Journey Inside the Atom Class 9 Notes, Diagrams and Questions

What Journey Inside the Atom is about

Weightage: This chapter is part of Unit II, Matter - Its Nature and Behaviour, which carries 25 of the 80 theory marks in the school annual exam, shared with Exploring Mixtures and their Separation and Atomic Foundations of Matter.Source: CBSE Science (086) syllabus, Class IX 2026-27, course structure, Unit II Matter - Its Nature and Behaviour

Key concepts

From parmanu to Dalton's atoms

NCERT section 8.1 Rediscovering the Roots of Atomic Theory

More than 2000 years ago thinkers asked what everything is made of.

• Acharya Kanada (India) said that dividing matter (*dravya*) again and again ends in tiny particles that cannot be divided further: parmanu. Two parmanus form a dyad, three a triad, and so on. His ideas are in the *Vaisesika Sutras*.
• Leucippus and Democritus (Greece) called such particles atomos, meaning 'indivisible'.
• These were ideas from thinking, not experiments.

John Dalton (1808) gave the first scientific atomic theory, based on experiments: all matter is made of tiny indivisible atoms, the basic building blocks.

Late in the 19th century, radioactivity (some elements giving out invisible radiation and particles) showed that atoms are made of still smaller particles, so they are not indivisible after all. Scientists then asked: what are atoms made of, what do they look like, and what makes atoms of different elements different?

Atoms are tiny: about $10^{-10}$ m across. A sheet of paper 0.1 mm thick is about a million atoms deep.

Worked example

An atom is about $10^{-10}$ m across. How many atoms would lie side by side across a pencil line 0.5 mm wide?

Try it first, then show the solutionSolution

1. 0.5 mm $=0.5\times 10^{-3}$ m $=5\times 10^{-4}$ m.
2. Number $=\frac{5\times 10^{-4}}{10^{-10}}=5\times 10^6$.

Answer: About 5 million atoms.

The electron and Thomson's model

NCERT section 8.2 A Short Historical Journey Through Atomic Models

In 1897, J. J. Thomson passed electricity through a gas at very low pressure in a glass tube with two electrodes and a high voltage. Rays travelled from the cathode (−) to the anode (+): cathode rays.

• Electric and magnetic fields showed they are streams of negatively charged particles, much lighter than atoms: electrons.
• The rays were the same whatever the cathode metal or gas, so electrons are present in all atoms.
• Charge of an electron $=-1.602\times 10^{-19}$ C, written as −1 (relative charge).

Thomson's model (plum pudding): atoms are neutral, so he pictured the atom as a sphere of positive charge with electrons spread through it, like plums in a pudding, or seeds in a watermelon (red pulp = positive charge, seeds = electrons). Total positive charge = total negative charge.

It was the first attempt to explain how charges in an atom balance, but it was replaced after the gold foil experiment. Thomson won the 1906 Nobel Prize in Physics.

Worked example

Cathode rays behaved the same whether the tube had a copper or an aluminium cathode and whether it held air or hydrogen. What did this tell Thomson? In his model, what total positive charge must an atom with 6 electrons carry?

Try it first, then show the solutionSolution

1. Particles with the same properties came from every material tried, so they must be a part of all atoms: electrons.
2. Atoms are neutral, so the positive sphere must balance the electrons.
3. Six electrons carry a total charge of −6, so the sphere must carry +6.

Answer: Electrons are present in all atoms; the positive sphere carries +6 to balance 6 electrons.

The gold foil experiment and Rutherford's model

NCERT section 8.2 A Short Historical Journey Through Atomic Models

In 1911, Geiger and Marsden, working with Rutherford, fired a narrow beam of alpha (α) particles (positively charged, from radioactive elements; later known to be helium nuclei with 2 protons and 2 neutrons) at a very thin gold foil.

Thomson's model predicted that all would pass with little deflection. Instead:

Observation	Conclusion
Most α-particles passed straight through	most of the atom is empty space
Some were deflected through large angles	the positive charge is concentrated in a tiny region that repels them
Very few (about 1 in 20 000 in later counts) bounced straight back	that region is very small, dense, and holds nearly all the mass: the nucleus

Rutherford's nuclear (planetary) model:

• A tiny, dense, positively charged nucleus at the centre with almost all the mass.
• Electrons revolve round the nucleus, like planets round the Sun.
• The nucleus is about $10^5$ times smaller than the atom: atom ≈ $10^{-10}$ m, nucleus ≈ $10^{-15}$ m. If the atom were a cricket ground about 100 m across, the nucleus would be a peppercorn at the centre.

Rutherford also showed the nuclear charge comes from protons: charge +1, much heavier than electrons. In a neutral atom, number of protons = number of electrons (helium 2 and 2, sodium 11 and 11).

Worked example

Suppose that out of 100 000 α-particles fired at a gold foil, about 99 900 pass straight through, 95 are deflected through large angles and 5 bounce back. What does each group tell you about the atom?

Try it first, then show the solutionSolution

1. Almost all pass straight: most of the atom is empty space.
2. A few are deflected sharply: the positive charge is concentrated in a small region that repels them strongly when they pass close.
3. A handful bounce back: that region is tiny, dense and holds nearly all the mass (the nucleus); a spread-out charge, as in Thomson's model, could never turn them back.

Answer: Empty space; concentrated positive charge; a tiny, dense, massive nucleus that rules out the plum pudding model.

Limitation of Rutherford's model and Bohr's model

NCERT section 8.2 A Short Historical Journey Through Atomic Models

Problem with Rutherford's model: an electron moving in a circle is always changing direction, so it is accelerating (Chapter 4). A charged particle that accelerates should lose energy, spiral inwards and fall into the nucleus. Atoms would collapse in a moment, but matter is stable. So the model could not explain stability.

Bohr's model (1913), by Niels Bohr:

• Electrons move only in certain fixed circular orbits called stationary states, shells or energy levels.
• Shells are named K, L, M, N… or numbered n = 1, 2, 3, 4…, starting nearest the nucleus.
• While in a shell, an electron does not lose energy (this was Bohr's postulate; it explains stability).
• Each shell has a definite energy; energy increases outwards (K lowest).
• An electron can jump between shells only by absorbing or giving out exactly the energy difference between them. It cannot be between shells.
• Each shell holds only a limited number of electrons.

The letters K, L, M come from Charles Barkla's X-ray work; he started at K in case earlier series were found. Bohr's model too was later improved by the quantum mechanical model, where electrons form 'clouds' in regions where they are likely to be found. Bohr won the 1922 Nobel Prize.

Worked example

In a hydrogen atom an electron jumps from the L shell to the K shell. Does the atom absorb or give out energy? Could the electron stop halfway between the two shells? Explain using Bohr's model.

Try it first, then show the solutionSolution

1. Energy increases outwards, so the K shell has less energy than the L shell.
2. Moving to a lower level, the electron gives out exactly the energy difference between L and K.
3. Electrons can exist only in fixed energy levels, so it cannot stop between them.

Answer: It gives out energy equal to the L-K difference; it cannot stop between shells, since only fixed energy levels are allowed.

The neutron, atomic number and mass number

NCERT section 8.5 Atomic Number

A puzzle: helium has 2 protons, hydrogen 1, yet helium's atom is about four times as heavy. In 1932 James Chadwick found the neutron (n): mass about equal to a proton's, no charge. Neutrons are in every nucleus except ordinary hydrogen.

Particle	Symbol	Relative charge	Relative mass	Where
Electron	e⁻	−1	about 1/1840 (negligible)	shells
Proton	p⁺	+1	1	nucleus
Neutron	n⁰	0	1	nucleus

• Atomic number (Z) = number of protons. It fixes the identity of an element. In a neutral atom, it also equals the number of electrons.
• Mass number (A) = protons + neutrons (nucleons). Electrons are ignored because they are so light.
• Number of neutrons = A − Z.
• Notation: ${}_Z^A\text{X}$, e.g. ${}_6^{12}\text{C}$: 6 protons, 6 neutrons, 6 electrons.

Light atoms often have equal protons and neutrons (carbon 6 and 6, oxygen 8 and 8); heavy ones have many more neutrons (iron 26 p, 30 n; uranium 92 p, 146 n), which help hold the nucleus together against proton-proton repulsion through the nuclear force.

Symbols: first letter capital, second small (Al, Co, Cl, Zn). Some come from Latin or other names: Na (natrium), K (kalium), Fe (ferrum), Cu (cuprum), Ag (argentum), Au (aurum), Pb (plumbum), Hg, W. Dalton first used picture symbols (1803); Berzelius proposed letters (1813); IUPAC now approves names. About 118 elements are known.

Worked example

An atom of silver has 47 protons and mass number 108. Find its neutrons and electrons, and write its notation (symbol Ag).

Try it first, then show the solutionSolution

1. Electrons = protons = 47 (neutral atom).
2. Neutrons $=A-Z=108-47=61$.
3. Notation: ${}_{47}^{108}\text{Ag}$; Ag comes from the Latin argentum.

Answer: 61 neutrons, 47 electrons; ${}_{47}^{108}\text{Ag}$.

Distribution of electrons in shells (first 18 elements)

NCERT section 8.7 How Are Electrons Distributed in Different Energy Levels?

Bohr–Bury rules:
1. A shell can hold at most $2n^2$ electrons: K ($n=1$) 2, L ($n=2$) 8, M ($n=3$) 18.
2. The outermost shell can hold at most 8 electrons (K alone: 2).
3. Shells fill in order K, L, M…: a shell starts only after the inner one is full (true for the first 18 elements).

The distribution of electrons in shells is the electronic configuration.

Z	Element	K, L, M	Z	Element	K, L, M
1	H	1	10	Ne	2, 8
2	He	2	11	Na	2, 8, 1
3	Li	2, 1	12	Mg	2, 8, 2
4	Be	2, 2	13	Al	2, 8, 3
5	B	2, 3	14	Si	2, 8, 4
6	C	2, 4	15	P	2, 8, 5
7	N	2, 5	16	S	2, 8, 6
8	O	2, 6	17	Cl	2, 8, 7
9	F	2, 7	18	Ar	2, 8, 8

To draw a Bohr diagram: a nucleus labelled with protons and neutrons, then concentric circles for K, L, M with dots or crosses for electrons.

Worked example

Write the electronic configurations of magnesium (Z = 12), sulphur (Z = 16) and argon (Z = 18), and state how many valence electrons each has.

Try it first, then show the solutionSolution

1. Mg: K takes 2, L takes 8, remaining 2 go to M: 2, 8, 2 → 2 valence electrons.
2. S: 2, 8, 6 → 6 valence electrons.
3. Ar: 2, 8, 8 → 8 valence electrons (a complete octet).

Answer: Mg 2, 8, 2 (2); S 2, 8, 6 (6); Ar 2, 8, 8 (8).

Valence electrons and valency

NCERT section 8.8 Combining Capacity of an Atom: Valency

The valence shell is the outermost shell with electrons; its electrons are valence electrons.

Atoms with 8 valence electrons (an octet), or 2 for helium, are stable and largely unreactive (noble gases He, Ne, Ar). Other atoms lose, gain or share electrons to get a complete outer shell.

Valency = the number of electrons an atom loses, gains or shares to complete its octet; it is its combining capacity.

• 1 to 3 valence electrons: usually lose them; valency = number of valence electrons (Na 2, 8, 1 → 1; Mg → 2; Al → 3).
• 5 to 7 valence electrons: usually gain; valency = 8 − valence electrons (N → 3; O 2, 6 → 2; F, Cl → 1).
• 4 valence electrons: hard to lose or gain 4, so share: carbon's valency is 4.
• 8 (or 2 for He): valency 0.

Combining capacity can also be read from hydrogen or chlorine compounds, as both have combining capacity 1: in H${}_2$O oxygen joins 2 H (valency 2); in NH${}_3$ nitrogen joins 3 H (valency 3); in MgCl${}_2$ magnesium joins 2 Cl (valency 2).

Worked example

Find the valency of (i) aluminium (Z = 13) (ii) nitrogen (Z = 7) (iii) chlorine (Z = 17) (iv) neon (Z = 10), showing the configuration.

Try it first, then show the solutionSolution

1. (i) Al 2, 8, 3: loses 3 → valency 3.
2. (ii) N 2, 5: gains 3 to make 8 → valency 3.
3. (iii) Cl 2, 8, 7: gains 1 → valency 1.
4. (iv) Ne 2, 8: octet complete → valency 0.

Answer: Al 3, N 3, Cl 1, Ne 0.

Isotopes, their uses, and average atomic mass

NCERT section 8.9 A Deeper Look into Atomic Structure

Dalton thought all atoms of an element were identical. In fact, atoms of one element can have different numbers of neutrons.

Isotopes are atoms of the same element with the same atomic number but different mass numbers.

• Hydrogen: protium ${}_1^1\text{H}$ (0 n, about 99.98%), deuterium ${}_1^2\text{H}$ (1 n), tritium ${}_1^3\text{H}$ (2 n, traces).
• Carbon: ${}_6^{12}\text{C}$ (most common), ${}_6^{13}\text{C}$, ${}_6^{14}\text{C}$.
• Isotopes have the same chemical properties (same electrons and configuration) but different physical properties (mass, density, melting and boiling points).

Uses: uranium-235 as nuclear fuel for electricity; cobalt-60 in cancer radiotherapy; iodine-131 to treat goitre and thyroid cancer; carbon-14 to date fossils and ancient objects.

Average atomic mass (in unified atomic mass units, u) takes the abundance of each isotope into account:

average $=\sum (\text{mass}\times \frac{\%\text{ abundance}}{100})$

Chlorine: 75% ${}^{35}\text{Cl}$ and 25% ${}^{37}\text{Cl}$: $35\times 0.75+37\times 0.25=35.5$ u. No single chlorine atom has a mass of 35.5 u; it is the average over many atoms. The simple average (36 u) is wrong because it ignores abundance.

India's Bhabha Atomic Research Centre (BARC), founded through Homi Bhabha's vision, uses reactors like Dhruva for neutron research.

Worked example

Bromine has two isotopes: ${}_{35}^{79}\text{Br}$ (49.7%) and ${}_{35}^{81}\text{Br}$ (50.3%). Find the average atomic mass, and the number of neutrons in each isotope.

Try it first, then show the solutionSolution

1. Average $=79\times 0.497+81\times 0.503$.
2. $=39.263+40.743=80.006\approx 80.0$ u.
3. Neutrons: $79-35=44$ and $81-35=46$.

Answer: About 80.0 u; 44 and 46 neutrons.

Isobars

NCERT section 8.9 A Deeper Look into Atomic Structure

Isobars belong to different elements but share the same mass number; their atomic numbers differ.

Example: ${}_{18}^{40}\text{Ar}$, ${}_{19}^{40}\text{K}$ and ${}_{20}^{40}\text{Ca}$ all have 40 nucleons, but 18, 19 and 20 protons (and so 22, 21 and 20 neutrons).

	Isotopes	Isobars
Element	same	different
Atomic number Z	same	different
Mass number A	different	same
Chemical properties	same	different
Example	${}_{17}^{35}\text{Cl}$, ${}_{17}^{37}\text{Cl}$	${}_{18}^{40}\text{Ar}$, ${}_{20}^{40}\text{Ca}$

The journey is not over: modern microscopes (scanning tunnelling and transmission electron microscopes) can image individual atoms, and the quantum model describes electron clouds.

Worked example

Nuclei P, Q and R contain: P 20 p, 20 n; Q 19 p, 21 n; R 20 p, 22 n. State the relation between (i) P and R (ii) P and Q.

Try it first, then show the solutionSolution

1. Mass numbers: P $=40$, Q $=40$, R $=42$.
2. (i) P and R have the same Z (20) but different A (40, 42): isotopes (of calcium).
3. (ii) P and Q have the same A (40) but different Z (20, 19): isobars.

Answer: (i) Isotopes (ii) isobars.

Science in real life: Journey Inside the Atom

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

• Atomic Number, Electronic Configuration, Valency and Isotopes
• Thomson, Rutherford and Bohr Atomic Models

Journey Inside the Atom: formulas and key facts

Neutral atom

$Numberofprotons=numberofelectrons=atomicnumberZ.$

Mass number

$A=protons+neutrons;neutrons=A-Z.$

Notation

${}_Z^A\text{X}$, e.g. ${}_{11}^{23}\text{Na}$: 11 p, 12 n, 11 e.

Shell capacity

$2n^2$: K 2, L 8, M 18; outermost shell at most 8.

Valency

$Valenceelectrons1to3:valency=valenceelectrons;5to7:valency=8-valenceelectrons;4:shares4;8:zero.$

Average atomic mass

$\sum \text{isotope mass}\times \frac{\%\text{ abundance}}{100}$

Sizes

Atom about $10^{-10}$ m; nucleus about $10^{-15}$ m; atom is about $10^5$ times larger.

Charges

Electron −1 ($-1.602\times 10^{-19}$ C); proton +1; neutron 0.

Timeline

$Dalton1808\to Thomson(electron)1897\to Rutherford(nucleus)1911\to Bohr1913\to Chadwick(neutron)1932.$

Isotopes vs isobars

$Isotopes:sameZ,differentA.Isobars:sameA,differentZ.$

Journey Inside the Atom: diagrams to practise

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

How the school exam asks Journey Inside the Atom

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

Tricks to remember Journey Inside the Atom

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

• D-T-R-B

  Don't Tell Rahul Bohr: Dalton, Thomson, Rutherford, Bohr (then Chadwick adds the neutron).

  Use it when: Chronological order of models.

• Pudding, Planet, Pathway

  Thomson = plum pudding; Rutherford = planetary; Bohr = fixed pathways (shells).

  Use it when: Naming the models.

• 2-8-8 ladder

  For the first 18 elements, fill 2 then 8 then 8.

  Use it when: Electronic configurations.

• isotoPes and isobArs

  The P in isotoPes: same Protons (bottom number Z). The A in isobArs: same mAss number (top number A).

  Use it when: Telling isotopes from isobars.

• Neutrons = top − bottom

  In ${}_Z^A\text{X}$, subtract the bottom from the top to get neutrons.

  Use it when: Particle-count questions.

Journey Inside the Atom: 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 E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 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 Journey Inside the Atom

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

• ✗Mistake: Saying the gold foil experiment showed neutrons

  The nucleus is linked to neutrons in memory.

  Wrong working

  Rutherford's experiment proved neutrons exist in the nucleus.

  Right working

  It showed a small, dense, positive nucleus; neutrons were discovered by Chadwick in 1932.

  ✓Do this: Link each particle to its discoverer.

  Usually costs: 1 mark.

• ✗Mistake: Filling M before L is full or putting more than 8 in the outer shell

  Using $2n^2=18$ for M without the octet rule.

  Wrong working

  Argon (18): 2, 16.

  Right working

  Argon: 2, 8, 8.

  ✓Do this: K 2, L 8, then M; outermost shell at most 8.

  Usually costs: 1 mark per configuration.

• ✗Mistake: Using valence electrons as valency for non-metals

  Valency is learnt only for metals.

  Wrong working

  Oxygen 2, 6 has valency 6.

  Right working

  Oxygen gains 2 to complete its octet: valency $8-6=2$.

  ✓Do this: Valency is the number of electrons lost, gained or shared, whichever is fewer.

  Usually costs: 1 mark.

• ✗Mistake: Taking a simple average for atomic mass

  It looks natural.

  Wrong working

  Chlorine: $\frac{35+37}{2}=36$ u.

  Right working

  $35\times 0.75+37\times 0.25=35.5$ u.

  ✓Do this: Weight each isotope by its abundance.

  Usually costs: The numerical.

• ✗Mistake: Writing symbols with the wrong case

  Carelessness.

  Wrong working

  Cobalt CO; aluminium AL; chlorine CL.

  Right working

  Co, Al, Cl (CO is carbon monoxide).

  ✓Do this: First letter capital, second small.

  Usually costs: 1 mark in symbol questions.

NCERT exercise guide: Journey Inside the Atom

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Journey Inside the Atom

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

   The relative charge on a neutron is
   (a) +1
   (b) −1
   (c) 0
   (d) +2

   Hint

   Neutral.

   Answer

   (c) 0

2. Q2 · 1 mark · MCQExam level

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

   Hint

   $2n^2$ with n = 3.

   Answer

   (c) 18 (though as the outermost shell it holds at most 8)

3. Q3 · 1 mark · Assertion-reasonExam level

   Assertion (A): Isotopes of an element have the same chemical properties.
   Reason (R): Isotopes have the same number of electrons and the same electronic configuration.
   (a) A and R are both true, and R explains A correctly.
   (b) A and R are both true, but R does not explain A.
   (c) A is true, R is false.
   (d) A is false, R is true.

   Hint

   Chemistry depends on electrons.

   Answer

   (a)

4. Q4 · 2 marks · Very short answerExam level

   Write the symbols of sodium, potassium, iron and silver, and say why they do not match the English names.

   Hint

   Latin names.

   Answer

   Na (natrium), K (kalium), Fe (ferrum), Ag (argentum). They come from the Latin names of the elements.

5. Q5 · 2 marks · Very short answerHarder exam level

   An atom ${}^{56}\text{X}$ has 26 electrons. How many neutrons does it have? Write its full notation and name it.

   Hint

   Z = electrons.

   Answer

   Z = 26; neutrons $=56-26=30$; ${}_{26}^{56}\text{Fe}$ (iron).

6. Q6 · 2 marks · Very short answerHarder exam level

   Solve the riddle: 'My mass number is 27 and I have 14 neutrons. I am a light metal used for kitchen foil and aircraft bodies.' Who am I, what is my configuration, and what is my valency?

   Hint

   Z = A − n.

   Answer

   Z $=27-14=13$: aluminium (Al); configuration 2, 8, 3; it loses 3 electrons, so valency 3.

7. Q7 · 3 marks · Short answerHarder exam level

   In Rutherford's model atoms should collapse, but in Bohr's model they do not. Explain (i) which force keeps an electron bound to its atom (ii) why an electron in a Bohr shell does not spiral inwards (iii) what an electron must do to move to a higher shell.

   Hint

   Attraction, stationary states, energy jumps.

   Answer

   (i) The electrostatic attraction between the positive nucleus and the negative electron. (ii) Bohr proposed that in a fixed shell (stationary state) the electron does not lose energy, so it does not spiral in. (iii) It must absorb exactly the energy difference between the two shells.

8. Q8 · 3 marks · Short answerHarder exam level

   Boron has isotopes boron-10 (20%) and boron-11 (80%). Find its average atomic mass. Explain why no boron atom actually has this mass.

   Hint

   Weighted average.

   Answer

   $10\times 0.2+11\times 0.8=2+8.8=10.8$ u. Each atom is either about 10 u or 11 u; 10.8 u is the average of a large number of atoms in their natural proportions.

9. Q9 · 3 marks · Short answerHarder exam level

   Give the electronic configuration and valency of (i) carbon (ii) fluorine (iii) magnesium, and say whether each tends to lose, gain or share electrons.

   Hint

   Count to 8.

   Answer

   (i) C 2, 4: shares 4, valency 4. (ii) F 2, 7: gains 1, valency 1. (iii) Mg 2, 8, 2: loses 2, valency 2.

10. Q10 · 5 marks · Long answerHarder exam level

    Compare Thomson's, Rutherford's and Bohr's models on (i) where the positive charge is (ii) where the electrons are (iii) one limitation of each of the first two.

    Hint

    Pudding, planet, pathway.

    Answer

    Thomson: positive charge spread through a sphere, electrons embedded in it; could not explain large-angle scattering in the gold foil experiment. Rutherford: positive charge and mass in a tiny central nucleus, electrons revolving round it; could not explain why electrons do not lose energy and fall in (stability). Bohr: nucleus at centre, electrons in fixed shells K, L, M with definite energy, explaining stability.

11. Q11 · 3 marks · Short answerHOTS / competency

    Argon-40, potassium-40 and calcium-40 are isobars. Find the neutrons in each (Z = 18, 19, 20). Would you expect them to have similar chemical properties? Explain.

    Hint

    Chemistry follows electrons.

    Answer

    Neutrons: 22, 21, 20. No: they have different numbers of electrons (18, 19, 20) and configurations (2, 8, 8; 2, 8, 8, 1; 2, 8, 8, 2), so argon is inert, potassium very reactive with valency 1 and calcium reactive with valency 2.

12. Q12 · 3 marks · Short answerHOTS / competency

    Silicon has isotopes Si-28 (92%), Si-29 (5%) and Si-30 (3%). Find its average atomic mass and the number of neutrons in Si-29 (Z = 14). Why does the average lie just above 28?

    Hint

    Three terms.

    Answer

    $28\times 0.92+29\times 0.05+30\times 0.03=25.76+1.45+0.9=28.11$ u. Si-29 has $29-14=15$ neutrons. The average is just above 28 because Si-28 makes up 92% of the atoms.

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.What did Acharya Kanada call the smallest particles of matter?Show answer

   Parmanu.

2. 2.Who discovered the electron, and how?Show answer

   J. J. Thomson, in 1897, by studying cathode rays in a gas discharge tube.

3. 3.What is an alpha particle?Show answer

   A positively charged particle, the nucleus of a helium atom (2 protons, 2 neutrons).

4. 4.Why could Rutherford's model not explain stability?Show answer

   An orbiting electron accelerates, should lose energy and spiral into the nucleus.

5. 5.What did Bohr call the fixed orbits?Show answer

   Stationary states, shells or energy levels (K, L, M, N or n = 1, 2, 3, 4).

6. 6.Who discovered the neutron?Show answer

   James Chadwick, in 1932.

7. 7.Define atomic number.Show answer

   The number of protons in the nucleus of an atom, Z.

8. 8.What is the maximum number of electrons in the L shell?Show answer

   $2n^2=2\times 2^2=8$.

9. 9.Give one use each of iodine-131 and carbon-14.Show answer

   Iodine-131: treating goitre and thyroid cancer. Carbon-14: dating fossils and ancient objects.

10. 10.Define isobars with an example.Show answer

    Atoms of different elements with the same mass number, e.g. ${}_{18}^{40}\text{Ar}$ and ${}_{20}^{40}\text{Ca}$.

Quick revision checklist

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

• ☐Can you put the atomic models in order with one key idea each?
• ☐Can you list three gold foil observations and conclusions?
• ☐Can you explain why Rutherford's model was unstable and how Bohr fixed it?
• ☐Can you fill the particle table with charge, mass and location?
• ☐Can you find p, n and e from ${}_Z^A\text{X}$?
• ☐Can you write configurations for any element up to argon?
• ☐Can you draw a Bohr diagram of sodium or chlorine?
• ☐Can you work out valency from the configuration?
• ☐Can you compute an average atomic mass from abundances?
• ☐Can you tell isotopes from isobars with examples?

Journey Inside the Atom: frequently asked questions