Journey Inside the Atom · Lesson 10 of 14
Combining Capacity of an Atom: Valency
“Valence electrons and stable outer shells explain common combining capacities”
• Define combining capacity and valency. • Identify valence shell and valence electrons. • Explain octet stability. • Determine valency from configuration. • Explain electron loss, gain and sharing. • Recognise complete outer shells as largely unreactive.
Atoms combine to form molecules and compounds. The number of other atoms with which an atom can combine is closely connected with the electrons in its outermost occupied shell.
Combining Capacity of an Atom: Valency
The number of atoms of hydrogen or chlorine with which one atom of an element can combine, used as an expression of its combining behaviour.
Hydrogen and chlorine each have combining capacity one. In water, one oxygen atom combines with two hydrogen atoms, so oxygen has combining capacity two. Similar reasoning can be applied to ammonia and magnesium chloride.
Valence Shell and Valence Electrons
The outermost occupied shell of an atom.
The electrons present in the valence shell.
Octet and Stability
An outer-shell arrangement containing eight electrons.
Atoms with a complete octet are generally stable and largely unreactive. Helium is also stable with two electrons because its only shell is complete. Atoms with incomplete outer shells may lose, gain or share electrons to reach a stable arrangement.
Valency
The number of electrons gained, lost or shared by an atom to reach a stable outer-shell arrangement.
Losing Electrons
Sodium has configuration 2, 8, 1. Losing one electron leaves 2, 8, a complete outer shell. Its common valency is therefore one.
Problem
Determine valency from 2, 8, 1.
- 1.The valence shell contains 1 electron.
- 2.Losing 1 electron leaves 2, 8.
- 3.The number lost is 1.
- 4.Valency = 1.
Gaining Electrons
Oxygen has configuration 2, 6. It needs two electrons to reach 2, 8, so it has common valency two.
Problem
Determine valency from 2, 6.
- 1.Valence electrons = 6.
- 2.A complete octet needs 8.
- 3.The atom needs 8 − 6 = 2 electrons.
- 4.Valency = 2.
Sharing Electrons
Carbon has configuration 2, 4. In the chapter's treatment, carbon can share four electrons with other atoms to complete its octet, giving valency four.
Problem
Determine valency from 2, 4.
- 1.Valence electrons = 4.
- 2.Four more are needed for an octet.
- 3.Carbon can share four electrons.
- 4.Valency = 4.
| Valence electrons | Common valency reasoning |
|---|---|
| 1 | Lose 1 → valency 1 |
| 2 | Lose 2 → valency 2 |
| 3 | Lose 3 → valency 3 |
| 4 | Share 4 → valency 4 |
| 5 | Gain 3 → valency 3 |
| 6 | Gain 2 → valency 2 |
| 7 | Gain 1 → valency 1 |
| 8 | Already stable → valency 0 |
The chapter notes that some compounds do not follow the simple valency pattern. Use it here as a foundation for common combining behaviour.
Work valency out from the valence shell instead of treating it as an isolated fact. Ask how many electrons must be lost, gained or shared to reach a stable outer shell.
Quiz
Which description best matches Combining Capacity?
Which description best matches Valence Shell?
Which term matches this description: The number of atoms of hydrogen or chlorine with which one atom of an element can combine, used as an expression of its combining behaviour.
Which term matches this description: The outermost occupied shell of an atom.
Which statement is a key takeaway from this lesson?
Practice Problems
- Find the common valency of magnesium 2, 8, 2.
- Find the common valency of chlorine 2, 8, 7.
- Why is neon 2, 8 largely unreactive?
- Explain carbon's valency using sharing.
- Find common valencies of aluminium, phosphorus and sulfur from their configurations.
Key Takeaways
• Valence electrons are the electrons present in the outermost occupied shell. • Valency describes the combining capacity of an atom. • Atoms tend to lose, gain or share electrons to obtain a more stable outer-shell arrangement. • The number of valence electrons helps predict an element's common valency.