6 min readConcept sheetSection A MCQ (1 mark) and true or false (2 marks, two parts); Section B very short answer (2 marks)

Why atoms combine: noble gases and the octet rule

Noble gases have 8 electrons in their outer shell (helium has 2), so they hardly react. Other atoms combine to reach this stable octet.

Octet ruleAtoms lose, gain or share electrons until they have 8 electrons in the outermost shell, like a noble gas. Hydrogen needs only 2, like helium

Noble gasesHelium, neon, argon, krypton, xenon and radon. They have full outer shells and were once called inert gases

Chemical bondThe force of attraction that holds atoms together in a molecule or a compound

Why bonds formThe bonded atoms have less energy than the separate atoms, so the compound is more stable

Outer shells of the noble gases

Noble gas	Symbol	Atomic number	Configuration	Outer electrons
Helium	He	2	2	2
Neon	Ne	10	2, 8	8
Argon	Ar	18	2, 8, 8	8
Krypton	Kr	36	2, 8, 18, 8	8
Xenon	Xe	54	2, 8, 18, 18, 8	8
Radon	Rn	86	2, 8, 18, 32, 18, 8	8

Why noble gases do not react

Noble gases do not usually form compounds. Early chemists called them inert gases, which means unreactive. Their outer shell is full: 8 electrons, or 2 for helium. A full outer shell is very stable, which means it does not easily change. Note: radon's symbol is Rn. Your NIOS lesson's table prints Ra, which is the symbol of radium.

The octet rule

Atoms such as hydrogen, sodium and chlorine do not have a full outer shell, so they react. They lose, gain or share electrons until each has 8 electrons in its outer shell, like the nearest noble gas. Hydrogen needs only 2, like helium. This is the octet rule (octet means a group of eight). It is the basic cause of chemical bonding.

Two ways to complete the octet

Transfer of electrons from a metal to a non-metal gives an ionic bond. Example: sodium chloride.
Sharing of electrons between two non-metal atoms gives a covalent bond. Example: hydrogen, H₂.
Either way, the bonded atoms have less energy than the separate atoms. Lower energy means more stable.
Metal atoms usually have 1, 2 or 3 outer electrons, so they lose them. Non-metal atoms usually have 4 to 7, so they gain or share electrons.

Stable or reactive? Read the outer shell

Element (atomic number)	Configuration	Outer electrons	What it does
Neon (10)	2, 8	8	Stable: does not react
Argon (18)	2, 8, 8	8	Stable: does not react
Sodium (11)	2, 8, 1	1	Reacts: loses 1 electron
Magnesium (12)	2, 8, 2	2	Reacts: loses 2 electrons
Oxygen (8)	2, 6	6	Reacts: gains or shares 2 electrons
Chlorine (17)	2, 8, 7	7	Reacts: gains or shares 1 electron

In real life

Neon in shop signs glows when electricity passes, but it never burns or reacts inside the tube. Argon fills ordinary filament bulbs because it does not react with the hot filament.

Exam angle

1 marks: “State the octet rule.” Atoms combine by losing, gaining or sharing electrons so that each has 8 electrons in its outermost shell, like a noble gas (2 for hydrogen).

2 marks: “Why do noble gases hardly form compounds? Name the noble gas that is stable with only 2 outer electrons.” Their outermost shell is already full (8 electrons, or 2 in helium). This arrangement is very stable, so they do not need to lose, gain or share electrons. Helium has 2.

2 marks: “Element X has atomic number 18 and element Y has atomic number 19. Which one reacts easily, and why?” X is 2, 8, 8: a full octet, so it is stable (argon). Y is 2, 8, 8, 1: it has one outer electron to lose, so Y (potassium) reacts easily.

Remember it

Eight is great: atoms lose, gain or share electrons to reach the noble gas eight. Hydrogen is happy with two.