What is an atom?
An atom is the smallest unit of a chemical element: a tiny, dense nucleus of protons and neutrons, surrounded by electrons. The number of protons decides which element it is.
Watch it explained
What is an atom?
Shows an atom as a tiny nucleus inside a mostly empty electron cloud, zooms into a pencil tip to find carbon atoms in a hexagonal lattice, opens the nucleus to show protons and neutrons with electrons moving around outside, and ends with the proton count deciding the element: 1 is hydrogen, 6 is carbon.
Scenes, in order:
- Atoms are mostly empty space
- Everything is built from atoms
- Inside: protons and neutrons
- Electrons zip around outside
- Protons decide the element
The three particles
- Protons: positive charge, in the nucleus, mass about 1 u. Their number is the atomic number.
- Neutrons: no charge, in the nucleus, mass about 1 u. Changing their number makes a different isotope, not a different element.
- Electrons: negative charge, outside the nucleus, about 1/1836 of a proton's mass. They take part in bonding.
A neutral atom has as many electrons as protons. Hydrogen has 1 proton, carbon 6, oxygen 8. Gain or lose electrons and you get an ion of the same element; gain or lose protons and it becomes another element.
Mostly empty space
The nucleus is tens of thousands of times smaller across than the atom, yet it holds more than 99.9% of the mass. We know this from Rutherford's gold foil experiment: most alpha particles passed straight through a thin gold sheet, but a few bounced back sharply, which only made sense if the positive charge and mass were packed into a very small center.
Electrons do not circle the nucleus like planets. Quantum mechanics describes each one by an orbital, a region where it is likely to be found, which is why diagrams show a cloud. The Bohr model with neat rings is still useful for counting energy levels, but it is a simplification.
Working out an atom's parts
Take a neutral atom of sodium-23. The periodic table gives sodium's atomic number as 11, so it has 11 protons and, being neutral, 11 electrons. Neutrons = mass number minus protons = 23 − 11 = 12. The ion Na+ has the same 11 protons and 12 neutrons but only 10 electrons.
Common slip: reading the atomic mass (22.99 for sodium) as the number of neutrons. Subtract the atomic number from the mass number of a specific isotope instead.
Watch it
The film on this page starts with a mostly empty atom, zooms into a pencil tip to find carbon atoms in a hexagonal lattice, opens the nucleus, and ends on the proton count deciding the element. Next steps: what an isotope is, what a mole is, and the chemistry page.
Questions
- What holds the nucleus together if protons repel each other?
- The strong nuclear force, which attracts protons and neutrons to each other over very short distances and outweighs the electric repulsion between protons inside the nucleus.
- Are protons and neutrons made of anything smaller?
- Yes. Each is made of three quarks held together by the strong force. Electrons, as far as we know, are not made of smaller parts.