Foundations · B01
Element, atom, molecule: three different ideas
A label on a water bottle can mention hydrogen and oxygen, but it does not mean loose hydrogen and oxygen atoms are inside it.
Element: a kind of atom
An element is defined by its atoms having a particular number of protons. Every oxygen atom has eight protons; every carbon atom has six. An element name therefore labels a kind, not a particular sample or a single speck.
Isotopes are atoms of the same element with different numbers of neutrons. They remain the same element because their proton count does not change.
Atom: one particle-sized unit
An atom is a single unit of an element. It has a tiny, positively charged nucleus and electrons around it. In ordinary pictures, atoms are drawn as small balls, but this is a useful model rather than a literal scale image.
Atoms can exist on their own in some conditions, but many are more often found bonded to other atoms.
Molecule: atoms joined together
A molecule is a discrete group of atoms held together by chemical bonds. A water molecule, H₂O, has two hydrogen atoms joined to one oxygen atom. The formula says how many of each kind are present.
The same elements can make different molecules. Carbon and oxygen form carbon monoxide, CO, and carbon dioxide, CO₂; the different atom ratios and connections matter.