Octet Rule Calculator

About Octet Rule Calculator

The octet rule: atoms tend to form bonds to achieve 8 valence electrons (2 for hydrogen). Bonding electrons = needed - available.

How to use this calculator

1. Enter the number of atoms and their valence electron count, if the calculator asks for them.
2. Use the calculator to find the total electrons needed for octets or duets.
3. Subtract the available electrons from the needed electrons to get the bonding electrons.
4. Check whether the result is even, because bonding electrons are usually shared in pairs.

The formula explained

The formula \(Bonding\\ electrons = needed - available\) tells you how many electrons must be used in bonding to satisfy the octet rule. It compares the electrons required for stable outer shells with the electrons already present.

• needed = the total number of electrons required for all atoms to reach stable shells
• available = the total number of valence electrons already present in the structure
• Bonding electrons = the number of electrons that must be shared in bonds

Step by step method

1. Find the total electrons needed for the atoms, using \(8\) for most atoms and \(2\) for hydrogen.
2. Find the total electrons available from the atoms in the molecule or ion.
3. Compute \(Bonding\\ electrons = needed - available\).
4. Use the result to help determine the number of bonds, remembering that each bond uses \(2\) electrons.

Worked example

Problem. Find the bonding electrons for carbon dioxide, \(CO_2\).

1. Carbon needs \(8\) electrons and each oxygen needs \(8\), so the total needed is \(8 + 8 + 8 = 24\).
2. Carbon has \(4\) valence electrons and each oxygen has \(6\), so the total available is \(4 + 6 + 6 = 16\).
3. Subtract to get \(24 - 16 = 8\) bonding electrons.

Tips and common mistakes

• Remember that hydrogen follows the duet rule, so it needs only \(2\) electrons.
• A negative result usually means the electron count was set up incorrectly, so recheck the totals.

Frequently asked questions