Grade 9 INTEGRATED SCIENCE Study Notes

Welcome to Strand 1: Mixtures, Elements and Compounds

Everything around you — water, iron, air, salt — is made of matter. All matter, however different it looks, is built from invisible building blocks called atoms.

This strand has three sub-strands:

• 1.1 Structure of an atom — what atoms are made of, how their parts are arranged, and what that tells us about elements.
• 1.2 Metals and alloys — properties of metals, rusting, and how mixing metals creates alloys.
• 1.3 Water hardness — why water from different sources behaves differently, and how to treat hard water.

Structure and Meaning of an Atom (continued)

An atom is the smallest particle of an element that can take part in a chemical reaction. It is the smallest part that retains the physical and chemical properties of the element.

Characteristics of the three subatomic particles

In a neutral atom, the number of protons equals the number of electrons — so positive and negative charges cancel out exactly.

Structure and Meaning of an Atom

All matter contains atoms. Atoms are made up of smaller particles called subatomic particles.

Every atom has exactly three subatomic particles:

• Protons — found in the nucleus (centre of the atom). Carry a positive charge (+).
• Neutrons — also in the nucleus. Carry no charge.
• Electrons — outside the nucleus, revolving around it in regions called energy levels. Carry a negative charge (−).

The nucleus is the dense centre of the atom — it holds the protons and neutrons. Electrons move in the space surrounding it.

Atomic Number and Mass Number of Elements

The atomic number of an atom is the total number of protons in its nucleus. It is represented by the letter Z.

All atoms of the same element have the same atomic number. The atomic number is the number used to place each element in its position on the periodic table.

Worked examples

• Carbon has 6 protons → Z = 6
• Helium has 2 protons → Z = 2

Atomic numbers of elements 1–20

Atomic Number and Mass Number of Elements (continued)

The mass number of an atom is the sum of protons and neutrons in its nucleus. It is represented by the letter A.

A = protons + neutrons

Electrons are not counted — their mass is negligibly small.

Worked examples

1. Carbon: 6 protons + 6 neutrons → A = 12
2. Helium: 2 protons + 2 neutrons → A = 4

Finding neutrons

Neutrons = A − Z

Example — Sodium (A = 23, Z = 11): Neutrons = 23 − 11 = 12

Mass numbers of elements 1–20

Atomic Number and Mass Number of Elements (continued)

The atomic number and mass number are written together with the chemical symbol in a standard way:

• Mass number (A) → superscript, upper-left of symbol
• Atomic number (Z) → subscript, lower-left of symbol

Writing notation — worked steps

Example 1 — Carbon (Z = 6, A = 12):

1. Write the symbol: C
2. Place mass number upper-left: ¹²C
3. Place atomic number lower-left: ¹²₆C ✓

Example 2 — Sodium (Z = 11, A = 23):

1. Write the symbol: Na
2. Place mass number upper-left: ²³Na
3. Place atomic number lower-left: ²³₁₁Na ✓

Interpreting notation — worked steps

Read ²³₁₁Na:

1. Lower-left (subscript) = Z = 11 protons
2. Upper-left (superscript) = A = 23
3. Neutrons = 23 − 11 = 12
4. Electrons = protons = 11

Notation for elements 1–20

Electron Arrangement of Elements

Electrons do not move randomly around the nucleus. They occupy specific regions called energy levels (shells), each at a fixed distance from the nucleus.

Each energy level holds a specific maximum number of electrons. When one energy level is filled up, the extra electrons occupy the next energy level until it is filled up.

Electron Arrangement of Elements (continued)

Electron arrangement (electron configuration) is the distribution of electrons within the energy levels. It is written as numbers separated by dots — each number shows the electrons in that level, from innermost outward.

Writing electron arrangement — worked steps

Carbon (6 electrons):

1. 1st level: 2 (maximum — full)
2. 2nd level: 6 − 2 = 4
3. Written: C: 2.4

Sodium (11 electrons):

1. 1st level: 2
2. 2nd level: 8 (maximum — full)
3. 3rd level: 11 − 10 = 1
4. Written: Na: 2.8.1

Drawing an electron arrangement diagram

1. Draw a small circle at the centre — the nucleus. Label it with the element symbol.
2. Draw the 1st ring close around the nucleus. Place the correct number of dots.
3. Draw the 2nd ring further out. Place the correct number of dots.
4. Continue for as many rings as needed.

Arrangements: Hydrogen, Helium, Lithium

Electron Arrangement of Elements (continued)

As electrons are added one by one from beryllium to neon, the 2nd energy level fills up progressively until it reaches its maximum of 8 at neon.

Use the table above to draw electron arrangement diagrams for each element in your exercise book — one ring per energy level, dots on each ring matching the electrons in that level.

Electron Arrangement of Elements (continued)

Drawing an energy level diagram — steps

1. Draw a small circle at the centre — the nucleus. Label it with the element symbol.
2. Draw the 1st ring close around the nucleus. Place the correct number of dots (max 2).
3. Draw the 2nd ring further out. Place the correct number of dots (max 8).
4. Draw the 3rd ring if needed. Place remaining dots (max 8).

Reading diagrams — worked table

When the 2nd ring is full (8 electrons), a 3rd ring opens.

Electron Arrangement of Elements (continued)

Once the 3rd level is full (8 electrons), a 4th level opens. The same filling rule applies.

Worked examples

Chlorine (Z = 17):

1. 1st level: 2 → total used: 2
2. 2nd level: 8 → total used: 10
3. 3rd level: 17 − 10 = 7
4. Written: Cl: 2.8.7

Potassium (Z = 19):

1. 1st level: 2 → total: 2
2. 2nd level: 8 → total: 10
3. 3rd level: 8 (max — full) → total: 18
4. 4th level: 19 − 18 = 1
5. Written: K: 2.8.8.1

Complete reference: Hydrogen to Calcium

Modelling the Structure of Elements

An atom is far too small to see. A physical model makes its structure visible — nucleus at the centre, energy level rings around it, electrons on each ring.

Materials

Your model must show

1. Name of the element
2. Number of protons, neutrons, and electrons
3. Atomic number, mass number, and chemical symbol
4. Electrons placed correctly on each ring (right number per ring)

After showing your model to classmates and teacher and receiving feedback, make any necessary improvements before displaying it.