Covalent radius (
see figure below), which is defined as one-half the distance between the nuclei of two identical atoms when they are joined by a covalent bond (this measurement is possible because atoms within molecules still retain much of their atomic identity).
The covalent radius is half the distance between two atoms that share a covalent bond. Usually, you see covalent radius in units of picometers (pm) or angstroms (Å), where 1 Å = 100 pm. For example the average covalent radius for hydrogen is 31 pm and the average neon covalent radius is 58 pm.
Why Are There Different Numbers?
When you look at a table of covalent radius values, its numbers may differ from those found on another table. This is because there are different ways of reporting covalent radius.
In reality, the covalent radius depends on an atom’s hybridization, the nature of the two atoms sharing a covalent bond, and on the chemical environment surrounding the atoms. For example, the covalent radius of carbon is 76 pm for the sp3, 73 pm for the sp2 hybridization, and 69 pm for the sp hybridization.
Also, covalent radius depends on whether the atom forms a single bond, double bond, or triple bond. In general, a single bond is longer than a double bond, which is longer than a triple bond. **
A given table might generalize data or else offer values based on very specific conditions. Tables that cite an average value usually combine data for covalent bonds an atom forms in many different compounds. Some tables list the covalent radius for a homonuclear covalent bond. For example, this is the covalent radius for H2 or O2. Either use the idealized (calculated) or empirical average covalent radius for an atom for maximum transferability.
How Covalent Radius Is Measured
The most common methods of measuring covalent radius are x-ray diffraction and rotational spectroscopy. Neutron diffraction of molecular crystals is another method.
Covalent Radius Trend on the Periodic Table
Covalent radius displays a periodic table trend.
- Moving left to right across a period, covalent radius decreases.
- Moving top to bottom down a group, covalent radius increases.
Covalent radius decreases moving from left to right across a row or period because atoms gain more protons in their nucleus and electrons in their outer shells. Adding more protons increases the attractive pull on these electrons, drawing them in more tightly.
Covalent radius increases moving down a column or periodic table group. This is because increasing filled inner electron energy levels shield the outer electrons from the positive nuclear charge. So, the electrons are less attracted to the nucleus and increase their distance to it.
Covalent Radius vs Atomic Radius and Ionic Radius
Covalent radius, atomic radius, and ionic radius are three ways of measuring the sizes of atoms and their sphere of influence. The atomic radius is half the distance between the nuclei of atoms that are just touching each other, where “touching” means their outer electrons shells are in contact.
The ionic radius is half the distance between two atoms touching each other that share an ionic bond in a crystal lattice.
All three measures of atomic size follow a periodic table trend, where radius generally increases in size moving down an element group and decreases in size moving from left to right across a period. However, the covalent radius and ionic radius often are different sizes from the atomic radius.
The Largest and Small Covalent Radius
The element with the smallest covalent radius is hydrogen (32 pm). The atom with the largest covalent radius is francium (223 pm when it forms a single bond). Basically, this is another way of saying hydrogen is the smallest atom and francium is the largest atom.
Group-wise Covalent Radii
Group 1: Alkali Metals
- Atomic Number 1: H - 37 pm (in H₂)
- Atomic Number 3: Li - 140 pm
- Atomic Number 11: Na - 166 pm
- Atomic Number 19: K - 227 pm
- Atomic Number 37: Rb - 248 pm
- Atomic Number 55: Cs - 262 pm
Group 2: Alkaline Earth Metals
- Atomic Number 4: Be - 112 pm
- Atomic Number 12: Mg - 160 pm
- Atomic Number 20: Ca - 197 pm
- Atomic Number 38: Sr - 215 pm
- Atomic Number 56: Ba - 222 pm
- Atomic Number 88: Ra - 247 pm (estimated)
Group 13: Boron Group
- Atomic Number 5: B - 88 pm
- Atomic Number 13: Al - 143 pm
- Atomic Number 31: Ga - 135 pm
- Atomic Number 49: In - 156 pm
- Atomic Number 81: Tl - 156 pm
Group 14: Carbon Group
- Atomic Number 6: C - 77 pm
- Atomic Number 14: Si - 118 pm
- Atomic Number 32: Ge - 122 pm
- Atomic Number 50: Sn - 140 pm
- Atomic Number 82: Pb - 175 pm
Group 15: Nitrogen Group
- Atomic Number 7: N - 75 pm
- Atomic Number 15: P - 110 pm
- Atomic Number 33: As - 114 pm
- Atomic Number 51: Sb - 140 pm
- Atomic Number 83: Bi - 156 pm
Group 16: Chalcogens
- Atomic Number 8: O - 73 pm
- Atomic Number 16: S - 104 pm
- Atomic Number 34: Se - 116 pm
- Atomic Number 52: Te - 140 pm
- Atomic Number 86: Po - 150 pm (estimated)
Group 17: Halogens
- Atomic Number 9: F - 72 pm
- Atomic Number 17: Cl - 99 pm
- Atomic Number 35: Br - 114 pm
- Atomic Number 53: I - 133 pm
- Atomic Number 85: At - 202 pm (estimated)
Summary
- Covalent Radii typically decrease across a period from left to right and increase down a group due to the addition of electron shells and the effect of effective nuclear charge.
- These values can vary slightly depending on the chemical environment and the specific compounds formed.
| Atomic Number | Element | Covalent Radius (pm) |
|---|
