Definition
An atom is the smallest unit of a chemical element that retains the chemical properties of that element. Atoms are the fundamental building blocks of all ordinary matter — every solid, liquid, and gas around you is composed of atoms bonded together in different arrangements.
Key Characteristics
- Atoms consist of a dense central nucleus (containing protons and neutrons) surrounded by a cloud of electrons.
- The atomic number equals the number of protons and uniquely identifies each element.
- In a neutral atom, the number of protons equals the number of electrons.
- Atoms are extremely small — a single human hair is about one million atoms wide.
- Atoms of the same element can have different numbers of neutrons; these variants are called isotopes.
- Atoms bond with other atoms through chemical bonds (covalent, ionic, or metallic) to form molecules and compounds.
Real-World Example
Water is made of atoms bonded together. Each water molecule (H₂O) contains two hydrogen atoms (1 proton each) and one oxygen atom (8 protons). When you boil water, you are not breaking apart the atoms — you are only separating the water molecules from each other. The atoms themselves remain intact throughout everyday physical changes.
An atom has two main regions: the nucleus at the centre (containing positively charged protons and neutral neutrons) and electron shells surrounding the nucleus (containing negatively charged electrons). In a neutral atom the number of protons equals the number of electrons, giving an overall charge of zero.
Subatomic Particles at a Glance
- Proton — found in the nucleus; positive charge (+1); mass ≈ 1 atomic mass unit (amu).
- Neutron — found in the nucleus; no charge (0); mass ≈ 1 amu.
- Electron — orbits the nucleus in shells; negative charge (−1); mass ≈ 1/1836 amu (negligible).
The mass number is the total count of protons plus neutrons. For example, carbon-12 has 6 protons and 6 neutrons, giving a mass number of 12.
Practice Problems
An atom has 11 protons, 12 neutrons, and 11 electrons. What is its atomic number, mass number, and overall charge?
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An atom of carbon-14 has 6 protons and 8 neutrons. How does it differ from carbon-12, and what are they called?
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Why does a neutral atom have no overall electric charge?
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A student says 'If I cut a piece of iron in half repeatedly, I could eventually get a single iron atom.' Is the student correct, and what happens if you try to go smaller than one atom?