In 1905, Albert Einstein proved the reality of these molecules and their motions by producing the first Statistical physics analysis of Brownian motion.The Geiger–Marsden experiment Top: Expected results: alpha particles passing through the plum pudding model of the atom with negligible deflection. Thomson measured the mass of cathode rays, showing they were made of particles, but were around 1800 times lighter than the lightest atom, hydrogen.

The number of protons in the nucleus defines to what chemical element the atom belongs: for example, all copper atoms contain 29 protons.The black bar is one angstrom ( An atom is the smallest constituent unit of ordinary matter that has the properties of a chemical element.Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms.If the number of protons and electrons are equal, that atom is electrically neutral.If an atom has more or fewer electrons than protons, then it has an overall negative or positive charge, respectively, and it is called an ion.