The octet rule and chemical stability

Atoms seeking stability

An isolated atom is almost never in its most stable state. Look at the noble gases (helium, neon, argon...): they exist alone, never reacting, because their outer electron shell is complete. All other atoms "dream" of reaching this same configuration, and that is exactly what drives them to form chemical bonds.

The octet rule

The octet rule states that an atom tends to surround itself with 8 electrons on its valence shell (the outer shell), like the noble gases that follow or precede it in the periodic table. To achieve this, an atom can:

• give up one or more electrons,
• capture one or more electrons,
• or share electrons with another atom.

There is one frequent exception: hydrogen (and helium) do not aim for 8 electrons but only 2, because their single shell can hold only two. This is called the duet rule.

Why this creates bonds

Take chlorine (Cl): its outer shell has 7 electrons, it is missing just one to reach 8. Sodium (Na), for its part, has 1 extra electron on its outer shell. When they react together, sodium gives its extra electron to chlorine: both atoms each reach a stable configuration. This is the starting point of every chemical bond, whether covalent or ionic.

Classic mistake

Don't confuse "stable atom" with "motionless atom": an atom involved in a bond is not frozen, it vibrates and moves, but its electron distribution has become stable in energy terms. It is this stability, not the absence of motion, that explains why bonds form and are maintained.