Last reviewed: 7 November 2026
Updated November 2026. These Grade 9 structure of the atom notes explain protons, neutrons and electrons, atomic number and mass number, electron arrangement for the first 20 elements, dot and cross diagrams, and how electron arrangement helps classify elements as metals or non-metals. In short: the atomic number is the number of protons; mass number = protons + neutrons; electrons fill energy levels in the pattern 2, 8, 8 for the first 20 elements. Sodium (atomic number 11) has the arrangement 2.8.1.
The examples and questions are original practice for Kenyan junior school learners, not an official paper. For other Grade 9 science topics, see the Grade 9 Integrated Science revision guide and the Grade 9 notes hub.
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What the Grade 9 Curriculum Covers
Structure of the atom is the first sub-strand of Mixtures, Elements and Compounds in the Kenya Institute of Curriculum Development’s Grade 9 Integrated Science design. It covers protons, electrons and neutrons, atomic and mass numbers, electron arrangement, dot or cross diagrams, and metals and non-metals, using only the first 20 elements. The design notes that the periodic table itself is not introduced at this level. It is listed on the KICD Grade Nine curriculum designs page.
Particles in an Atom
An atom has a tiny central nucleus containing protons and neutrons, with electrons moving around it in energy levels (shells). This model is described in OpenStax Chemistry’s section on atomic structure and symbolism.
| Particle | Where | Charge | Relative mass |
|---|---|---|---|
| Proton | Nucleus | +1 | 1 |
| Neutron | Nucleus | 0 | 1 |
| Electron | Energy levels | −1 | about 1/1840 |
In a neutral atom, the number of electrons equals the number of protons, so the charges cancel.
Atomic Number and Mass Number
- Atomic number = number of protons. It identifies the element: every atom with 6 protons is carbon.
- Mass number = protons + neutrons.
- Number of neutrons = mass number − atomic number.
Worked examples
- Sodium: atomic number 11, mass number 23. Neutrons = 23 − 11 = 12.
- Aluminium: atomic number 13, mass number 27. Neutrons = 14.
- Chlorine-35: atomic number 17. Neutrons = 35 − 17 = 18.
Electron Arrangement of the First 20 Elements
At this level, the first energy level holds up to 2 electrons, the second up to 8 and the third up to 8, with the next electrons going into the fourth level (this applies to potassium and calcium). The simplified “2, 8, 8” rule works for the first 20 elements; OpenStax Chemistry’s section on electron configurations explains the fuller picture you will meet later.
| Element | Atomic number | Arrangement |
|---|---|---|
| Hydrogen | 1 | 1 |
| Carbon | 6 | 2.4 |
| Oxygen | 8 | 2.6 |
| Neon | 10 | 2.8 |
| Sodium | 11 | 2.8.1 |
| Magnesium | 12 | 2.8.2 |
| Chlorine | 17 | 2.8.7 |
| Argon | 18 | 2.8.8 |
| Potassium | 19 | 2.8.8.1 |
| Calcium | 20 | 2.8.8.2 |
Dot and Cross Diagrams
Draw the nucleus as a small circle labelled with the symbol (or the numbers of protons and neutrons). Draw each energy level as a larger circle around it, and place dots (or crosses) on the circles for the electrons. For magnesium (2.8.2): 2 dots on the first circle, 8 on the second, 2 on the third. Pair electrons neatly so they are easy to count. A model made from locally available materials, such as bottle tops for particles and wire rings for energy levels, is the project the curriculum suggests.
Classifying Metals and Non-Metals
The electrons in the outermost energy level give a strong clue:
- Atoms of metals in the first 20 elements usually have 1, 2 or 3 outer electrons (sodium 2.8.1, magnesium 2.8.2, aluminium 2.8.3).
- Atoms of non-metals usually have 4 to 7 outer electrons (carbon 2.4, oxygen 2.6, chlorine 2.8.7).
- Helium, neon and argon have full outer levels and are unreactive non-metals (noble gases).
Exceptions: hydrogen (1 electron), helium (2) and boron (2.3) are non-metals despite having few outer electrons. Treat the rule as a guide, not a law.
Original Practice Questions With Worked Answers
- An atom has 15 protons and 16 neutrons. Find its atomic number and mass number. 15 and 31.
- Write the electron arrangement of sulphur (atomic number 16). 2.8.6.
- How many neutrons are in potassium-39 (atomic number 19)? 39 − 19 = 20.
- Is the element with arrangement 2.8.2 a metal or a non-metal? A metal (magnesium), with 2 outer electrons.
- Why is an atom neutral? It has equal numbers of positive protons and negative electrons.
Cover the answers and attempt each question first. For timed practice, use the Grade 9 revision papers or the Grade 9 CBC Challenge. These notes are free; full revision access is available with the KSh 20 Day Pass or the KSh 500 Monthly Pass.
Frequently Asked Questions
What is the difference between atomic number and mass number?
Atomic number is the number of protons. Mass number is the number of protons plus neutrons.
How do you find the number of neutrons in an atom?
Subtract the atomic number from the mass number.
How many electrons go in each energy level for the first 20 elements?
Up to 2 in the first level, 8 in the second and 8 in the third, with any further electrons in the fourth level.
How does electron arrangement show whether an element is a metal?
Metals among the first 20 elements usually have 1 to 3 outer electrons, while non-metals usually have 4 to 8, with hydrogen, helium and boron as exceptions.