ChemistryICSE Class 9 · Atomic Structure and Chemical Bonding

Rutherford's and Bohr's Models, Electron Distribution and Isotopes (ICSE Class 9)

Updated 17 October 2026 · Fact-checked

Rutherford's gold-foil experiment showed a small, dense, positive nucleus with mostly empty space around it; Bohr's model placed electrons in fixed shells K, L, M, N holding at most 2n² electrons (2, 8, 18, 32), with at most 8 in the outermost shell. Atomic number Z is the number of protons, mass number A = protons + neutrons, and isotopes have the same Z but different A.

Each shell n holds at most 2n² electrons (2, 8, 18, 32), filled from the inside out, and the outermost shell never holds more than 8.

Illustration drawn for QuestionBuddy.

Understand Rutherford's and Bohr's Models, Electron Distribution and Isotopes

Particles. Electron (charge −1, negligible mass), proton (+1, mass 1), neutron (0, mass 1). Atomic number $Z$ = number of protons = number of electrons in a neutral atom. Mass number $A$ = protons + neutrons; neutrons $= A - Z$ . Notation $_{Z}^{A} X$ . In an ion, only the electrons change: electrons $= Z - charge$ (so for $A l^{3 +}$ , $13 - 3 = 10$ ; for $N^{3 -}$ , $7 + 3 = 10$ ).

Rutherford’s gold-foil experiment. Alpha particles were fired at thin gold foil, with a zinc sulphide screen to detect them.

Most passed straight through: most of the atom is empty space.
A few were deflected: the centre carries a positive charge that repels them.
Very few (about 1 in 20 000) bounced back: the positive charge and almost all the mass are packed into a tiny, dense nucleus.

Model: electrons revolve round a small, heavy, positive nucleus. *Merits:* discovered the nucleus. *Demerits:* a revolving electron should lose energy and spiral into the nucleus, so the atom would not be stable; it does not explain how electrons are arranged.

Bohr’s model.
1. Electrons revolve in fixed orbits (shells K, L, M, N: $n$ = 1, 2, 3, 4).
2. Each orbit has a fixed energy; an electron in its orbit does not radiate, so the atom is stable.
3. An electron absorbs energy to jump to a higher orbit and gives out energy when it falls.

Filling the shells. A shell can hold at most $2 n^{2}$ electrons: K 2, L 8, M 18, N 32. The outermost shell holds at most 8 (2 for K). Fill from the inside out. Examples: Al (13): 2, 8, 3; Cl (17): 2, 8, 7; K (19): 2, 8, 8, 1; Ca (20): 2, 8, 8, 2. Ions: $A l^{3 +}$ (2, 8), $C l^{-}$ (2, 8, 8), $K^{+}$ (2, 8, 8).

Octet and activity. Atoms with incomplete outer shells react to reach 8 (or 2). Noble gases have complete shells and are inert.

Isotopes. Atoms of the same element with the same $Z$ but different $A$ (different neutrons); same chemical properties. Hydrogen: protium $_{1}^{1} H$ , deuterium $_{1}^{2} H$ , tritium $_{1}^{3} H$ . The relative atomic mass of an element with isotopes is the weighted average:

$\overset{ˉ}{A} = \frac{\sum ( mass number \times percentage )}{100} .$

Chlorine’s 35.5 comes from the mixture of $^{35} C l$ and $^{37} C l$ in about a 3 : 1 ratio.

Rutherford's and Bohr's Models, Electron Distribution and Isotopes: formulas and key facts

Neutrons: $n = A - Z$; Nucleons = protons + neutrons.
Electrons in an ion: $e = Z - charge$; Positive ion: fewer; negative ion: more.
Shell capacity: $2 n^{2} : K = 2, L = 8, M = 18, N = 32$; Outermost shell ≤ 8.
Average atomic mass: $\overset{ˉ}{A} = \frac{\sum ( mass number \times % )}{100}$; Weighted by abundance.

How to solve Rutherford's and Bohr's Models, Electron Distribution and Isotopes questions

Start from Z, then adjust for charge.

1Protons = Z; neutrons = A − Z.
2Electrons = Z for an atom; Z − charge for an ion.
3Distribute electrons using 2, 8, 8, 2 … with at most 8 in the outer shell.
4Isotopes: same Z, different A; use percentages as weights for the average.
5Quote the observation and the matching conclusion for each part of Rutherford’s experiment.

Rutherford's and Bohr's Models, Electron Distribution and Isotopes in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Carbon dating isotope

Suppose a sample contains the carbon isotopes ¹²C, ¹³C and ¹⁴C.

State the numbers of protons and neutrons in each.

Try it first, then show the solutionSolution

All have $Z = 6$ , so 6 protons (and 6 electrons).
Neutrons: $12 - 6 = 6$ ; $13 - 6 = 7$ ; $14 - 6 = 8$ .

Answer: 6 protons each; 6, 7 and 8 neutrons.

Memory hook: Same Z, different A.

Tricks to remember Rutherford's and Bohr's Models, Electron Distribution and Isotopes

Short memory aids. Say each one aloud once, then use it on a question the same day.

Empty, positive, dense
Straight through: empty. Deflected: positive. Bounced back: dense nucleus.
Use it when: Rutherford conclusions.
2, 8, 18, 32
2n² gives the capacity of each shell.
Use it when: Configurations.
Same Z, different A
Isotopes differ only in neutrons.
Use it when: Isotope questions.

Rutherford's and Bohr's Models, Electron Distribution and Isotopes: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Capacity of the M shell · 1 mark · MCQBoard level

The maximum number of electrons the M shell can hold is:
(a) 2
(b) 8
(c) 18
(d) 32

Try it first, then show the solutionSolution

$2 n^{2} = 2 \times 9 = 18$ for $n = 3$ .

Answer: (c) 18

Example 2: Composition of an atom · 2 marks · Short answerBoard level

An atom has atomic number 15 and mass number 31. Find the number of protons, electrons and neutrons and write its electronic configuration.

Try it first, then show the solutionSolution

Protons $= 15$ ; electrons $= 15$ . (½)
Neutrons $= 31 - 15 = 16$ . (½)
Configuration 2, 8, 5. (1)

Answer: 15 p; 15 e; 16 n; 2, 8, 5

Example 3: Two ions · 3 marks · Short answerHarder board level

For (a) $A l^{3 +}$ (Z = 13, A = 27) and (b) $N^{3 -}$ (Z = 7, A = 14) give the numbers of protons, electrons and neutrons and the electronic configuration. Which noble gas has the same configuration?

Try it first, then show the solutionSolution

(a) $A l^{3 +}$ : 13 protons, 10 electrons, 14 neutrons; configuration 2, 8. (1½)
(b) $N^{3 -}$ : 7 protons, 10 electrons, 7 neutrons; configuration 2, 8. (1)
Both resemble neon (2, 8). (½)

Answer: Al³⁺: 13, 10, 14; N³⁻: 7, 10, 7; neon-like

Example 4: Average atomic mass · 3 marks · Short answerHarder board level

An element of atomic number 31 has two isotopes of mass numbers 69 and 71, present in the ratio 60% to 40%. Find the number of neutrons in each isotope and the average relative atomic mass.

Try it first, then show the solutionSolution

Neutrons: $69 - 31 = 38$ ; $71 - 31 = 40$ . (1)
$\overset{ˉ}{A} = \frac{69 \times 60 + 71 \times 40}{100}$ (1)
$= \frac{4140 + 2840}{100} = 69.8$ . (1)

Answer: 38 and 40 neutrons; 69.8

Example 5: Reading Rutherford’s experiment · 4 marks · Case-basedHOTS / competency

In an experiment of Rutherford’s type, $4.0 \times 1 0^{5}$ alpha particles are fired at a thin gold foil.
(a) About how many bounce back (1 in 20 000)?
(b) Name the observation that shows the atom is mostly empty.
(c) Name the observation that shows the nucleus is positive.
(d) State one demerit of Rutherford’s model and how Bohr’s model removes it.

Try it first, then show the solutionSolution

(a) $\frac{4.0 \times 1 0 ^{5}}{20 000} = 20$ . (1)
(b) Most alpha particles passed straight through. (1)
(c) A few were deflected (repelled) through small angles. (1)
(d) A revolving electron would lose energy and spiral in; Bohr proposed fixed orbits in which electrons do not lose energy. (1)

Answer: (a) 20 (b) straight through (c) deflections (d) energy loss; fixed orbits

Where students lose marks in Rutherford's and Bohr's Models, Electron Distribution and Isotopes

The slips examiners see most often, and what to write instead.

Mistake: Electrons counted as Z in an ion
Ions gain or lose electrons.
Wrong working
Al³⁺ has 13 electrons.
Right working
Al³⁺ has 10 electrons.
Do this: Z − charge.
Usually costs: 1 mark.
Mistake: Conclusion matched to the wrong observation
Each observation gives one conclusion.
Wrong working
Deflection shows the atom is empty.
Right working
Straight passage shows empty space; deflection shows a positive centre.
Do this: Use a two-column table.
Usually costs: 2 marks.
Mistake: Isotopes said to differ in protons
They differ in neutrons.
Wrong working
Isotopes have different numbers of protons.
Right working
Same protons, different neutrons.
Do this: Same Z.
Usually costs: 1 mark.

Exam tips

Present Rutherford’s answer as a three-row observation/conclusion table.
Show Z, A, protons, neutrons and electrons in a small table for composition questions.
Check that outer-shell electrons never exceed 8.
Weight isotopes by percentage in average-mass sums.

Practice questions on Rutherford's and Bohr's Models, Electron Distribution and Isotopes

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The number of neutrons in an atom of $_{15}^{31} P$ is:
(a) 15
(b) 16
(c) 31
(d) 46
Hint
A − Z.
Answer
(b) 16
Q2 · 1 mark · Very short answerBoard level
Who discovered the neutron?
Hint
Named in the table.
Answer
James Chadwick.
Q3 · 2 marks · Short answerHarder board level
Write the electronic configurations of (a) calcium (Z = 20), (b) the K⁺ ion (Z = 19).
Hint
Remove one electron for K⁺.
Answer
(a) 2, 8, 8, 2. (b) 2, 8, 8 (18 electrons).
Q4 · 3 marks · Short answerHarder board level
An ion X²⁺ has 18 electrons and 20 neutrons. Find the atomic number, the mass number and the electronic configuration of the neutral atom.
Hint
Add back the charge.
Answer
Z = 18 + 2 = 20; A = 20 + 20 = 40; configuration of the atom 2, 8, 8, 2 (calcium).
Q5 · 2 marks · Assertion-reasonHarder board level
Assertion (A): Noble gases are chemically inert.
Reason (R): Their outermost shells are completely filled.
(a) Both true, R explains A.
(b) Both true, R does not explain A.
(c) A true, R false.
(d) A false, R true.
Hint
Octet/duplet.
Answer
(a)
Q6 · 3 marks · Short answerHOTS / competency
Deuterium (²H) combines with oxygen-16 to form "heavy water" D₂O. Find the number of protons, neutrons and electrons in one molecule of D₂O and in one molecule of H₂O (¹H, ¹⁶O), and the relative molecular masses (by adding mass numbers).
Hint
Oxygen has Z = 8.
Answer
D₂O: protons $2 \times 1 + 8 = 10$ , neutrons $2 \times 1 + 8 = 10$ , electrons 10; $M_{r} = 2 \times 2 + 16 = 20$ . H₂O: protons 10, neutrons $0 + 8 = 8$ , electrons 10; $M_{r} = 18$ . Same chemical makeup but different mass.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Who discovered the electron?Show answer
J. J. Thomson.
2.Shell capacity formula?Show answer
2n².
3.Mass number?Show answer
Protons + neutrons.
4.Name the three isotopes of hydrogen.Show answer
Protium, deuterium, tritium.
5.Why is Cl’s relative atomic mass 35.5?Show answer
It is the weighted average of ³⁵Cl and ³⁷Cl.

Rutherford's and Bohr's Models, Electron Distribution and Isotopes: frequently asked questions

What did Rutherford's gold foil experiment show?

Most alpha particles passed through (mostly empty space), a few were deflected (positive centre) and very few rebounded (small dense nucleus).

What is Bohr's model of the atom?

Electrons revolve in fixed shells (K, L, M, N) with fixed energies, without losing energy while in a shell; the shell n holds at most 2n² electrons.

What are isotopes?

Atoms of the same element with the same atomic number but different mass numbers.