What Information Does the Electronic Configuration of an Atom Provide?

Electronic configuration tells us:

1. Number of shells occupied — indicates the period (row in periodic table) Example: 2,8,1 has 3 shells → Period 3

2. Number of valence electrons — electrons in the outermost shell Example: 2,8,1 has 1 valence electron

3. Group of the element — based on valence electrons Elements with 1 valence electron → Group 1 (Alkali metals) Elements with 7 valence electrons → Group 17 (Halogens) Elements with 8 valence electrons → Group 18 (Noble gases)

4. Valency — combining capacity of the atom If valence electrons ≤ 4: Valency = number of valence electrons If valence electrons > 4: Valency = 8 − (number of valence electrons)

5. Chemical nature (metal, non-metal, metalloid) 1–3 valence electrons: metals (tend to lose electrons) 5–7 valence electrons: non-metals (tend to gain electrons) 4 valence electrons: metalloids (e.g., Si, Ge)

Period: • Count the number of occupied electron shells. • That number is the period.

Examples: • Na: 2,8,1 → 3 shells → Period 3 • Cl: 2,8,7 → 3 shells → Period 3 • Ca: 2,8,8,2 → 4 shells → Period 4

Group (for s and p block elements): • 1 valence electron → Group 1 • 2 valence electrons → Group 2 • 3 valence electrons → Group 13 • 4 valence electrons → Group 14 • 5 valence electrons → Group 15 • 6 valence electrons → Group 16 • 7 valence electrons → Group 17 • 8 valence electrons → Group 18 (noble gases)

Valency is the combining capacity of an atom:

For elements with valence electrons 1 to 4: Valency = number of valence electrons

For elements with valence electrons 5 to 7: Valency = 8 − valence electrons

For noble gases (8 valence electrons): Valency = 0

Examples: • Na (2,8,1) → 1 valence electron → Valency = 1 • Mg (2,8,2) → 2 valence electrons → Valency = 2 • Al (2,8,3) → 3 valence electrons → Valency = 3 • C (2,4) → 4 valence electrons → Valency = 4 • N (2,5) → 5 valence electrons → Valency = 8−5 = 3 • O (2,6) → 6 valence electrons → Valency = 8−6 = 2 • Cl (2,8,7) → 7 valence electrons → Valency = 8−7 = 1 • Ar (2,8,8) → 8 valence electrons → Valency = 0

Electronic configuration also reveals:

1. Metallic or non-metallic character: • 1–3 valence electrons → metallic (tends to lose electrons, forms cations) • 5–7 valence electrons → non-metallic (tends to gain electrons, forms anions) • 4 valence electrons → can be either (e.g., C forms covalent bonds)

2. Type of bond formed: • Metals + non-metals → ionic bond (transfer of electrons) • Non-metals + non-metals → covalent bond (sharing of electrons)

3. Stability: • Atoms with completely filled shells (like noble gases with 8 valence electrons) are chemically stable and unreactive. • Atoms try to achieve noble gas configuration by gaining, losing, or sharing electrons.

4. Oxidation state: • Can be predicted from the number of electrons an atom tends to lose or gain.