The three particles
| Particle | Charge | Relative mass | Where |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 (very slightly more) | Nucleus |
| Electron | −1 | About 1/1836 | Shells around the nucleus |
The nucleus contains almost all of an atom’s mass but is tiny — roughly 100,000 times smaller than the atom itself. The rest of the atom is the region where electrons are found.
Atomic and mass numbers
- The atomic number (Z) is the number of protons. It defines the element: every carbon atom has 6 protons, every oxygen atom has 8.
- The mass number (A) is the number of protons plus neutrons.
A = Z + N number of neutrons N = A − ZChemists write an atom as a symbol with A at the top left and Z at the bottom left, for example ¹²₆C.
Isotopes
Atoms of the same element can have different numbers of neutrons. These versions are calledisotopes. They behave almost identically in chemical reactions because they have the same electrons, but their masses differ. Some combinations of protons and neutrons are stable; others areradioactive and eventually decay. Carbon-12 and carbon-13 are stable, while carbon-14 is radioactive with a half-life of about 5,700 years. Too many or too few neutrons and a nucleus cannot hold together at all.
Ions
In a neutral atom, the number of electrons equals the number of protons, so the charges cancel. If an atomloses electrons it becomes a positive ion (cation), such as Na⁺. If itgains electrons it becomes a negative ion (anion), such as Cl⁻.
charge = protons − electronsElectron shells
In the simple shell model, electrons occupy shells at increasing distances from the nucleus. For the first 20 elements, the shells fill in the pattern 2, 8, 8, 2. The electrons in the outermost shell, calledvalence electrons, decide how an element reacts. Atoms with full outer shells, such as neon and argon, are very unreactive (noble gases). Sodium, with one valence electron, readily loses it; chlorine, one short of a full shell, readily gains one. That is why sodium and chlorine combine to form sodium chloride.
Energy levels and light
Each shell corresponds to an energy level. An electron can absorb exactly the right amount of energy to jump to a higher level, then fall back and release that energy as a photon of light. Because each element has its own pattern of energy levels, each produces its own colours — sodium streetlights glow orange-yellow, and astronomers identify elements in stars from their light.
Key terms
- Element
- A substance made of atoms that all have the same number of protons.
- Isotope
- Atoms of the same element with different numbers of neutrons.
- Ion
- An atom or group of atoms with an electric charge because electrons were gained or lost.
- Valence electrons
- Electrons in the outermost shell, involved in chemical bonding.
- Radioactive decay
- The spontaneous change of an unstable nucleus, releasing radiation.
Real-world applications
- Radiocarbon dating uses carbon-14 to find the age of wood, bone and other once-living material up to about 50,000 years old.
- Medical imaging uses radioactive isotopes such as technetium-99m as tracers (an educational example — see our disclaimer).
- Nerves and muscles work using ions such as Na⁺, K⁺ and Ca²⁺ moving across cell membranes.
- Fireworks and neon signs get their colours from electrons dropping between energy levels.
Common misconceptions
- “Electrons orbit like planets.” The Bohr picture is a useful model, but electrons are better described as clouds of probability called orbitals.
- “Changing neutrons changes the element.” Only protons decide the element; neutrons give different isotopes.
- “Ions are different elements.” Na⁺ is still sodium — only its electron count changed.
What the model simplifies
The Bohr model, proposed in 1913, places electrons on circular shells. Modern quantum mechanics replaces these with orbitals of different shapes, and from potassium onward the filling order becomes more complex. The nucleus here is drawn hugely enlarged, and stability is simplified to three categories using the known stable isotopes of the first 20 elements.
Key takeaways
- Protons set the element, neutrons set the isotope, electrons set the charge.
- Mass number = protons + neutrons.
- Ions form when atoms gain or lose electrons.
- Valence electrons in the outer shell control chemical behaviour.