What is an Atom?
Everything around you — your desk, the air you breathe, the water you drink — is made of atoms. Atoms are the fundamental building blocks of all matter. They are incredibly tiny: a single human hair is about one million atoms wide!
The smallest unit of an element that retains the chemical properties of that element. Atoms cannot be broken down into simpler substances by ordinary chemical means.
For a long time, scientists thought atoms were solid, indivisible spheres (like tiny billiard balls). We now know this is not the case — atoms have internal structure, made up of even smaller particles called subatomic particles.
Think of an atom like a tiny solar system. The nucleus sits at the centre like the Sun, and electrons move around it like planets orbiting. Of course, this is a simplified model, but it helps us visualise something we cannot directly see!
Understanding atomic structure is the foundation for everything in chemistry — from why elements behave the way they do, to how chemical bonds form, to why some materials conduct electricity.
Subatomic Particles
Atoms are made up of three types of subatomic particles: protons, neutrons, and electrons. Each has a specific location within the atom and a specific charge and mass.
A positively charged subatomic particle found in the nucleus of an atom. It has a relative charge of +1 and a relative mass of 1.
A neutral (no charge) subatomic particle found in the nucleus of an atom. It has a relative charge of 0 and a relative mass of 1.
A negatively charged subatomic particle that orbits the nucleus in shells (energy levels). It has a relative charge of −1 and a relative mass of approximately 0 (1/1836 of a proton's mass).
Here is a summary table:
| Particle | Location | Relative Charge | Relative Mass |
|---|---|---|---|
| Proton | Nucleus | +1 | 1 |
| Neutron | Nucleus | 0 | 1 |
| Electron | Shells | −1 | ≈ 0 |
Because electrons have such a tiny mass compared to protons and neutrons, the mass of an atom is essentially concentrated in its nucleus. The electrons contribute almost nothing to the total mass.
In a neutral atom, the number of protons always equals the number of electrons, so the positive and negative charges cancel out perfectly.
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