helium atom
A helium atom also has two neutrons in its nucleus. Usually, no atom can change its number of protons or electrons and remain the same kind of atom—but it can change its number of neutrons.
Atoms of the same element, but with different numbers of neutrons in the nucleus, are called isotopes. Some elements have only one isotope, some have as many as eight or ten.
an atom of Helium
Uranium has three main isotopes. The most common kind of uranium has 92 protons, 146 neutrons, and—of course—92 electrons. (Did you remember that the number of electrons has to match the number of protons?)
Adding up the total particles in the nucleus, we see that 92 + 146 = 238, so this kind of uranium is called uranium-238. There is an isotope that has 143 neutrons, so this is uranium-235 because 92 + 143 = 235. The last isotope has 142 neutrons and is uranium-234.
The heaviest part of any atom is the nucleus. Protons and neutrons are very much heavier than electrons. And then there is lots and lots of empty space. If the nucleus of the hydrogen atom (which has only one proton) were enlarged to the size of a tennis ball, the electron would be a half mile away!
A whole atom is so tiny that it is almost impossible to imagine anything so small. It would take 250 million of some kinds of atoms to measure one inch. But it would take fifty thousand times as many electrons to cover the same inch!
