S1 Chemistry Practice Book · Unit 5: Atoms, elements and compounds

Unit 5 Atoms, elements and compounds

Key unit competence: Analyse the structure, properties and interactions of atoms, elements and compounds to explain matter formation and behaviour.

Key ideas

An atom is the smallest particle of an element that can take part in a chemical reaction. An element is made of only one kind of atom. A molecule is two or more atoms chemically joined. A compound is made of two or more different elements chemically combined in a fixed ratio.
An atom has a nucleus containing protons (charge +1, mass 1) and neutrons (charge 0, mass 1), surrounded by electrons (charge −1, mass about 1/1840) in shells.
Atomic number (Z) = number of protons. Mass number (A) = protons + neutrons. Neutrons = A − Z. In a neutral atom, electrons = protons. Isotopes are atoms of the same element with different numbers of neutrons, e.g. ¹²₆C and ¹⁴₆C.
Electrons fill shells in order: 2, 8, 8, 2 for the first 20 elements. Example: sodium (Z = 11) is 2,8,1; chlorine (Z = 17) is 2,8,7; calcium (Z = 20) is 2,8,8,2.
Atoms lose or gain electrons to get a full outer shell. Metals lose electrons to form positive ions (Na⁺, Mg²⁺); non-metals gain electrons to form negative ions (Cl⁻, O²⁻).
Radicals are groups of atoms that act as one unit: OH⁻ hydroxide, NO₃⁻ nitrate, SO₄²⁻ sulfate, CO₃²⁻ carbonate, NH₄⁺ ammonium. Formulae are written by balancing charges: CaCl₂, Al₂O₃, Na₂CO₃, (NH₄)₂SO₄.

Common misconceptions

"The mass number is the number of particles around the nucleus." No: Mass number counts protons and neutrons in the nucleus.
"Isotopes are different elements." No: Isotopes are the same element (same protons) with different neutrons.
"A molecule is always a compound." No: O₂ and N₂ are molecules of elements, not compounds.

Worked examples

Example 1. Find the number of protons, neutrons and electrons in ⁴⁰₂₀Ca and write its electronic configuration.

Solution

Protons = 20 (atomic number).

Neutrons = 40 − 20 = 20.

Electrons = 20 (neutral atom).

Configuration: 2,8,8,2.

Example 2. Write the formula of magnesium nitrate.

Solution

Ions: Mg²⁺ and NO₃⁻.

Two nitrate ions are needed to balance +2.

Formula: Mg(NO₃)₂.

Practice questions

Define an element.
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A substance made of only one kind of atom that cannot be split into simpler substances by chemical means.
Common mistake: Saying 'a substance with one molecule'.
Give the charge and relative mass of a neutron.
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Charge 0; relative mass 1.
Common mistake: Giving the charge as +1.
Write the symbols of iron, copper and lead.
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Fe, Cu, Pb.
Common mistake: Writing Ir, Co, Le.
What is meant by isotopes?
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Atoms of the same element with the same number of protons but different numbers of neutrons.
Common mistake: Saying 'different elements with the same mass'.
Write the electronic configuration of aluminium (Z = 13).
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2,8,3.
Common mistake: Writing 2,11 or 8,5.
How many neutrons are in ⁵⁶₂₆Fe?
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56 − 26 = 30.
Common mistake: Adding 56 + 26.
Explain why a sodium atom forms an Na⁺ ion.
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Sodium (2,8,1) loses one outer electron to get the stable configuration 2,8.
Common mistake: Saying it gains a proton.
Write the formula of potassium sulfate.
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K₂SO₄.
Common mistake: Writing KSO₄.
Name: Na₂CO₃, CuO, NH₄NO₃.
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Sodium carbonate; copper(II) oxide; ammonium nitrate.
Common mistake: Calling NH₄NO₃ 'ammonia nitrate'.
Copper has isotopes Cu-63 (69%) and Cu-65 (31%). Calculate its relative atomic mass.
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(63 × 69 + 65 × 31) ÷ 100 = (4347 + 2015) ÷ 100 = 63.62 ≈ 63.6.
Common mistake: Taking the simple average (64).

Matching and drag-and-drop

Matching

Match each item in column A with the correct answer in column B. Write the letter next to each number.

Column A	Column B
1. The formula of the sulfate radical is:	a. In the nucleus
2. What is the electronic configuration of oxygen (Z = 8)?	b. SO₄²⁻
3. Where are protons and neutrons found?	c. 2,6
4. An ion has 12 protons, 12 neutrons and 10 electrons. It is:	d. Mg²⁺

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1 → b (SO₄²⁻) · 2 → c (2,6) · 3 → a (In the nucleus) · 4 → d (Mg²⁺)

Sulfate is SO₄²⁻; SO₃²⁻ is sulfite.
The first shell holds 2, the second holds the remaining 6.
The nucleus contains protons and neutrons.
12 protons is magnesium; 2 fewer electrons gives a 2+ charge.

Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
(NH₄)₂SO₄ · Fe₂(SO₄)₃ · Fe₃(SO₄)₂ · protons
1. Isotopes of an element have the same number of .
2. The formula of iron(III) sulfate is .
3. The formula of ammonium sulfate is .
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1: protons · 2: Fe₂(SO₄)₃ · 3: (NH₄)₂SO₄
- They differ only in their number of neutrons.
- Step 1. Fe³⁺ and SO₄²⁻.
  Step 2. 2 × (+3) = 3 × (−2), so Fe₂(SO₄)₃.
- Two NH₄⁺ balance one SO₄²⁻; brackets show two ammonium groups.

Matching

Match each item in column A with the correct answer in column B. Write the letter next to each number.

Column A	Column B
1. How many neutrons are in ²⁷₁₃Al?	a. gaining one electron to form Cl⁻
2. Chlorine (2,8,7) forms an ion by:	b. CaCl₂
3. The correct formula of calcium chloride is:	c. 14
4. The formula of aluminium oxide is:	d. Al₂O₃

Show answer

1 → c (14) · 2 → a (gaining one electron to form Cl⁻) · 3 → b (CaCl₂) · 4 → d (Al₂O₃)

Neutrons = 27 − 13 = 14.
Gaining one electron completes the outer shell of 8.
Ca²⁺ needs two Cl⁻ ions to balance its charge.
Two Al³⁺ (+6) balance three O²⁻ (−6).

Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
23 · sodium oxide · sodium hydroxide · protons
1. An atom has 11 protons and 12 neutrons. Its mass number is .
2. The name of NaOH is .
3. The atomic number of an element is the number of .
Show answer
1: 23 · 2: sodium hydroxide · 3: protons
- Mass number = 11 + 12 = 23.
- OH⁻ is hydroxide.
- Each element has its own number of protons.

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