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NCERT Solutions for Class 9th Science Chapter 8 .7.1 Building up atoms — Pause and Ponder

Book page 1528 Updated on2026-09-08

Q14.
Identify the number of electrons in the outermost shell of the following elements: (i) ¹²₆C (ii) ¹⁹₉F (iii) ²⁸₁₄Si
Answer

(i) 4 (ii) 7 (iii) 4.

Method: the lower number in AZX is the atomic number Z, which equals the number of electrons in a neutral atom. Fill the shells K, L, M in turn, using the maximum 2n² and the rule that the outermost shell may not hold more than 8.

AtomZ = electronsK (max 2)L (max 8)MOutermost shell
(i) 126C624–L, with 4 electrons
(ii) 199F927–L, with 7 electrons
(iii) 2814Si14284M, with 4 electrons

For silicon, check the order: K takes 2, then L must be filled completely (8) before M begins, leaving 14 – 2 – 8 = 4 for the M-shell.

Why it happens: the mass number on top plays no part here. Electrons are counted by Z alone, so 12C and 14C have exactly the same outermost shell — which is precisely why isotopes have identical chemical properties.
Tip: carbon and silicon both end with 4 valence electrons, and both form four bonds. That family resemblance is what puts them in the same group of the periodic table.
Q15.
Write the electronic configuration of the elements having atomic numbers 12, 16 and 18.
Answer

Z = 12 → 2, 8, 2 | Z = 16 → 2, 8, 6 | Z = 18 → 2, 8, 8.

Rules used: maximum electrons in a shell = 2n²; the outermost shell holds at most 8; shells fill in order K → L → M.

Capacities: K (n = 1) = 2 × 1² = 2    L (n = 2) = 2 × 2² = 8    M (n = 3) = 2 × 3² = 18

Z = 12 (magnesium): 12 – 2 = 10 left after K; 10 – 8 = 2 left after L → K 2, L 8, M 2 = 2, 8, 2
Z = 16 (sulfur): 16 – 2 = 14; 14 – 8 = 6 → K 2, L 8, M 6 = 2, 8, 6
Z = 18 (argon): 18 – 2 = 16; 16 – 8 = 8 → K 2, L 8, M 8 = 2, 8, 8

Note argon carefully. The M-shell can hold 18 electrons, but here it is the outermost shell, and an outermost shell is limited to 8. So filling stops at 8 — giving argon a complete octet, which is why it is an unreactive noble gas.

ZElementConfigurationValence electronsValency
12Magnesium2, 8, 222 (loses 2)
16Sulfur2, 8, 662 (gains 2)
18Argon2, 8, 880 (octet complete)
Q16.
Solve this riddle: I am an atom with a mass number of 23 and 11 protons. I am a soft metal and react vigorously with water. Who am I and how many neutrons do I have? You can also create one such riddle.
Answer

I am sodium, 2311Na, and I have 12 neutrons.

Number of protons = 11 ⇒ atomic number Z = 11
Z = 11 identifies the element as sodium (Na)

Number of neutrons = A – Z
= 23 – 11
= 12 neutrons

Do the clues fit? Sodium's configuration is 2, 8, 1 — a single valence electron, loosely held. It gives that electron away very easily, so sodium is a soft metal that reacts vigorously with water, releasing hydrogen gas. Both clues match.

Sample riddle of your own:

Try This: “My atomic number is 17 and I have 18 neutrons. I am a greenish-yellow gas, I need just one electron to complete my octet, and a little of me is added to drinking water to make it safe. Who am I, and what is my mass number?”

Answer: chlorine. A = 17 + 18 = 35, so 3517Cl; configuration 2, 8, 7; valency 1.
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