An atom has a central nucleus containing protons and neutrons, surrounded by electrons moving in shells. The proton has relative mass 1 and charge +1, the neutron relative mass 1 and no charge, and the electron negligible mass and charge -1.
In a neutral atom the number of protons equals the number of electrons.
This page covers a single Form 4 content standard: atomic structure and subatomic particles. It turns the historical models into the working picture of the atom you will use for the rest of the syllabus, for isotopes, electron arrangement, bonding and the Periodic Table. Get the three particles and their properties fixed here and everything downstream in SPM Chemistry becomes easier.
The structure of the atom
An atom is made of a very small central nucleus surrounded by electrons. The nucleus contains two kinds of particle, protons and neutrons (together called nucleons), and it holds almost all of the atom’s mass. The electrons move around the nucleus in regions called shells (energy levels). Because the electrons take up the space around the nucleus while the nucleus itself is tiny, most of the atom is actually empty space.
The three subatomic particles
You must know each particle’s location, relative mass and relative charge. Because the actual masses and charges are extremely small, chemists compare them using relative values:
- Proton, found in the nucleus; relative mass 1; relative charge +1.
- Neutron, found in the nucleus; relative mass 1; relative charge 0 (no charge).
- Electron, found in the shells around the nucleus; relative mass approximately 1/1840 (so small it is usually taken as negligible); relative charge −1.
Key idea. Almost all of an atom’s mass is in the nucleus, because protons and neutrons each have a relative mass of 1 while an electron’s mass is negligible. But the atom’s volume is mostly the space occupied by the electrons.
Why an atom is electrically neutral
In a neutral atom, the number of protons is exactly equal to the number of electrons. Since each proton carries a charge of +1 and each electron a charge of −1, the positive and negative charges cancel out, so the atom has no overall charge. This single rule, protons equal electrons in a neutral atom, is used constantly, so make it automatic. (When an atom gains or loses electrons it becomes a charged ion, but that comes later in the bonding chapter.)
Worked example
Question. An atom of aluminium has 13 protons and 14 neutrons. State (a) the number of electrons in the neutral atom, (b) where each type of particle is located, and (c) which particles make up almost all of the atom’s mass, with a reason.
Step 1, Find the number of electrons. In a neutral atom, number of electrons = number of protons = 13 electrons.
Step 2, State the locations. The 13 protons and 14 neutrons are in the nucleus; the 13 electrons move in shells around the nucleus.
Step 3, Identify where the mass is. Almost all the mass is from the protons and neutrons in the nucleus, because each has a relative mass of 1, while the 13 electrons have negligible mass (about 1/1840 each).
Answer. (a) 13 electrons. (b) Protons and neutrons in the nucleus; electrons in the shells. (c) The protons and neutrons (the nucleons) account for almost all the mass, because each has relative mass 1 whereas the electron’s mass is negligible.
Practice question
Question. Complete the following comparison of the three subatomic particles by giving the relative mass and relative charge of each: proton, neutron, electron. Then state one reason why an atom carries no overall electric charge.
Answer. Proton: relative mass 1, relative charge +1. Neutron: relative mass 1, relative charge 0. Electron: relative mass 1/1840 (negligible), relative charge −1. An atom carries no overall charge because it contains an equal number of protons and electrons, so the total positive charge exactly cancels the total negative charge.
Exam tip
Draw yourself a small three-row table, particle, relative mass, relative charge, and reproduce it from memory before you start a question; almost every question on this standard is answered directly from it. Two ideas are worth stressing because they are marked precisely: the electron’s mass is negligible (never write “0” as its relative mass unless the question asks you to round, say “negligible” or “1/1840”), and a neutral atom always has equal numbers of protons and electrons. If a question mentions an ion or a charge, remember it is the electrons that are gained or lost, the number of protons in the nucleus never changes.
Why we use “relative” values
The actual mass of a proton is extremely small, and its actual charge is a tiny fraction of a coulomb, numbers that are awkward to work with in every calculation. Chemists therefore set the mass of a proton as 1 unit and its charge as +1 unit, then describe the other particles relative to it. This is why the neutron, which has almost the same real mass as a proton, is given a relative mass of 1, and why the electron, far lighter, is given a negligible relative mass. The same idea returns later when you meet relative atomic mass, so getting comfortable with “relative” thinking now pays off across the whole subject.
Representing an atom in a diagram
You may be asked to draw or interpret a simple atomic-structure diagram. Show the nucleus as a small circle at the centre, write the numbers of protons and neutrons inside it, and draw the electrons as dots or crosses on curved shells around the nucleus, filling the innermost shell first. A neat diagram makes it obvious that the protons and neutrons are packed in the tiny nucleus while the electrons occupy the much larger space around it, a picture that reinforces both the “mass in the nucleus” and the “mostly empty space” ideas at once. Always label the particles clearly, because a mark is often given specifically for correct labelling.
Where this fits
This standard is the heart of the Matter and the Atomic Structure chapter and leads directly into proton number, nucleon number and isotopes. Revise it with the revision notes, and check your understanding with the practice questions. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers make sure you can build the particle table from memory and apply the “protons equal electrons” rule without hesitating, because these are the foundations for isotopes and bonding.
Quick recap
- An atom = a central nucleus (protons + neutrons) surrounded by electrons in shells.
- Proton: relative mass 1, charge +1. Neutron: relative mass 1, charge 0. Electron: negligible mass, charge −1.
- Almost all the mass is in the nucleus; most of the volume is empty space around it.
- A neutral atom has equal numbers of protons and electrons.
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