What Is an Atom?
An atom is the smallest particle of an element that still has the chemical properties of that element. Atoms are incredibly tiny — a single grain of sand contains around 40 billion billion (4 × 10¹⁹) atoms.
All matter is made of atoms. The air you breathe, the food you eat, the chair you sit on — everything is built from atoms bonded together in different ways.
Structure of an Atom
An atom has two main regions:
The nucleus — at the very centre of the atom. It contains:
- Protons: positively charged particles.
- Neutrons: particles with no charge (neutral).
The nucleus is tiny but contains almost all the mass of the atom.
The electron shells — surrounding the nucleus. They contain:
- Electrons: negatively charged particles that orbit the nucleus in layers called shells or energy levels.
In a neutral atom, the number of protons equals the number of electrons, so the overall charge is zero.
If an atom were the size of a football stadium, the nucleus would be about the size of a marble in the centre. Almost all the space inside an atom is empty! This was discovered by Ernest Rutherford in 1909 through his famous gold foil experiment.
Atomic Number and Mass Number
Each element has a unique atomic number (also called the proton number), which is the number of protons in its nucleus. This is what makes each element unique:
- Hydrogen: 1 proton → atomic number 1
- Carbon: 6 protons → atomic number 6
- Oxygen: 8 protons → atomic number 8
The mass number is the total number of protons and neutrons in the nucleus. Electrons are so light that they are not included in the mass number.
Electron Shells
Electrons are arranged in shells around the nucleus. Each shell can hold a limited number of electrons:
- 1st shell: maximum 2 electrons
- 2nd shell: maximum 8 electrons
- 3rd shell: maximum 8 electrons (for elements 1–18)
The arrangement of electrons determines how an atom reacts chemically. For example, carbon has 6 electrons arranged as 2 in the first shell and 4 in the second shell.
The chemical symbol for an element is a shorthand way of representing one atom of that element. For example, O represents one atom of oxygen, and Fe represents one atom of iron. You can find all the symbols in the periodic table.
Isotopes
Atoms of the same element always have the same number of protons, but they can have different numbers of neutrons. These variations are called isotopes. Carbon-12 (6 protons, 6 neutrons) and carbon-14 (6 protons, 8 neutrons) are both carbon, but are different isotopes. Carbon-14 is mildly radioactive and is used in carbon dating to estimate the age of ancient objects.
Real world: Atoms in metals
The iron atoms in a steel beam are packed tightly in a regular lattice. Each iron atom has 26 protons and 26 electrons. When iron is shaped into beams for buildings, no individual atoms are created or destroyed — they are just rearranged. A single iron nail contains roughly 10²³ (100 billion trillion) iron atoms.
Real world: Carbon dating
Carbon-14 is a radioactive isotope of carbon (6 protons, 8 neutrons) found in tiny amounts in living organisms. When an organism dies, its carbon-14 decays at a known rate. Archaeologists measure the remaining carbon-14 in ancient bones or wood to estimate how old they are — a direct application of atomic structure and isotopes.
Real world: Atoms in water
A single drop of water (0.05 mL) contains approximately 1.7 × 10²¹ water molecules. Each water molecule (H₂O) contains 2 hydrogen atoms (1 proton each) and 1 oxygen atom (8 protons). This illustrates the unimaginably small size of atoms and the vast numbers in everyday objects.
Where do you see this in real life? Every physical object around you — your desk, your clothes, the air you breathe — is made of atoms. The type of atoms determines what the material is. Iron atoms make iron; gold atoms make gold; carbon atoms form diamonds or graphite depending on how they are arranged.
Practice Problems
An atom has 8 protons, 8 neutrons, and 8 electrons. What is its atomic number, mass number, and name?
Show answer
Why does a neutral atom have the same number of protons and electrons?