Question

Which of the following has the smallest atomic radius?
a.) K
b.) Ca
c.) Br
d.) Kr
e.) Zn

Answer 1

Hint: As you go across a period, electrons are added to the same energy level. At the same time, protons are being added to the nucleus. The concentration of more protons in the nucleus creates a higher effective nuclear charge. Complete answer:
Let us first understand what the atomic radius really means before trying to understand what the trend of atomic size is across a period.
The atomic radius is one-half the distance between the nuclei of two atoms (just like a radius is half the diameter of a circle). This distance is measured in picometers. Atomic radius patterns are observed throughout the periodic table.
Atomic size gradually decreases from left to right across a period of elements. This is because, within a period or family of elements, all electrons are added to the same shell. However, at the same time, protons are being added to the nucleus, making it more positively charged. The effect of increasing proton number is greater than that of the increasing electron number; therefore, there is a greater nuclear attraction.
This means that the nucleus attracts the electrons more strongly, pulling the atom's shell closer to the nucleus. The valence electrons are held closer towards the nucleus of the atom. As a result, the atomic radius decreases.
Now, since all of the given elements belong to the same period i.e. the 4th period, therefore we can conclude that the atom with the smallest atomic radius would be the noble gas of the period, which is, Krypton.
Thus, we can conclude that the answer to this question is d) Kr.
Note:
The idea of atomic radii is complicated by the fact that not all atoms are normally bound together in the same way. Some are bound by covalent bonds in molecules, some are attracted to each other in ionic crystals, and others are held in metallic crystals.
Nevertheless, it is possible for a vast majority of elements to form covalent molecules in which two like atoms are held together by a single covalent bond. The covalent radii of these molecules are often referred to as atomic radii.