exactly alike, and all protons are exactly alike. All protons carry a certain amount of positive electricity, and all electrons carry an equal amount of negative electricity. But the mass of a proton is about 1835 times that of an electron: it takes 1835 electrons to weigh as much as one proton. Protons repel each other, and electrons repel each other, but an electron and a proton attract each other. Every atom is a structure consisting of electrons and protons. The hydrogen atom, which is the simplest, consists of one proton with one electron going round it as a planet goes round the sun. The electron may be lost, and the proton left alone; the atom is then positively electrified. But when it has its electron, it is, as a whole, electrically neutral, since the positive electricity of the proton is exactly balanced by the negative electricity of the electron.
The second element, helium, has already a much more complicated structure. It has a nucleus, consisting of four protons and two electrons very close together, and in its normal state it has two planetary electrons going round the nucleus. But it may lose either or both of these, and it is then positively electrified.
All the later elements consist, like helium, of a nucleus composed of protons and electrons, and a number of planetary electrons going round the nucleus. There are more protons than electrons in the nucleus, but the excess is balanced by the planetary electrons when the atom is unelectrified. The number of protons in the nucleus gives the "atomic weight" of the element: the excess of protons over electrons in the nucleus gives the "atomic number", which is also the number of planetary electrons when the atom is unelectrified. Uranium, the last element, has 238 protons and 146 electrons in the nucleus, and when unelectrified it has 92 planetary electrons. The arrangement of the planetary electrons in atoms other than hydrogen is not accurately known, but it is clear that, in some sense, they form different rings, those in the outer rings being more easily lost than those nearer the nucleus.
I come now to what Bohr added to the theory of atoms
