In about 400 BC the Greek philosopher/scientist Democritus developed the theory that all matter was made of tiny units. The different properties of matter were caused by interactions between the tiny units, such as whether they were smooth or rough. Matter made of smooth units formed liquids, because the units could move around freely. Matter made of units that stuck together formed metals and other solids.
This theory was verified much later, when chemistry was again advanced in Europe in the 19th century AD. In the 20th century it was clarified that the "hooks" theorized by the ancient Greeks were electrons. Some atoms had their electrons available to hook onto other atoms, and some had their electrons sealed up so they could not interact. How available they are to "hook up" determined how reactive an element is. And like the Greeks theorized, liquids are made of particles that don't hook to their neighbors very well.
The modern science of chemistry was started when John Dalton revived the ancient Greek theory in order to explain chemical observations. In 1808 he proposed that an element was componsed of tiny particles called atoms, with each atom of a certain element having the same properties as other atoms of the same element, and each atom being indivisible into smaller units. Science today observes that atoms are practically indivisible, but under extreme conditions they can be broken into even smaller units. Dalton's theory met fierce resistence by the scientific establishment, but gradually became a central theory of science.