Orbitals are like volumes of space around the atomic nucleus in which electrons are likely to be at any instant in time. Different energy levels are associated with the orbitals and each orbital can contain either one or two electrons.
Orbitals can be thought of as occupying shells around the nucleus. The shell closest to the nucleus has one ortibal holding a maximum of two electrons. The next shell can have four orbitals with two electrons each for a total of eight electrons.
In the periodic table, elements in the same period (row) have the same number of shells, and the regular change in the number of electrons from one element to the next leads to a fairly regular pattern of change in the chemical properties of the elements across the period. All elements in the same group (column) have the same number of electrons in their outer shell, and so have similar chemical properties.
Hydrogen, oxygen, carbon and nitrogen all have electron vacancies in their outermost shells. Atoms with vacancies in their shells (less than 2 electrons for hydrogen, and less than 8 for the others) try to fill their vacancies by combining with, or forming bonds with other atoms.
Atoms normally are neutral and have equal numbers of protons and electrons. Atoms with unfilled orbitals in their outermost shell tend to be reactive with other atoms. The number or the distribution of its electrons changes when an atom gives up, gains, or shares electrons. A chemical bond is a union between the electron structures of atoms.
A molecule is a bonded unit of two or more (same or different) atoms. A compound is a substance in which the relative percentages of two or more elements never vary. In a mixture, two or more elements intermingle in proportions that can vary.
This animation (Audio - Impportant) is a good review of atomic orbitals.
REVIEW: Which includes the other four?
a. atoms
b. molecules
c. electrons
d. elements
e. protons
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