Microbiology Demystified

In its simplest form, a chemical element is made up of one atom. In its more

complex form, a chemical element is made up of two or more atoms, which is

called a molecule of the chemical element. For example, binding together two

hydrogen atoms forms a hydrogen molecule.

H +H →H 2

In order to make different things you need to combine different atoms and

molecules of different chemical elements. This combination is called a com-

pound. For example, combining two hydrogen molecules to an oxygen atom

results in the compound we know as water.

H 2 +O →H 2 O

Molarity: Hey, There’s a Mole Amongst Us

It seems nearly impossible to measure a molecule’s mass or size. Fortunately,

there is Avagadro’s number,which is the number of particles in a mole of a sub-

stance. The number is 6.022 × 1023. Amadeo Avagadro was an Italian physicist

for whom the value was named.

Scientists can measure molecules using units called a mole. Abbreviated as

mol. One moleis equal to the atomic weight of an element expressed in grams.

Amole is the weight in grams of a substance that is equal to the sum of the

atomic weights of the atoms in a molecule of the substance. This is referred to

as a gram molecular weight.

Let’s look at a water molecule to determine how many moles there are in a

liter of water.

• Find the atomic mass for each chemical element that makes up water.
  Water has two chemical elements. These are hydrogen and oxygen.


• Look up the symbol for each element on the Periodic Table. These are H
  and O for hydrogen and oxygen.


• Note the bottom number alongside the symbol. This is the atomic mass for
  the chemical element. These are 1 for hydrogen and 16 for oxygen.


• Multiply the number of atoms of each chemical element in the molecule by
  its atomic mass to determine the value for one mole for the chemical ele-
  ment. For water, there are two hydrogen atoms so this will be 2 ×1 g. One

(^36) CHAPTER 2 Chemical Elements of Microorganisms