Atomic Structure and Chemical Bonding ICSE Class 9: Rutherford, Bohr, Ionic and Covalent Bonds

What Atomic Structure and Chemical Bonding is about

Weightage: CISCE publishes no chapter weightage for ICSE Chemistry. Atomic Structure and Chemical Bonding is topic 4 of the Class IX part of the 2028 syllabus; the Class IX paper (80 marks, 2 hours) is set and marked by the school.Source: CISCE ICSE Chemistry (Science Paper 2) syllabus, examination year 2028 (Class IX)

Key concepts

Constituents of an atom and their discovery

Particle	Symbol	Charge	Relative mass	Where	Discovered by
Electron	e	-1	1/1837 (negligible)	in shells around the nucleus	J. J. Thomson (cathode rays)
Proton	p	+1	1	nucleus	E. Goldstein (anode rays); named by Rutherford
Neutron	n	0	1	nucleus	James Chadwick

• Atomic number (Z) = number of protons = number of electrons in a neutral atom. It identifies the element.
• Mass number (A) = protons + neutrons (the nucleons). Neutrons = A - Z.
• Notation: ${}_Z^{A}X$, e.g. ${}_{11}^{23}Na$ has 11 p, 11 e, 12 n.

In an ion the number of electrons changes, not protons: Na⁺ has 10 electrons; Cl⁻ has 18.

Worked example

Find the protons, neutrons and electrons in (a) ${}_{13}^{27}Al$, (b) ${}_{13}^{27}A{l}^{3+}$, (c) ${}_{16}^{32}{S}^{2-}$.

Try it first, then show the solutionSolution

1. (a) p = 13, e = 13, n = 27 - 13 = 14.
2. (b) p = 13, n = 14, e = 13 - 3 = 10.
3. (c) p = 16, n = 16, e = 16 + 2 = 18.

Answer: As above.

Rutherford's gold-foil experiment

Experiment: a narrow beam of fast, positively charged alpha particles from a radioactive source was aimed at a very thin gold foil. A circular zinc sulphide screen flashed wherever an alpha particle struck.

Observations and conclusions:

Observation	Conclusion
Most alpha particles passed straight through	Most of the atom is empty space
A few were deflected through small angles	There is a positively charged region that repels them
Very few (about 1 in 20,000) bounced back	The positive charge and almost all the mass are in a tiny, dense nucleus

Rutherford's model: a small, heavy, positive nucleus at the centre; electrons revolve around it like planets round the Sun; the atom is neutral overall.

Merits: discovered the nucleus; showed the atom is mostly empty space; correctly placed the mass and positive charge at the centre.

Demerits: a revolving charged electron should lose energy continuously and spiral into the nucleus, so the atom would be unstable; and the model says nothing about how electrons are arranged.

Worked example

What would Rutherford have seen if the positive charge were spread evenly through the atom?

Try it first, then show the solutionSolution

1. A spread-out charge would give only weak, small deflections.
2. No alpha particle would bounce back.

Answer: Almost all particles passing through with slight deflection; none rebounding.

Bohr's model and electron distribution

Bohr's model (simplified):
1. Electrons revolve round the nucleus in certain fixed circular paths called orbits, shells or energy levels, named K, L, M, N (n = 1, 2, 3, 4) outwards.
2. Each orbit has a fixed energy; energy increases away from the nucleus.
3. While an electron stays in its orbit it does not lose energy, so the atom is stable (this removes Rutherford's problem).
4. An electron absorbs energy to jump to a higher orbit and gives out energy (light) when it falls back.

2n² rule: the maximum number of electrons in shell n is $2n^2$: K = 2, L = 8, M = 18, N = 32.
Further rule: the outermost shell holds at most 8 electrons (and the next-to-last at most 18). Shells fill from the inside out.

Element	Z	K	L	M	N
C	6	2	4
O	8	2	6
Na	11	2	8	1
Cl	17	2	8	7
Ar	18	2	8	8
K	19	2	8	8	1
Ca	20	2	8	8	2

Worked example

Why is the electronic configuration of potassium 2, 8, 8, 1 and not 2, 8, 9?

Try it first, then show the solutionSolution

1. The M shell could hold 18 by the 2n² rule.
2. But the outermost shell cannot hold more than 8.
3. So the 19th electron starts the N shell.

Answer: Outermost shell maximum 8: 2, 8, 8, 1.

Octet rule, chemical activity and isotopes

Octet rule: atoms combine by losing, gaining or sharing electrons so that each gets 8 electrons in its outermost shell (or 2, a duplet, like helium, for hydrogen and for small atoms such as Li that reach helium's configuration), like the nearest noble gas.

Reason for chemical activity: an atom with an incomplete outer shell is unstable and reacts to complete it. Noble gases (He 2; Ne 2, 8; Ar 2, 8, 8) already have a stable configuration and are almost inert.

Isotopes are atoms of the same element with the same atomic number but different mass numbers (different numbers of neutrons). They have the same chemical properties (same electrons) but slightly different physical properties.

• Hydrogen: protium ${}_1^{1}H$ (0 n), deuterium ${}_1^{2}H$ (1 n), tritium ${}_1^{3}H$ (2 n).
• Carbon: ${}_6^{12}C$ (6 n), ${}_6^{13}C$ (7 n), ${}_6^{14}C$ (8 n; used in carbon dating).
• Chlorine: ${}_{17}^{35}Cl$ (18 n), ${}_{17}^{37}Cl$ (20 n), in about a 3 : 1 ratio, which is why chlorine's relative atomic mass is 35.5, not a whole number.

Worked example

Natural chlorine is 75% chlorine-35 and 25% chlorine-37. Show that its average atomic mass is 35.5.

Try it first, then show the solutionSolution

1. Average = (35 × 75 + 37 × 25) ÷ 100
2. = (2625 + 925) ÷ 100 = 35.5

Answer: 35.5

Electrovalent (ionic) bond

An electrovalent bond is formed by the complete transfer of one or more electrons from a metal atom to a non-metal atom; the oppositely charged ions formed are held by electrostatic attraction.

• Metals (1-3 outer electrons) lose electrons → cations. Non-metals (5-7 outer electrons) gain them → anions.
• Electrovalency = number of electrons lost or gained.

Sodium chloride: Na (2, 8, 1) gives 1 electron to Cl (2, 8, 7).
Na → Na⁺ (2, 8) + e⁻; Cl + e⁻ → Cl⁻ (2, 8, 8). Formula NaCl.

Magnesium chloride: Mg (2, 8, 2) gives one electron to each of two Cl atoms → Mg²⁺ and 2Cl⁻. Formula MgCl₂.

Calcium oxide: Ca (2, 8, 8, 2) gives two electrons to O (2, 6) → Ca²⁺ (2, 8, 8) and O²⁻ (2, 8). Formula CaO.

In the orbit structure, draw both atoms with all shells, show the transferred electron(s) with an arrow, then draw the ions in square brackets with their charges.

Worked example

Describe, using electronic configurations, the formation of magnesium chloride. (Mg = 12, Cl = 17)

Try it first, then show the solutionSolution

1. Mg: 2, 8, 2; it loses 2 electrons to form Mg²⁺ (2, 8).
2. Each Cl: 2, 8, 7; each gains 1 electron to form Cl⁻ (2, 8, 8).
3. One Mg²⁺ attracts two Cl⁻: MgCl₂.

Answer: Transfer of 2 electrons from Mg to two Cl atoms; MgCl₂.

Covalent bond and radical formation

A covalent bond is formed by the sharing of a pair of electrons between two atoms (usually non-metals), each atom contributing one electron. One shared pair = single bond, two = double, three = triple.

Molecule	Sharing	Bonds	Lone pairs on central atom(s)
H₂	H shares 1 pair (duplet each)	H-H single	0
Cl₂	1 pair	Cl-Cl single	3 on each Cl
O₂	2 pairs	O=O double	2 on each O
N₂	3 pairs	N≡N triple	1 on each N
HCl	1 pair	H-Cl single	3 on Cl
H₂O	O shares 1 pair with each H	2 single	2 on O
NH₃	N shares 1 pair with each H	3 single	1 on N
CH₄	C shares 1 pair with each H	4 single	0
CCl₄	C shares 1 pair with each Cl	4 single	0 on C (3 on each Cl)

In a structure, use dots for one atom's electrons and crosses for the other's.

Radical formation: when a covalent bond breaks evenly (each atom takes one of the two shared electrons), two free radicals with unpaired electrons form. For example, in ultraviolet light, $C{l}_2\stackrel{UV}{\to }2C{l}^{\bullet }$; methane can split into a methyl radical and a hydrogen atom, $C{H}_3^{\bullet }$ and $H^{\bullet }$. These radicals are very reactive (see topic 1).

Worked example

How many shared pairs and lone pairs are there in a molecule of (a) ammonia, (b) water, (c) nitrogen?

Try it first, then show the solutionSolution

1. (a) NH₃: 3 shared pairs, 1 lone pair on N.
2. (b) H₂O: 2 shared pairs, 2 lone pairs on O.
3. (c) N₂: 3 shared pairs (triple bond), 1 lone pair on each N.

Answer: As above.

Chemistry in real life: Atomic Structure and Chemical Bonding

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.

Topics in this chapter

• Electrovalent and Covalent Bonds: Electron Transfer, Sharing and Dot Structures
• Rutherford's and Bohr's Models, Electron Distribution and Isotopes

Atomic Structure and Chemical Bonding formula sheet

Neutrons

$n=A-Z$

Mass number minus atomic number.

Maximum electrons in shell n

$2n^2$

K 2, L 8, M 18, N 32; outermost at most 8.

Electrons in an ion

$e=Z-\text{charge}$

Cation: fewer; anion: more (subtract a negative charge).

Average atomic mass

$\bar{A}=\frac{\sum (\text{mass number}\times \%)}{100}$

E.g. chlorine 35.5.

Atomic Structure and Chemical Bonding: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Atomic Structure and Chemical Bonding

The question patterns to expect, and the method that works for each.

Tricks to remember Atomic Structure and Chemical Bonding

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

• Discoverers: 'The Good Chap'

  Thomson electron, Goldstein proton (anode rays), Chadwick neutron.

  Use it when: Discovery of particles.

• Rutherford's three S's

  Straight through (empty Space), Small deflection (positive charge), Sent back (tiny dense nucleus).

  Use it when: Observations and conclusions.

• 2-8-18-32 with a cap of 8

  2n² gives the room; the outermost shell is capped at 8.

  Use it when: Electron distribution up to Z = 20.

• Metals Lose, Non-metals Gain

  'LEO the lion': Loses Electrons → positive ion (metal).

  Use it when: Electrovalent bonding.

Atomic Structure and Chemical Bonding: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 5-mark question split into 2 + 2 + 1 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Atomic Structure and Chemical Bonding

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

• ✗Mistake: Same symbol for both atoms' electrons

  CISCE examiners' comments on the ICSE Class 10 paper, 2025: teachers were asked to stress using distinct symbols, dots for one element and crosses for the other, in electron dot structures, because candidates' diagrams were unclear.

  Wrong working

  Water drawn with all electrons as dots.

  Right working

  O's electrons as dots, H's as crosses; shared pairs shown as dot-cross.

  ✓Do this: Two elements, two symbols.

  Usually costs: 1 mark.

• ✗Mistake: Missing lone pairs on nitrogen

  CISCE examiners' comments on the ICSE Class 10 paper, 2024: in the structure of a nitrogen molecule some candidates drew the triple bond but forgot the two electrons on each nitrogen that are not in the bond.

  Wrong working

  N≡N with only six electrons drawn.

  Right working

  Triple bond (six shared electrons) plus one lone pair on each N: 10 valence electrons in all.

  ✓Do this: Count: every atom must end with 8 around it.

  Usually costs: 1 mark.

• ✗Mistake: Changing protons in an ion

  Students alter the atomic number when an ion forms.

  Wrong working

  Na⁺ has 10 protons.

  Right working

  Na⁺ has 11 protons and 10 electrons.

  ✓Do this: Ions gain or lose electrons only.

  Usually costs: 1 mark.

• ✗Mistake: Overfilling the M shell

  The 2n² rule is applied without the outermost-shell limit.

  Wrong working

  Ca: 2, 8, 10.

  Right working

  Ca: 2, 8, 8, 2.

  ✓Do this: Outermost shell: maximum 8.

  Usually costs: 1 mark.

• ✗Mistake: Mixing up Rutherford's observations and conclusions

  Students give conclusions when asked for observations, or pair them wrongly.

  Wrong working

  Observation: the atom has a nucleus.

  Right working

  Observation: a very few alpha particles bounced back. Conclusion: there is a tiny dense nucleus.

  ✓Do this: Observations are what happened to the alpha particles.

  Usually costs: 1 mark each.

Practice questions on Atomic Structure and Chemical Bonding

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

1. Q1 · 1 mark · MCQExam level

   Which pair are isotopes?
   (a) ${}_{18}^{40}Ar$ and ${}_{20}^{40}Ca$
   (b) ${}_{17}^{35}Cl$ and ${}_{17}^{37}Cl$
   (c) O₂ and O₃
   (d) Na and Na⁺

   Hint

   Same Z.

   Answer

   (b)

2. Q2 · 1 mark · MCQExam level

   The number of lone pairs on the central atom of methane is:
   (a) 0
   (b) 1
   (c) 2
   (d) 4

   Hint

   Four bonds.

   Answer

   (a)

3. Q3 · 3 marks · Short answerExam level

   State the charge, relative mass and location of the electron, proton and neutron.

   Hint

   Table.

   Answer

   Electron: -1, about 1/1837, shells. Proton: +1, 1, nucleus. Neutron: 0, 1, nucleus.

4. Q4 · 3 marks · Short answerHarder exam level

   Write the electronic configurations of (a) Al, (b) S²⁻, (c) K⁺. (Al = 13, S = 16, K = 19) Which two are isoelectronic with argon?

   Hint

   Count electrons.

   Answer

   (a) 2, 8, 3 (b) 2, 8, 8 (c) 2, 8, 8. S²⁻ and K⁺ have argon's configuration.

5. Q5 · 3 marks · Short answerHarder exam level

   Describe the formation of carbon tetrachloride, stating the type of bond and the number of shared pairs. Why is it not electrovalent?

   Hint

   C has 4 outer electrons.

   Answer

   C (2, 4) shares one electron pair with each of four Cl (2, 8, 7): four single covalent bonds, four shared pairs; each Cl has three lone pairs. Carbon cannot easily lose or gain four electrons, so it shares instead of transferring.

6. Q6 · 3 marks · Short answerHarder exam level

   Describe the formation of an oxygen molecule and of a hydrogen chloride molecule.

   Hint

   Double; single.

   Answer

   O₂: each O (2, 6) shares two electrons: two shared pairs, a double bond; two lone pairs on each O. HCl: H shares its one electron and Cl one of its seven: one shared pair, a single bond; H gets a duplet, Cl an octet (three lone pairs).

7. Q7 · 3 marks · Short answerHarder exam level

   Chlorine gas is exposed to sunlight. (a) What happens to the Cl-Cl bond? (b) What are the particles formed called? (c) Why are they reactive?

   Hint

   Even break.

   Answer

   (a) It breaks evenly, each atom taking one electron of the pair. (b) Chlorine free radicals (atoms), $C{l}^{\bullet }$. (c) Each has an unpaired electron, which it tends to pair by reacting.

8. Q8 · 2 marks · Short answerExam level

   Why are noble gases unreactive while sodium is very reactive? Use configurations of Ne (10), Ar (18) and Na (11).

   Hint

   Octet.

   Answer

   Ne (2, 8) and Ar (2, 8, 8) have complete outer octets: stable. Na (2, 8, 1) has one outer electron, which it loses easily to reach the stable 2, 8.

9. Q9 · 3 marks · Short answerHOTS / competency

   An ion X²⁺ has 10 electrons and 12 neutrons. Find (a) Z, (b) A, (c) the configuration of the atom X. Name the element.

   Hint

   Add back the lost electrons.

   Answer

   (a) Z = 12 (b) A = 24 (c) 2, 8, 2: magnesium.

10. Q10 · 3 marks · Short answerHOTS / competency

    Copper has two isotopes, Cu-63 and Cu-65, and an average atomic mass of 63.5. Find the percentage of each isotope.

    Hint

    Let Cu-63 be x%.

    Answer

    63x + 65(100 - x) = 6350; 6500 - 2x = 6350; x = 75. Cu-63 75%, Cu-65 25%.

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. 1.Who discovered the neutron?Show answer

   James Chadwick.

2. 2.What did Rutherford's experiment discover?Show answer

   The nucleus.

3. 3.Maximum electrons in the M shell?Show answer

   18 (but at most 8 if it is the outermost shell).

4. 4.Configuration of chlorine?Show answer

   2, 8, 7.

5. 5.Isotopes of hydrogen?Show answer

   Protium, deuterium, tritium.

6. 6.Number of lone pairs on O in water?Show answer

   Two.

7. 7.Bond in N₂?Show answer

   Triple covalent bond.

8. 8.What forms when a covalent bond breaks evenly?Show answer

   Two free radicals.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you give p, n and e for any atom or ion up to Z = 20?
• ☐Can you pair each Rutherford observation with its conclusion?
• ☐Can you state two merits and two demerits of Rutherford's model?
• ☐Can you state Bohr's postulates without maths?
• ☐Can you draw electron dot structures of H₂O, NH₃, CH₄, CCl₄, N₂, O₂?
• ☐Can you explain why chlorine's atomic mass is 35.5?

Atomic Structure and Chemical Bonding: frequently asked questions