ScienceVerseLearning path: Atoms
The concepts to learn before Atoms, in order. Each step links to its page and, where one exists, its quiz.
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No prerequisites
Atoms has no prerequisite concepts: you can start here.
- AtomsYour goal
An atom is the smallest unit of an element that keeps that element's chemical identity. Every atom has a tiny, dense nucleus of protons and neutrons, which holds nearly all of its mass, surrounded by electrons that fill almost all of its volume. The number of protons fixes which element the atom is; changing the number of electrons makes an ion, and changing the number of neutrons makes a different isotope.
Where to go next
Concepts that build directly on Atoms:
- Chemical Bonds: A chemical bond is the attraction that holds atoms together in a substance. In an ionic bond, oppositely charged ions attract after electrons have moved from one atom to another; in a covalent bond, two atoms share a pair of electrons; in a metal, outer electrons are shared across the whole structure. Bonding lowers a system's energy, so breaking a bond always costs energy and forming one releases it.
- Elements: A chemical element is a substance made of atoms that all have the same number of protons — the atomic number. Carbon is every atom with 6 protons, oxygen every atom with 8. Atoms of one element can differ in their number of neutrons (isotopes), which is why an element's listed atomic mass is a weighted average rather than a whole number. 118 elements are known; 94 occur naturally on Earth.
- Semiconductors: Semiconductors such as silicon conduct electricity better than insulators but far less than metals, because only a small energy gap separates their filled and empty electron bands. Adding tiny amounts of impurity (doping) creates n-type material, rich in electrons, or p-type material, rich in holes. Joining the two makes diodes and transistors — the switches and amplifiers from which every computer chip is built.
- States of Matter: Matter is normally found as a solid, a liquid or a gas, and at very high temperatures as plasma. The state depends on a contest between the attractions holding particles together and the particles' energy of motion, which rises with temperature. Melting, boiling and condensing change how the particles are arranged, not what they are, so they are physical changes rather than chemical reactions.