An atom is the smallest particle of an element that still has the chemical properties of that element; it has a positively charged nucleus of protons and neutrons surrounded by electrons in shells.
An atom is the smallest particle of an element that still has the properties of that element. This is the definition the marking scheme expects, so learn the exact phrase. Malay: atom · Chinese: 原子.
Every atom has the same basic structure: a small, dense, positively charged nucleus at the centre, made of protons and neutrons, surrounded by electrons that move in shells (energy levels). Almost all of the mass sits in the nucleus, because a proton and a neutron each have a relative mass of 1 while an electron is so light its mass is usually ignored.
Example. A single atom of copper, written Cu, is the smallest amount of copper that is still copper. What decides that it is a copper atom, and not iron or gold, is the number of protons in its nucleus, its proton number. Change that number and you have a different element altogether. Every box in the Periodic Table therefore stands for one kind of atom.
Confusion to avoid. Do not mix up an atom with a molecule or an ion. A molecule is two or more atoms chemically joined, such as O₂ or H₂O; an ion is a particle that has gained or lost electrons and so carries a charge. A neutral atom has an equal number of protons and electrons, so it has no overall charge, that balance is exactly what separates an atom from an ion.
The atom is the central idea of the whole Matter and the Atomic Structure chapter: proton number, nucleon number, isotopes and electron arrangement are all ways of describing what is inside one atom. In our online one-to-one lessons we get students drawing labelled atomic-structure diagrams early, because every later chapter of SPM Chemistry builds on this one.
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