Start with the recap, study the fully worked examples, then use the practice problems to
check your understanding of Octet Rule.
This page combines explanation, solved examples, and follow-up practice so you can move
from recognition to confident problem-solving in Chemistry.
Concept Recap
A chemical bonding principle stating that atoms tend to gain, lose, or share electrons in order to achieve a stable configuration of 8 electrons in their outermost (valence) shell.
8 is the magic number. Atoms 'want' a full outer shell like noble gases.
Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.
What to Focus On
Core idea:Octet Rule starts by identifying valence electrons, likely charges or sharing, and the structure that follows.
Common stuck point:Students often know a formula related to octet rule but skip the recognition step: Am I explaining a substance by electron behavior, bond type, molecular shape, polarity, or attractions between particles? That leads to a correct-looking substitution attached to the wrong chemical model.
Sense of Study hint:Ask: Am I explaining a substance by electron behavior, bond type, molecular shape, polarity, or attractions between particles?
Worked Examples
Example 1
easy
State the octet rule and explain why atoms tend to follow it.
Answer
Atoms seek 8 valence electrons for noble gas stability.
First step
1
The octet rule states that atoms tend to gain, lose, or share electrons until they have 8 electrons in their outermost shell (valence shell).
Full solution
2
This configuration matches the electron arrangement of the noble gases, which are extremely stable.
3
Having a full outer shell (8 electrons) represents the lowest energy state, so atoms naturally tend toward this configuration through chemical bonding.
The octet rule is a guiding principle in chemistry that explains why atoms form bonds. It applies primarily to main-group elements in periods 2 and 3. Hydrogen is an exception — it needs only 2 electrons (a duet).
Example 2
medium
Use the octet rule to explain why sodium forms Na+ and chlorine forms Cl− when they react to form NaCl.Sodium has 1 valence electron — it loses this to achieve the neon octet
Example 3
medium
Use the octet rule to predict the formula of magnesium chloride.
Example 4
medium
Predict the formula for calcium chloride using the octet rule.
Example 5
hard
Use the octet rule to predict the simplest compound formula between gallium (3 valence e−) and sulfur.
Practice Problems
Try these problems on your own first, then open the solution to compare your method.
Example 1
medium
Use the octet rule to explain why oxygen forms two covalent bonds in a water molecule (H2O).
Example 2
hard
Identify two exceptions to the octet rule and give a specific molecular example of each.
Example 3
easy
According to the octet rule, how many valence electrons do atoms 'want' in their outer shell?
Example 4
easy
Why do noble gases not readily form bonds, in octet-rule terms?
Example 5
easy
To reach an octet, will chlorine (7 valence electrons) gain or lose electrons? How many?Chlorine has 7 valence electrons — it gains 1 to reach the argon octet
Example 6
easy
To reach an octet, will sodium (1 valence electron) gain or lose electrons? How many?Sodium has 1 valence electron — it loses this electron to achieve a noble-gas octet
Example 7
easy
Which 'rule' do hydrogen and helium follow instead of the octet rule?
Example 8
easy
How does oxygen achieve an octet when it has 6 valence electrons?
Example 9
easy
Why does the octet rule make group 18 a useful reference for stability?
Example 10
easy
Magnesium has 2 valence electrons. To follow the octet rule, what does it do?Magnesium has 2 valence electrons — it loses both to form Mg²⁺ with a neon-like octet
Example 11
medium
Use the octet rule to predict the ion charges of magnesium and oxygen, then their compound formula.
Example 12
medium
Carbon shares electrons with 4 hydrogens in CH4. Show how the octet (and duet) rules are satisfied.CH₄: carbon forms 4 bonds (octet); each hydrogen shares 1 bond (duet)
Example 13
medium
Why does the octet rule predict that group 1 and group 17 elements react readily together?
Example 14
medium
Boron in BF3 has only 6 electrons around it. Why is this an octet-rule exception?BF₃: boron has only 6 electrons (incomplete octet) — an exception to the octet rule
Example 15
medium
Sulfur in SF6 has 12 electrons around it. Why can sulfur 'expand its octet' but oxygen cannot?
Example 16
medium
Predict the most likely ion for nitrogen (5 valence electrons) using the octet rule, and explain the choice.Nitrogen has 5 valence electrons — it gains 3 to reach the neon octet, forming N³⁻
Example 17
medium
Why is the molecule NO (nitric oxide) an exception to the octet rule?
Example 18
medium
Use the octet rule to predict the ion charges of sodium and sulfur, then their compound formula.
Example 19
medium
Water H2O has oxygen sharing with two hydrogens. Show how the octet and duet rules are satisfied.H₂O: oxygen forms 2 bonds (octet via sharing); each hydrogen has a duet
Example 20
challenge
Use the octet rule to derive the formula of aluminum oxide from Al (3 valence e-) and O (6 valence e-).
Example 21
challenge
Phosphorus pentachloride PCl5 has 10 electrons around phosphorus. Reconcile this with the octet rule and identify why it is allowed.PCl₅: phosphorus has 10 electrons (expanded octet) — allowed because period-3 elements have d-orbitals
Example 22
challenge
Predict the compound formed when calcium (2 valence e-) reacts with nitrogen (5 valence e-) using the octet rule.
Example 23
easy
Fluorine has 7 valence electrons. To follow the octet rule, will it gain or lose, and how many?Fluorine has 7 valence electrons — it gains 1 to complete its octet
Example 24
easy
Aluminum has 3 valence electrons. What ion does it form to satisfy the octet rule?Aluminum has 3 valence electrons — it loses all 3 to form Al³⁺ with a neon-like octet
Example 25
easy
Why doesn't lithium follow the full octet rule when it bonds?
Example 26
easy
True or false: noble gases generally don't form bonds because they already have an octet.
Example 27
easy
How many bonds does carbon typically form to satisfy the octet rule?
Example 28
medium
Show how the octet rule predicts F2 (a single bond between two fluorines).
Example 29
medium
Use the octet rule to predict the simplest compound between K and Br.
Example 30
medium
Explain how N2 satisfies the octet rule.
Example 31
medium
Why is CO2 drawn with two double bonds rather than two single bonds?
Example 32
medium
Use the octet rule to predict the formula of lithium nitride.
Example 33
medium
Phosphorus and chlorine combine. Use the octet rule to predict the simplest compound (without expanding the octet).
Example 34
medium
Explain why O2 has a double bond using the octet rule.
Example 35
medium
Why does SF4 violate the octet rule for sulfur, and why is this allowed?
Example 36
medium
How many bonds does a typical oxygen atom form in covalent molecules?
Example 37
hard
Use the octet rule to predict the formula of magnesium nitride.
Example 38
hard
Draw on the octet rule to explain why XeF2 exists despite Xe being a noble gas.
Example 39
hard
Explain why NO2 is an octet-rule exception and predict a typical reaction outcome.
Example 40
hard
Predict the formula of strontium phosphide from the octet rule.
Example 41
challenge
ClF3 has 10 electrons around the central Cl. Explain how this fits the octet rule framework and why ClF3 exists but NF3 has only 8 around the central N.