Atoms are very small; typical sizes are around 100 picometers (a ten-billionth of a meter, in the short scale).Atoms are small enough that attempting to predict their behavior using classical physics – as if they were billiard balls, for example – gives noticeably incorrect predictions due to quantum effects.Dalton also believed atomic theory could explain why water absorbs different gases in different proportions.For example, he found that water absorbs carbon dioxide far better than it absorbs nitrogen.).In the early 1800s, John Dalton used the concept of atoms to explain why elements always react in ratios of small whole numbers (the law of multiple proportions).For instance, there are two types of tin oxide: one is 88.1% tin and 11.9% oxygen and the other is 78.7% tin and 21.3% oxygen (tin(II) oxide and tin dioxide respectively).

Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms.

The nucleus is made of one or more protons and typically a similar number of neutrons. More than 99.94% of an atom's mass is in the nucleus.

The protons have a positive electric charge, the electrons have a negative electric charge, and the neutrons have no electric charge.

This common pattern in chemistry suggested to Dalton that elements react in whole number multiples of discrete units—in other words, atoms.

In the case of tin oxides, one tin atom will combine with either one or two oxygen atoms.