In short
An atom is the smallest unit of an element that keeps the properties of that element. It has a tiny central nucleus of protons and neutrons, with electrons around it.
The number of protons is the atomic number and fixes the element. A neutral atom has equal numbers of protons and electrons. Nearly all the mass is in the nucleus, because a proton or neutron is about times heavier than an electron. The atom is mostly empty space: its radius is about and the nucleus is about .
Carbon has atomic number . A carbon-12 atom has protons, neutrons and electrons.
Also searched as: atoms and molecules, majority of an atom's mass, atoms to molecules.
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