Grade 9 Integrated Science Notes: Structure of the Atom (with Practice Questions)

Grade 9 Integrated Science · Strand 1.0 Mixtures, Elements and Compounds · Sub-strand 1.1 Structure of the Atom

These are free study notes for Grade 9 learners, written to the KICD curriculum design and taken from our full Grade 9 Integrated Science notes. This topic is the foundation for the rest of the strand and appears often in KJSEA papers, usually as a table to complete or an electron arrangement to draw.

Everything around us, from the air we breathe to the iron sheets on our roofs, is made of tiny particles called atoms. In this topic we study what an atom is made of, how to find its atomic number and mass number, how electrons are arranged, how to draw atoms with dot and cross diagrams, and how electron arrangement tells us whether an element is a metal or a non-metal.

Atoms and elements

Particles in an atom

In a neutral atom the number of electrons equals the number of protons, so the charges cancel.

Atomic number and mass number

Number of neutrons = mass number - atomic number. In notation, the mass number is written at the top left of the symbol and the atomic number at the bottom left.

Electron arrangement

For the first 20 elements, the first energy level holds up to 2 electrons, the second up to 8, the third up to 8, and any more go into the fourth.

Drawing a dot and cross diagram (sodium)

1. Draw the nucleus and write 11p and 12n inside it.
2. Draw three circles (energy levels) around it.
3. Put 2 crosses on the first, 8 on the second and 1 on the third.
4. Arrange the electrons in pairs so they are easy to count.

Metals and non-metals

Metals around us: aluminium sufurias, copper wires, iron nails and sheets. Non-metals: oxygen we breathe, carbon in charcoal and pencils, chlorine in water treatment. Knowing atomic structure helps scientists make medicines, fertilisers, batteries and electronics.

Test yourself: multiple choice

1. Which particle has no charge?

   1. Proton
   2. Neutron
   3. Electron
   4. Nucleus
   Show the answer

   B: Neutrons are neutral.

2. An atom has mass number 27 and atomic number 13. How many neutrons does it have?

   1. 14
   2. 13
   3. 27
   4. 40
   Show the answer

   A: 27 - 13 = 14.

3. What is the electron arrangement of chlorine (atomic number 17)?

   1. 2.8.8
   2. 2.7.8
   3. 2.8.7
   4. 8.8.1
   Show the answer

   C: 2 + 8 + 7 = 17.

4. An element has the arrangement 2.8.2. It is most likely

   1. a gas.
   2. a non-metal.
   3. a metalloid.
   4. a metal.
   Show the answer

   D: It has 2 outer electrons: magnesium, a metal.

5. The atomic number of an element is the number of

   1. neutrons.
   2. protons.
   3. shells.
   4. protons plus neutrons.
   Show the answer

   B: Atomic number = number of protons.

Practice questions

1. Copy and complete: an atom of aluminium has atomic number 13 and mass number 27. Protons = ..., neutrons = ..., electrons = ..., arrangement = ... (4 marks)

   Show the answer

   Protons 13, neutrons 14, electrons 13, arrangement 2.8.3.

2. Draw a dot and cross diagram of an oxygen atom (8 protons, 8 neutrons). (3 marks)

   Show the answer

   Nucleus labelled 8p, 8n; 2 electrons on the first shell and 6 on the second.

3. Classify as metal or non-metal and give a reason: (a) potassium 2.8.8.1 (b) sulphur 2.8.6. (4 marks)

   Show the answer

   (a) Metal: 1 outer electron. (b) Non-metal: 6 outer electrons.

4. Explain why a neutral atom has no overall charge. (2 marks)

   Show the answer

   It has equal numbers of protons (+) and electrons (-), so the charges cancel.