Unit notes and practice questions

Grade 9 Chemistry unit 3: Structure of the atom

Everything here is for unit 3 of the Grade 9 Chemistry textbook: what the chapter covers, the short notes, practice questions with their answers, and the lesson video where one exists.

5
Cards: 10
Practice questions on unit 3: 53–110
The textbook pages for unit 3

What unit 3 covers

1.Historical Development of the Atomic
2.Fundamental Laws of Chemical Reactions
3.Atomic Theory
4.Discoveries of Fundamental Subatomic
5.Composition of an Atom and the Isotopes

Short notes for unit 3

The cards filed under this chapter on the Grade 9 Chemistry page. Open one at a time.

Discovery of Fundamental Subatomic ParticlesReference tableCathode rays (electrons) · Anode rays (protons)

Ray / Particle	Discovered by	Key Properties
Cathode rays (electrons)	J. J. Thomson (1897)	Travel in straight lines, turn paddle wheels, deflect toward positive electric plates, independent of gas type.
Anode rays (protons)	Eugene Goldstein (1886)	Travel through perforated cathodes, deflect toward negative plates, properties and e/m ratio depend on gas.

When you use it

Use when reviewing the historical experiments and properties of cathode and anode rays.

Watch out

Cathode ray properties remain identical regardless of the gas used in the tube, whereas anode ray properties and their charge-to-mass ratio depend directly on the gas used.

Drafted from Grade 9 Chemistry, pages 53-110, then checked twice before it went up

Fundamental Laws of Chemical ReactionsReference tableLaw of Conservation of Mass · Law of Definite Proportions · Law of Multiple Proportions

Law	Scientist	Key Principle
Law of Conservation of Mass	Antoine Lavoisier (1789)	Mass is neither created nor destroyed during a chemical reaction.
Law of Definite Proportions	Joseph Louis Proust (1799)	A compound always contains the same elements in fixed proportions by mass.
Law of Multiple Proportions	John Dalton	Masses of one element combining with a fixed mass of another are in small whole-number ratios.

When you use it

Use when identifying or comparing the experimental laws that govern chemical combinations.

Watch out

The Law of Definite Proportions applies to one compound, while the Law of Multiple Proportions compares two or more different compounds formed from the same elements.

Drafted from Grade 9 Chemistry, pages 53-110, then checked twice before it went up

Number of Moles CalculationFormula

n = \frac{m}{M}

n = \frac{m}{M}

$n$: Number of moles $\text{mol}$
$m$: Given mass $\text{g}$
$M$: Molecular mass $\text{g}\,\text{mol}^{-1}$

When you use it

Use to calculate the amount of substance in moles from a known mass and molecular mass.

Watch out

Ensure the given mass is expressed in grams so that the unit cancels properly with molecular mass in grams per mole.

Drafted from Grade 9 Chemistry, pages 53-110, then checked twice before it went up

Average Atomic MassFormula

m_{\text{avg}} = \frac{p_1 m_1 + p_2 m_2}{100}

m_{\text{avg}} = \frac{p_1 m_1 + p_2 m_2}{100}

$m_{\text{avg}}$: Average Atomic Mass of an element $\text{amu}$
$p_1$: Percentage abundance of the first isotope $\%$
$m_1$: Atomic mass of the first isotope $\text{amu}$
$p_2$: Percentage abundance of the second isotope $\%$
$m_2$: Atomic mass of the second isotope $\text{amu}$

When you use it

Use to calculate the average atomic mass of an element from the isotopic masses and their natural abundances.

Watch out

Divide the total sum by 100 if using percentage values directly.

Drafted from Grade 9 Chemistry, pages 111-142, then checked twice before it went up

Mass NumberFormula

A = Z + N

A = Z + N

$A$: Mass number, the total number of protons and neutrons $1$
$Z$: Atomic number, the number of protons in the nucleus $1$
$N$: Number of neutrons in the nucleus $1$

When you use it

Use when finding the mass number, atomic number, or number of neutrons in an atom.

Watch out

Do not count electrons in the mass number. Electrons contribute negligibly to total atomic mass.

Drafted from Grade 9 Chemistry, pages 111-142, then checked twice before it went up

Practice questions on unit 3

10 multiple-choice questions written against this chapter, each with its answer and the reason. Try one before you open the answer.

Question 1

The word atom comes from the ancient Greek word atomos, which means:

A.Uncuttable or indivisible
B.Positively charged
C.Liquid substance
D.Continuous matter

Show the answer and why

Answer: A Uncuttable or indivisible

Why

In Greek, the prefix a- means 'not' and tomos means 'cut'. Therefore, atomos means uncuttable or indivisible.

Question 2

According to the ancient Greek philosopher Empedocles, all materials are made up of which four elements?

A.Earth, air, water, and fire
B.Gold, silver, iron, and copper
C.Wood, stone, salt, and clay
D.Carbon, hydrogen, oxygen, and nitrogen

Show the answer and why

Answer: A Earth, air, water, and fire

Why

Empedocles proposed that all materials in the universe are formed from four basic elements: earth, air, water, and fire.

Question 3

Which scientist discovered the electron in 1897 using a cathode ray tube?

A.J. J. Thomson
B.John Dalton
C.James Chadwick
D.Ernest Rutherford

Show the answer and why

Answer: A J. J. Thomson

Why

The British physicist J. J. Thomson discovered the electron in 1897 and determined its charge-to-mass ratio using cathode ray experiments.

Question 4

Who discovered the Law of Conservation of Mass in 1789?

A.Antoine Lavoisier
B.Joseph Louis Proust
C.John Dalton
D.Niels Bohr

Show the answer and why

Answer: A Antoine Lavoisier

Why

Antoine Lavoisier discovered the Law of Conservation of Mass in 1789 by showing that mass is neither created nor destroyed during chemical reactions.

Question 5

In a laboratory in Hawassa, a chemistry student reacts 224.4 g of potassium hydroxide (KOH) with 88.0 g of carbon dioxide (CO₂). If 36.0 g of water is formed, what mass of potassium carbonate (K₂CO₃) is produced?

A.276.4 g
B.312.4 g
C.136.4 g
D.348.4 g

Show the answer and why

Answer: A 276.4 g

Why

By the Law of Conservation of Mass, total mass of reactants equals total mass of products. Total reactants = 224.4 g + 88.0 g = 312.4 g. Mass of K₂CO₃ = 312.4 g - 36.0 g = 276.4 g.

Question 6

What modification did Eugen Goldstein make to the cathode ray tube in 1886 to discover canal rays (anode rays)?

A.He used a perforated cathode
B.He inserted a rotating paddle wheel
C.He removed all electric electrodes
D.He replaced the glass tube with a metal box

Show the answer and why

Answer: A He used a perforated cathode

Why

Goldstein used a cathode with small holes (a perforated cathode), which allowed positively charged rays to pass through and produce a glow behind the cathode.

Question 7

When J. J. Thomson placed a light paddle wheel between the cathode and the anode, the paddle wheel rotated. What did this show about cathode rays?

A.They consist of material particles having mass and kinetic energy
B.They are composed purely of light waves with no mass
C.They carry a positive electrical charge
D.They can only travel in circular paths

Show the answer and why

Answer: A They consist of material particles having mass and kinetic energy

Why

The rotation of the paddle wheel showed that cathode rays have particulate nature, meaning they are material particles that carry mass and kinetic energy.

Question 8

Dalton believed that atoms of different elements always have different masses. Which pair of elements disproves this by having the same mass number of 40 amu (isobars)?

A.Argon and calcium
B.Carbon and oxygen
C.Hydrogen and helium
D.Sodium and chlorine

Show the answer and why

Answer: A Argon and calcium

Why

Argon and calcium are isobars because both have an atomic mass of 40 amu, proving that atoms of different elements can sometimes have the same mass.

Question 9

Nitrogen forms different compounds with oxygen. In one compound, 2.62 g of nitrogen combines with 1.50 g of oxygen. In a second compound, 0.656 g of nitrogen combines with 1.50 g of oxygen. What is the small whole-number ratio of nitrogen masses combining with the fixed mass of oxygen?

A.4:1
B.2:1
C.3:1
D.1:2

Show the answer and why

Answer: A 4:1

Why

Divide the larger mass of nitrogen by the smaller mass: 2.62 / 0.656 = 4. This gives a small whole-number ratio of 4:1, demonstrating the Law of Multiple Proportions.

Question 10

According to the historical timeline of atomic discoveries in the textbook, which subatomic particle was discovered by James Chadwick in 1932?

A.Neutron
B.Electron
C.Proton
D.Alpha particle

Show the answer and why

Answer: A Neutron

Why

In 1932, James Chadwick discovered the neutron, which is the neutral subatomic particle in the atomic nucleus.

The app holds 12 questions on this unit, marked as you go. These are the first 10.

The textbook pages for unit 3

Unit 3 runs from page 53 to page 110 of the Ministry of Education edition.

Ministry of Education edition, English, 186 pages

Download the PDF

All Grade 9 Chemistry short notes

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