the real atom.
So, as the atomic number of the elements in a group increases, the number of occupied levels increases, while the effective nuclear charge on the outermost electron is the same. Consequently, the atomic radius increases.
In other words, when going down in a group of the periodic table, the number of electrons added with respect to the previous element in the same group increases in a complete shell, so that each time they distance themselves more from the atomic nucleus. Then, as the distance from the electrons to the nucleus increases the electrical attraction rapidly decreases, the size increases greatly.
In summary, the radius of an atom will increase as a group goes down on the periodic table.
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