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Elements are sub-divided into sub-groups called isotopes based on the number of protons AND neutrons in the nucleus. All atoms of an element with the same number of neutrons in the nucleus are of the same type of isotope. An element's isotopes will have very nearly the same chemical properties but they can behave very differently in nuclear reactions. For example, all of the isotopes of the element Hydrogen have one electron orbiting the nucleus and behave the same way in chemistry reactions. The ordinary Hydrogen isotope has 0 neutrons + 1 proton while another Hydrogen isotope called Deuterium has 1 neutron + 1 proton and another Hydrogen isotope called Tritium has 2 neutrons + 1 proton in the nucleus. Tritium is radioactive---its nucleus spontaneouly changes into another type of nucleus. In the figure above, atoms (c), (d), and (e) are different isotopes of the same element called Lithium.

Most atoms in nature are neutral, the negative charges exactly cancel the positive charges. But sometimes an atom has a hard collision with another atom or absorbs an energetic photon so that one or more electrons are knocked out of the atom. In some rare cases, an atom may temporarily hold onto an extra electron. In either case, the atom has an extra positive or negative charge and is called an ion. For example, the carbon ion C+ has 6 protons and 5 electrons and the iron ion Fe2+ has 26 protons and 24 electrons. Because the number of electrons are different, an ion of an element will behave differently in chemical reactions than its neutral cousins. In the figure above atom (d) is a Li+ ion [compare it with atom (c) or (e)].

In order to explain discrete spectra, Bohr found that atoms obey three basic rules:


Electrons have only certain energies corresponding to particular distances from nucleus. As long as the electron is in one of those energy orbits, it will not lose or absorb any energy. The energy orbits are analogous to rungs on a ladder: electrons can be only on rungs of the ladder and not in between rungs.


The orbits closer to the nucleus have lower energy.

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