Atomic Radii

Atomic radius refers to the size of an atom. It is generally defined as the distance from the centre of the nucleus to the outermost shell of electrons. Since atoms are extremely small and the position of electrons is not fixed, it is difficult to measure the exact boundary of an atom. Therefore, atomic radius is usually determined by measuring the distance between the nuclei of two atoms that are bonded together.

It helps us understand how the size of atoms changes in the periodic table and how atoms interact with each other during chemical bonding. Atomic radii vary for different elements depending on factors such as the number of electron shells, nuclear charge, and shielding effect.

Formula

Atomic Radius = Distance between the two nuclei / 2

Types of Atomic Radii

Based on how atoms are bonded or arranged, atomic radii are classified into the following types:

1) Covalent Radius

• The covalent radius is defined as half the distance between the nuclei of two identical atoms that are joined together by a covalent bond.
• It is commonly used for non-metal elements that form covalent compounds.

Example:
In a hydrogen molecule (H₂), the distance between the two hydrogen nuclei is about 74 pm, so the covalent radius of hydrogen is 37 pm.

2) Ionic Radius

• The ionic radius refers to the size of an ion formed when an atom loses or gains electrons.
• When an atom loses electrons, it forms a cation, and its size becomes smaller.
• When an atom gains electrons, it forms an anion, and its size becomes larger.

Example:

• Na⁺ (sodium ion) is smaller than a neutral sodium atom because it loses one electron.
• Cl⁻ (chloride ion) is larger than a neutral chlorine atom because it gains one electron.

3) Metallic Radius

• The metallic radius is half the distance between the nuclei of two adjacent metal atoms in a metallic crystal.
• In metals, atoms are arranged in a closely packed structure, so this radius represents the size of metal atoms.

Example:
Metals such as sodium (Na) and copper (Cu) have metallic radii measured from the distance between neighbouring atoms in their metallic lattice.

4) Van der Waals Radius

• The van der Waals radius is half the distance between the nuclei of two atoms that are not chemically bonded but are very close to each other.
• This radius is usually larger than the covalent radius because the atoms are not sharing electrons.

Example:
Noble gases such as neon (Ne) and argon (Ar) do not form covalent bonds, so their size is measured using the van der Waals radius.

Atomic Radii for Elements

The values are given in picometers (pm), where 1 picometer is equal to 1×10^-12 meters.

Trends in Periodic Table

In the periodic table, the atomic radius shows a regular pattern when moving across a period or down a group. These patterns are known as periodic trends of atomic radii.

1) Trend Across a Period

• When moving from left to right across a period, the atomic radius decreases.
• This happens because the number of protons in the nucleus increases, which increases the nuclear charge.
• The stronger attraction between the nucleus and electrons pulls the electrons closer to the nucleus, reducing the size of the atom.

Example:
The atomic radius gradually decreases from sodium (Na) to chlorine (Cl) in the same period.

2) Trend Down a Group

• When moving from top to bottom in a group, the atomic radius increases.
• This is because a new electron shell is added for each element as we move down the group.
• The increase in the number of shells and the shielding effect pushes the outer electrons farther from the nucleus, making the atom larger.

Example:
The atomic radius increases from lithium (Li) to sodium (Na) to potassium (K) in the same group.

Factors affecting Atomic Radii

1) Number of Electron Shells

• The number of electron shells greatly affects the atomic radius.
• When the number of shells increases, the distance between the nucleus and the outermost electrons also increases, making the atom larger.

Example:
The atomic size increases from lithium (Li) to sodium (Na) to potassium (K) because each element has an additional electron shell.

2) Nuclear Charge

• Nuclear charge refers to the number of protons present in the nucleus.
• A higher nuclear charge increases the attraction between the nucleus and the electrons, pulling the electrons closer to the nucleus and decreasing the atomic radius.

Example:
Across a period, the atomic radius decreases from sodium (Na) to chlorine (Cl) because the nuclear charge increases.

3) Shielding Effect

• The shielding effect occurs when inner electron shells block or reduce the attraction between the nucleus and the outermost electrons.
• Greater shielding causes the outer electrons to be less strongly attracted to the nucleus, increasing the atomic radius.

Example:
In heavier atoms like potassium (K), inner electrons shield the outer electron from the nucleus, making the atom larger.

Related Articles:

• Atomic Structure
• Atomic Number
• Rutherford Atomic Model