http://www.ck12.org Chapter 22. The Nucleus
22.1 Isotopes
- Define atomic number.
- Define mass number.
- State the charge and mass of a nucleus.
- Define isotopes.
- Given the atomic and mass numbers of a nuclide, find the numbers of protons, electrons, and neutrons.
The nuclei of atoms contain protons and neutrons while electron clouds surrounding the nuclei contain the electrons.
The positively charged protons are very close together in an atomic nucleus and the repulsive force of the like charges
is tremendous. The strong force is a type of interaction that binds together protons and neutrons. Without the strong
force, the positively charged protons would blow the nucleus apart.
Isotopes
Until 1932, the positively chargednucleusof an atom was known to exist, but it was believed to contain only protons.
The nucleus was known to be surrounded by enough negatively charged electrons to make the atom electrically
neutral. Most of the atom was empty space, with its mass concentrated in the tiny nucleus.
Twelve years earlier, Lord Ernest Rutherford, a pioneer in atomic structure, had postulated the existence of a third,
neutral, sub-atomic particle, with the approximate mass of a proton, that could result from the capture of an electron
by a proton. This postulation stimulated a search for the particle. However, its electrical neutrality complicated the
search because almost all experimental techniques of this period measured charged particles.
James Chadwick bombarded hydrogen atoms in paraffin with beryllium emissions. By comparing the energies
of recoiling charged particles from different targets, he proved that the beryllium emissions contained a neutral
component with a mass approximately equal to that of the proton. He called it theneutronin a paper published in
1932.