Bottom: Observed results: a small portion of the particles were deflected by the concentrated positive charge of the nucleus. Therefore, they were not atoms, but a new particle, the first subatomic particle to be discovered, which he originally called "corpuscle" but was later named electron, after particles postulated by George Johnstone Stoney in 1874.He also showed they were identical to particles given off by photoelectric and radioactive materials.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.

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.

They also could not convince everybody, so atomism was but one of a number of competing theories on the nature of matter.

It was not until the 19th century that the idea was embraced and refined by scientists, when the blossoming science of chemistry produced discoveries that only the concept of atoms could explain.

The number of neutrons defines the isotope of the element.

The number of electrons influences the magnetic properties of an atom.

Under certain circumstances, the repelling electromagnetic force becomes stronger than the nuclear force, and nucleons can be ejected from the nucleus, leaving behind a different element: nuclear decay resulting in nuclear transmutation.

The number of protons in the nucleus defines to what chemical element the atom belongs: for example, all copper atoms contain 29 protons.

This means that 100g of tin will combine either with 13.5g or 27g of oxygen.

13.5 and 27 form a ratio of 1:2, a ratio of small whole numbers.

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