Average distance between the nucleus of the atom and its outermost shell provided that the atom is assumed to be spherical is called atomic radius.
Units of Atomic Radius:
Atomic radius is usually expressed in:
- Picometres (pm) → 1 pm=10−12 m
- Angstroms (Å) → 1 Å=10−10 m
Explanation:
The precise measurement of the size of the atom is difficult. This is due to the following reasons:
- There are no sharp boundaries for the atomic orbitals. The reason is if the electron is at a large distance from the nucleus.
- The nearby atoms affect the probability distribution of electron.
The techniques have been developed which can measure the distance in between the centers of two adjacent atoms of any element. Half of this distance is considered to be the radius of the atom. The atomic radii are expressed in pm.
$$
\begin{aligned}
1\, \text{pm} &= 10^{-12}\, \text{m} \\
1\, \text{nm} &= 10^{3}\, \text{pm} \\
1\, \text{Å} &= 10^{2}\, \text{pm}
\end{aligned}
$$
Trends in Atomic Radius in the Periodic Table:
Across a Period (→)
- Atomic radius decreases from left to right across a period.
- Reason:
- Nuclear charge increases.
- Electrons are added in the same shell.
- Effective nuclear attraction increases, pulling electrons closer to the nucleus.
Example:
Atomic radius decreases from Li → F in Period 2.
| Elements | Li | Be | B | C | N | O |
| Radius (pm) | 152 | 111 | 88 | 77 | 70 | 66 |
Down a Group (↓)
Atomic radius increases down the group.
Reason:
- New electron shells are added.
- Distance between nucleus and outermost shell increases.
- Shielding effect reduces nuclear attraction.
Example:
Atomic radius increases from F → Cl → Br → I.
Periodic Variation Summary:
| Trend | Across a Period | Down a Group |
| Atomic Radius | Decreases → | Increases ↓ |
| Nuclear Charge | Increases → | Increases ↓ |
| Shielding Effect | Slight increase → | Strong increase ↓ |
Variation of atomic radii in periodic table:
Atomic radii vary in the periodic table systematically.
Variation in periods:
Atomic radii decrease in a period from left to the right in the periodic table:
Reason:
- Due to increasing nuclear charge, the outermost orbitals come closer to the nucleus.
- The number of shells remain the same.
- Shielding effect remains the same.
Variation in groups:
Atomic radii increase in a group from upper to the downward direction.
Reason:
- Number of shells increase down the group.
- Shielding effect increases down the group.
Variation of atomic radii in transition elements:
d- and f-block elements are transition elements. Whenever, we go from left to the right in transition elements series, then there happens a decrease in atomic radii for first four members, but after that atomic radii show non-systematic behaviour.
FAQ — Atomic Radius
Does the atomic radius increase from left to right?
No. Generally the atomic radius decreases from left to right across a period. As you move across a period the nuclear charge (number of protons) increases while electrons are added to the same principal shell — the stronger attraction pulls the outer electrons closer to the nucleus. Small exceptions occur in transition metals or due to electron-electron repulsion effects, but the overall trend is a decrease.
What is the difference between ionic radii and atomic radii?
Atomic radius refers to the size of a neutral atom (measured in different ways — covalent, metallic, van der Waals). Ionic radius is the effective size of an ion. Cations (positive ions) are smaller than their parent atoms because they lose electrons and the remaining electrons are pulled in tighter; anions (negative ions) are larger because added electrons increase electron-electron repulsion. Ionic radius also depends on coordination number and the method used to measure it.
How is atomic radius measured?
Because atoms have no sharp boundary, several operational definitions are used:
- Covalent radius — half the distance between nuclei of two identical atoms joined by a covalent bond.
- Metallic radius — half the distance between adjacent atoms in a metal lattice.
- Van der Waals radius — half the distance between non-bonded atoms in contact.
Experimental techniques that provide the distances used above include X-ray crystallography, electron diffraction and spectroscopy; the chosen distance is then halved to report the radius.
Why does atomic radius increase down a group?
Atomic radius increases down a group because each successive element has an additional electron shell (higher principal quantum number). These extra shells place valence electrons farther from the nucleus. Inner-shell electrons also increase shielding (screening) of the nuclear charge, so the effective attraction felt by outer electrons is reduced — allowing the atomic size to grow.