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đź’ˇ One at a time
Add one proton and one electron at a time and the periodic pattern of atoms appears before your eyes — hydrogen, helium, lithium…
What is electronic configuration?
- The way electrons are distributed among the shells K, L, M, N…
- Filled from the innermost shell outward .
- Written as numbers per shell — e.g. sodium (11) = 2, 8, 1 .
| Name of the element | Symbol | Atomic number | Number of protons | Number of neutrons | Number of electrons | Distribution | |||
|---|---|---|---|---|---|---|---|---|---|
| K | L | M | N | ||||||
| Hydrogen | H | 1 | 1 | - | 1 | 1 | - | - | - |
| Helium | He | 2 | 2 | 2 | 2 | 2 | - | - | - |
| Lithium | Li | 3 | 3 | 4 | 3 | 2 | 1 | - | - |
| Beryllium | Be | 4 | 4 | 5 | 4 | 2 | 2 | - | - |
| Boron | B | 5 | 5 | 6 | 5 | 2 | 3 | - | - |
| Carbon | C | 6 | 6 | 6 | 6 | 2 | 4 | - | - |
| Nitrogen | N | 7 | 7 | 7 | 7 | 2 | 5 | - | - |
| Oxygen | O | 8 | 8 | 8 | 8 | 2 | 6 | - | - |
| Fluorine | F | 9 | 9 | 10 | 9 | 2 | 7 | - | - |
| Neon | Ne | 10 | 10 | 10 | 10 | 2 | 8 | - | - |
| Sodium | Na | 11 | 11 | 12 | 11 | 2 | 8 | 1 | - |
| Magnesium | Mg | 12 | 12 | 12 | 12 | 2 | 8 | 2 | - |
| Aluminium | Al | 13 | 13 | 14 | 13 | 2 | 8 | 3 | - |
| Silicon | Si | 14 | 14 | 14 | 14 | 2 | 8 | 4 | - |
| Phosphorus | P | 15 | 15 | 16 | 15 | 2 | 8 | 5 | - |
| Sulfur | S | 16 | 16 | 16 | 16 | 2 | 8 | 6 | - |
| Chlorine | Cl | 17 | 17 | 18 | 17 | 2 | 8 | 7 | - |
| Argon | Ar | 18 | 18 | 22 | 18 | 2 | 8 | 8 | - |
NCERT Question 9 — Find the following information for
Find the following information for the elements shown in Fig. 8.17: (i) Name of the element (ii) Symbol (iii) Total number of electrons (iv) Number of valence electrons (v) Valency of the element (vi) Number of protons (vii) Atomic number
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⏸▶ Pause and Ponder
- 14. Outermost-shell electrons: (i) 12 6 C → config 2, 4 → 4 ; (ii) 19 9 F → 2, 7 → 7 ; (iii) 28 14 Si → 2, 8, 4 → 4 .
- 15. Configurations: Z=12 → 2, 8, 2 ; Z=16 → 2, 8, 6 ; Z=18 → 2, 8, 8 .
- 16. Mass number 23, 11 protons, soft metal reacting with water → Sodium (Na) ; neutrons = 23 − 11 = 12 .
âś… Test Yourself
-
Write the Bohr–Bury formula for the maximum electrons in a shell.
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2n 2 , where n is the shell number. -
How many electrons can the L-shell hold?
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8, because 2× 2 2 = 8 . -
What is the maximum number of electrons allowed in the outermost shell?
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8. -
Write the electronic configuration of chlorine (Z = 17).
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2, 8, 7.
Important Definitions
- Electronic configuration — the distribution of the electrons of an atom among its shells K, L, M, N.
