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high school Covalent Bond

A covalent bond forms when nonmetal atoms share a pair of electrons. As two atoms approach, their electron clouds overlap and the energy drops, but if they get too close the nuclei repel and the energy rises again, so the distance with the lowest energy becomes the bond length. The more electron pairs two atoms share, going from single to double to triple bonds, the larger the bond energy and the shorter the bond length. In the pictures, changing the distance between the nuclei moves the point along the energy curve, and changing the bond order changes the length and strength of the bond between two carbon atoms.

How Does a Covalent Bond Form?

1 r₀
💡 An Analogy: Holding One Ball Together
①It is like two people holding one ball (the electron pair) together
②As the atoms approach, their electron clouds overlap and the energy drops, but if they get too close the nuclei push each other away and the energy rises again
③The distance with the lowest energy is the bond length
④Unlike an ionic bond, electrons are not handed over but shared

Single, Double and Triple Bonds

1
Bond Order and Properties
Bond order ↑ → bond energy ↑, bond length ↓
Between the same two atoms, triple > double > single in bond strength
🔍 Comparing the Three Bonds
①Single bond: one σ bond (H₂, Cl₂, C−C)
②Double bond: a σ bond + a π bond (O₂, C=C)
③Triple bond: a σ bond + two π bonds (N₂, C≡C)
④Carbon-carbon bonds are 154 pm single, 134 pm double and 120 pm triple: the higher the bond order, the shorter and stronger the bond

Electronegativity and Polarity

Polar Covalent Bond
ΔEN > 0 → the electron pair shifts to one side
ΔEN is the difference in electronegativity between the two atoms
Nonpolar Covalent Bond
ΔEN = 0 → the electron pair is shared equally
Bonds between atoms of the same element (H₂, O₂, N₂)
💡 Bonding Is a Continuum
①ΔEN = 0: nonpolar covalent bond (H−H)
②ΔEN smaller than about 1.7: polar covalent bond (H−Cl)
③ΔEN larger than about 1.7: close to an ionic bond (Na−Cl)
④Covalent and ionic bonding are not cleanly separated but run into each other

Work It Out

Example 1
How do the two hydrogen atoms bond in a hydrogen molecule (H₂)? How many shared electron pairs are there?
1
Each H atom contributes one electron, and the two share one electron pair (a single bond).
H· + ·H → H:H (H−H)
2
Count the shared electron pairs.
1 shared electron pair (single bond)
▸ A single bond with 1 shared electron pair
By sharing one electron pair, each hydrogen atom reaches a stable arrangement like that of helium.
Example 2
Find the bond type and the numbers of shared and lone (unshared) electron pairs in a nitrogen molecule (N₂).
1
An N atom has 5 valence electrons, so each contributes 3 and they share 3 electron pairs in a triple bond.
N≡N (triple bond)
2
Count the shared pairs and the lone pairs that remain.
3 shared electron pairs, 2 lone pairs
▸ Triple bond (3 shared pairs, 2 lone pairs)
Each N atom keeps one lone pair, 2 in all, and because the bond is triple it is very short and strong.

Summary

Covalent Bond
non-metal + non-metal → electron pair sharing
Forms between elements with a small difference in electronegativity
2023 KICE mock Chemistry I type, adapted
Which of the following molecules contains a double bond?
①H₂
②Cl₂
③O₂
④N₂
⑤HCl
▸ ③ O₂
1
Compare the bond order of each molecule. H₂, Cl₂ and HCl have single bonds, and N₂ has a triple bond.
2
Check the bond order of the remaining O₂.
O₂: O=O double bond (2 shared electron pairs)
🎯 Exam Points
①A covalent bond is one in which nonmetal atoms share electrons
②The higher the bond order, the larger the bond energy and the shorter the bond length
③A σ bond is an overlap of orbitals along the bond axis, and a π bond is a side-by-side overlap
④ΔEN > 0 gives a polar covalent bond, ΔEN = 0 a nonpolar one
⑤A coordinate bond is one in which one atom supplies both electrons of the pair (e.g. NH₄⁺)
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Ionic Bond
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Molecular Structure

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