Formation of compounds and the stability of atoms

This page covers a single Form 4 content standard: the formation of compounds and the stability of atoms. It is the foundation of the whole Chemical Bond chapter, because every kind of bond you meet later, ionic, covalent, dative and hydrogen, exists for the same reason: atoms are driven towards a stable electron arrangement. Get this idea clear now and the rest of the chapter becomes far easier to explain in the way the marking scheme wants.

Why noble gases are the standard of stability

The noble gases in Group 18, helium, neon, argon and the rest, are famously unreactive. They do not readily form compounds because their outermost shells are already full: helium has a full first shell of 2 electrons (a duplet), and the others have 8 electrons in their outermost shell (an octet). A full outer shell is a low-energy, stable arrangement, which is exactly why these elements have so little tendency to react.

Every other atom starts with an incomplete outer shell, so it is comparatively unstable and tends to react. When atoms react and bond, they rearrange their outer electrons until each atom has reached a noble-gas arrangement. This is the single most important idea in the chapter.

The duplet and octet rules

The syllabus expects you to use two simple rules:

• The duplet rule: very small atoms become stable when their outer shell holds 2 electrons, matching helium. Hydrogen, lithium and beryllium follow the duplet rule.
• The octet rule: most other atoms become stable when their outer shell holds 8 electrons, matching neon or argon.

An atom can reach this stable arrangement in one of three ways: by losing electrons, by gaining electrons, or by sharing electrons with another atom.

Losing, gaining or sharing electrons

Whether an atom loses, gains or shares depends on how many valence electrons it has:

• Metals have few valence electrons (1, 2 or 3). It is easier for them to lose these electrons and expose a full inner shell. Losing electrons produces a positive ion (cation).
• Non-metals have many valence electrons (5, 6 or 7). It is easier for them to gain electrons to complete the octet. Gaining electrons produces a negative ion (anion).
• When two non-metals meet, neither wants to give up electrons, so instead they share electrons. Shared pairs of electrons let both atoms count a full outer shell at the same time.

Transferring electrons from a metal to a non-metal produces an ionic bond; sharing electrons between non-metals produces a covalent bond. Both routes exist for the same purpose, to give every atom a stable, noble-gas-like arrangement. Because the bonded arrangement is lower in energy than the separate atoms, energy is released when a bond forms, and the compound is more stable than the elements it came from.

Worked example

Question. Sodium has the electron arrangement 2.8.1 and chlorine has the arrangement 2.8.7. Explain, in terms of electron arrangement, how a sodium atom and a chlorine atom each achieve a stable arrangement when sodium chloride forms.

Step 1, Look at each atom’s outer shell. Sodium (2.8.1) has 1 valence electron; chlorine (2.8.7) has 7 valence electrons. Neither has a full outer shell, so neither is stable on its own.

Step 2, Decide the easier route for each. Sodium has only 1 outer electron, so it is easier to lose it than to gain 7. Chlorine has 7 outer electrons, so it is easier to gain 1 than to lose 7.

Step 3, Transfer the electron. The sodium atom transfers its single valence electron to the chlorine atom.

Step 4, Check the new arrangements. Sodium becomes Na⁺ with arrangement 2.8 (an octet, like neon). Chlorine becomes Cl⁻ with arrangement 2.8.8 (an octet, like argon).

Answer. Sodium loses 1 electron to form Na⁺ (2.8) and chlorine gains that electron to form Cl⁻ (2.8.8). Both ions now have a stable octet arrangement like a noble gas, which is why the compound sodium chloride forms and is stable.

Practice question

Question. Magnesium has the electron arrangement 2.8.2 and oxygen has the arrangement 2.6. Explain how each atom achieves a stable electron arrangement when magnesium oxide is formed, and give the arrangement of each ion produced.

Answer. Magnesium has 2 valence electrons, so it loses both to form Mg²⁺ with the arrangement 2.8 (an octet like neon). Oxygen has 6 valence electrons, so it gains 2 to form O²⁻ with the arrangement 2.8 (also an octet). The two electrons lost by magnesium are exactly the two gained by oxygen, so both ions reach a stable octet and magnesium oxide (MgO) forms.

Exam tip

When a question asks you to explain how atoms become stable, always write the electron arrangement before and after (for example 2.8.1 → 2.8) and name the number of electrons transferred, gained or shared. State clearly that the atom reaches a duplet or octet like a noble gas, the words “duplet” and “octet” are what the marking scheme rewards. Do not just write “it becomes stable”; show how many electrons move and which stable arrangement results. Remember the shortcut: metals with 1–3 valence electrons lose them, non-metals with 5–7 valence electrons gain or share.

Where this fits

This standard is the doorway to the whole Chemical Bond chapter. Once you can explain why atoms bond, the next standards simply show how: electron transfer gives the ionic bond and electron sharing gives the covalent bond. Consolidate the electron-arrangement reasoning with the revision notes and practise the “explain the stability” answers with the worked examples. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers check that your before-and-after electron arrangements are correct and that you always name the noble-gas configuration achieved, which is exactly what secures full marks on this standard across SPM Chemistry.

Quick recap

• Noble gases are stable because their outer shell is full (duplet of 2 or octet of 8).
• Other atoms bond to reach a noble-gas arrangement.
• Metals lose electrons (form cations); non-metals gain or share electrons.
• Electron transfer → ionic bond; electron sharing → covalent bond.
• The bonded compound is lower in energy and more stable than the separate atoms.