Answer:

The atomic radius is the distance from the nucleus of an atom to the outermost electrons. Since the orbitals around an atom are defined in terms of a probability distribution in quantum mechanics, and do not have fixed boundaries, determining where an atom "stops" is not very straightforward. By comparing the bond lengths of a number of representative compounds of an element, an average size for most atoms can be determined.

The atomic radius can also be defined in other ways. The van der Waals radius (also known as the nonbonding atomic radius) is the radius of an atom which is not bonded to other atoms; this is determined by measuring the distance between atomic nuclei which are in direct but nonbonding contact with each other in a crystal lattice. The covalent atomic radius (also known as the bonding atomic radius) is determined for metals by taking one-half of the distance between two adjacent atoms in a metallic crystal, or one-half the distance between like bonded atoms for nonmetals.

Unfortunately, it is not possible to determine the radius for every element on the periodic table in the same way, and consequently, it is sometimes difficult to make comparisons between different sets of data. In the table above, most of the atomic radii listed are average atomic radii, while for the halogens (Group 7A) and the noble gases (Group 8A) the covalent radius is used.

Atomic radii vary in a predictable way across the periodic table. As can be seen in the figures below, the atomic radius increases from top to bottom in a group, and decreases from left to right across a period. Thus, helium is the smallest element, and francium is the largest.

From top to bottom in a group, orbitals corresponding to higher values of the principal quantum number (n) are being added, which are on average further away from the nucleus, thus causing the size of the atom to increase.

From left to right across a period, more protons are being added to the nucleus, but the electrons which are being added are being added to the valence shell, not to the lower energy levels. As more protons are added to the nucleus, the electrons in the valence shell feel a higher effective nuclear charge — the sum of the charges on the protons in the nucleus and the charges on the inner, core electrons. (See figure below.) The valence electrons are therefore held more tightly, and the size of the atom contracts across a period.

Explanation:

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