What Is the Valence Bond Theory?
A bond forms between two atoms when an orbital on one atom comes to occupy a portion of the same region of space as an orbital on the other.
The two orbitals are said to overlap.

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sigma bonds
HYDROGEN

The total number of electrons in both orbitals is no more than two.

1s1
H

+

σ
sigma bonds

Sigma bonds are formed by head-on overlap of two orbitals.

A double overlap in P orbital makes a pi (π) bond.

π
pi bonds

Py ORBITAL
pi (π) bond

Pz ORBITAL
pi (π) bond

Bond Strength Examples

Bond strength depends on the amount of orbital overlap.
The bigger the overlap, the stronger the bond.

Exception:
Two Px orbitals make a sigma (σ) bond, not a pi (π) bond.
Orbital overlap is single, not double.

Complete S orbital overlap.

STRONGEST

A.

Complete S orbital overlaps half of P orbital.

WEAKEST

Only half of each P orbital overlaps.

B.

The bigger the overlap, the stronger the bond.

C.

P ortibals bond ONLY along the same axis.

Example E

Complete orbital overlap.

two S orbitals

Example A

Complete S orbital overlaps half of P orbital.

Example B

one S orbital; one P orbital

two P orbitals

Example C

Orbitals other than S orbitals bond ONLY along the same axis or in a given direction.

Example D

Remember: Two Px orbitals make a sigma (σ) bond, not a pi (π) bond.

The bigger the overlap, the stronger the bond.

Example F

Example G

The bigger the overlap, the stronger the bond.

Practice

Problems

Incorrect!
Think about how overlap and bond strength relate.

Incorrect!
Think about how overlap and bond strength relate.
Try again

Click the stronger sigma (σ) bond.

Try again

Correct!
Two S orbitals have complete overlap. The bigger the overlap, the stronger the bond.
close

Correct!
The complete S orbital bonds with half the P orbital
close

Incorrect!
Think about how overlap and bond strength relate.
Try again

Tap/Click the stronger sigma (σ) bond.

sigma bonds
HELIUM

1s2
He