Definition of a Molecule
Molecules are formed when two or more atoms are held together by chemical bonds.
Molecules Formed by Covalent Bonds
For
example, the oxygen molecule consists of two atoms of oxygen. In the
case of oxygen, the bond that holds the two atoms together is known as a
covalent bond, and here is how it works.
The oxygen atom has 8
each of protons, neutrons, and electrons. The protons and neutrons are
found in the center of the atom, known as the nucleus, and the electrons
surround the nucleus in layers, or shells. The oxygen atom has 2
electrons in its first shell, and 6 in its second and outer shell.
However, in chemistry, there exists the octet rule, which states that
atoms generally strive to have 8 electrons in their outer shell. The
oxygen therefore is 2 electrons away from a complete outer shell. When
it binds with another oxygen atom, they can share two pairs of electrons
and so each will have 8. When atoms share electrons like that, they
form a molecule through a covalent bond.
Molecules Formed by Ionic Bonds
Molecules
can also be formed through other kinds of chemical bonds, for example,
an ionic bond. An example this is sodium chloride, or table salt. In an
ionic bond, one atom has a much greater ability to attract electrons
than the other atom. In this case, the chlorine, which is only 1
electron short of a complete outer shell, will steal that electron from
the sodium, which has only 1 outermost electron. This turns the sodium
into a positively charged ion, and the chlorine into a negatively
charged ion, and the two atoms end up being held together by an
electrostatic charge.
Meaning of a Mole
Now what is a
mole? A mole is simply a counting number, much like a dozen. When we say
"dozen", we mean 12; well, when we say "a mole" we mean 6.022 x 10 to
the 23. Moles make it easier to quantify chemical substances. The
periodic table lists the weights of all the elements in grams per mole.
Examples of Finding Molar Mass
For
example, oxygen has molar mass of 16, which means 6.022 x 10 to the 23
atoms of oxygen weigh 16 grams. Hydrogen has a molar mass of 1. To find
out the molar mass of a molecule, such as water, we need to add up the
molar masses of the atoms it is composed of. Since water has two atoms
of hydrogen and 1 atom of oxygen, its molar mass is 18 grams per mole.
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