Fig. 9-1

photo credit: EMSL

Every material object or substance consists of atoms and molecules. For the most part, they are too small to be seen, but quantitative chemical  evidence for their existence is overwhelming. As Linus Pauling wrote, “Atoms are the units which retain their identity when chemical reactions take place.”  To appreciate events in the natural world, including functions of the human body, you (yes, you) must have some minimal knowledge of atoms and molecules. Even the artistic-creative and poetic sort need this.

A chemical element consists of atoms of only one kind; there are 92 natural elements, with names like hydrogen, oxygen, carbon, nitrogen, each with a symbol such as H, O, C, N. A chemical compound consists of atoms of more than one kind, such as H₂O = water, CO₂ = carbon dioxide, NH₃ = ammonia. These formulas represent molecules of compounds.  Symbols such as O₂ – oxygen or N2 – nitrogen  represent molecules composed of two atoms of the same element. Molecules are very small, but some are sufficiently large that images of them can be made with special equipment. Now, the concept of atoms and molecules is beginning to make sense to you.

Analysis of the composition of molecules formed in chemical reactions has enabled chemists to put the elements in order of atomic number in a chart called the periodic table. This is printed in many textbooks. Real chemists have it in their heads. The word atom means indivisible, but atomic physics has shown that atoms actually have a very dense nucleus of protons and neutrons, surrounded by a lot of space with fast-moving electrons. Chemical reactions are the result of behavior of the outer electrons.  Atomic numbers are equal to the number of protons in the nucleus of all atoms of a specific element, while the sum of protons and neutrons determines the atomic weight. These simple facts sound like dogma, and they are. However, their origin is not revelation, but rather the result of many experiments in many laboratories. If you want to get to the bottom of this dogma, take a chemistry course, and question the instructor.

Chemists are fond of a quantity called the mole, which is the molecular weight (MW) of a substance, taken in grams. The molecular weight is the sum of the atomic weights of the atoms forming the molecule. For example, one mole of water weighs 18 g:

2 H = 2,  O = 16, H₂O = 18;

one mole of carbon dioxide weighs 44 g:

C = 12, 2 O = 32, CO₂ = 44.  

Now, consider  some really big and really small numbers. The number of molecules in a mole = 6.02 X 10²³; this was determined by experiments  that were followed by calculations based of the charge of an electron:

1.60 X 10^-10coulomb

Hey, Jennifer, what is the charge on a mole of electrons? Oh, Mike, that is so easy!

6.2 X 10²³( 1.60 X 10^-10) = 9.632 X 10¹³coulombs.

Note that when numbers like this are multiplied, the exponents are added. When one number is divided by another, the lower (divisor) exponent is subtracted from the upper (dividend) to get the answer (quotient). If a mole of water weighs 18.0g, how much does each molecule weigh?

18.0 X 10⁰g / 6.02 X 10²³ = 2.99 X 10^-23 g

There is no way to isolate one molecule of water, nor any balance to weigh it. Such questions are really just a way to have fun with numbers.  Other than a wish to communicate with numbers freaks, is there any reason to be familiar with the rules of exponents? With the US national debt in the trillions of dollars, and world population over six billion, the answer is yes.

10⁶ = 1 million =  1 000 000   

10⁹ = 1 billion =  1 000 000 000

10¹² = 1 trillion = 1 000 000 000 000                    

The confusion associated with long strings of zeroes is eliminated.

Numbers with negative powers of ten express small quantities, and are reciprocals of the large numbers as follows:

10^-6 = 1/ 10⁶ = 0.000 001 = 1 one-millionth part

10^-9 = 1/ 10⁹ = 0.000 000 001 = 1 one-billionth part

In chemistry, it is often more convenient to work with less than one mole of a substance, and a terminology has developed to describe extra small amounts, such as chemicals in blood, or pollutants.

10^-3 mole is a millimole = mmol

10^-6 mole = 1 micromole = µmol

A micromole is not a tiny burrowing mammal. Molar does not refer to a tooth. Molar refers to concentration: a 1 molar solution of C₆H₁₂O₆ (MW 180.156) contains 180 grams dissolved in 1 liter. The label on the bottle reads 1.0 M glucose.

The blood plasma of a healthy fasting adult contains 0.910 g glucose / L. The molar concentration is:

0.910 g / 180.156 g/mol = 5.05 X 10^-3 M = 5.05 mM.

Below 10^-6 -micro are additional prefixes: 10^-9 -nano, and 10^-12 -pico. A normal plasma concentration of insulin, the hormone that regulates blood sugar, may range up to 500 picomolar:

500 pM = 500 X 10^-12 M = 0.5 X 10^-9 M = 0.5 nM

Hormones are highly effective at very low concentrations. With MW = 5 808, a 1M solution of insulin would contain 5 808 g/L, an obvious impossibility, because 5.8 kg cannot dissolve in 1 kg water. On the other hand,

500 pM = 0.5 nM = 0.5 X 10^-9 X 5.808 X 10³ = 2.9 X 10^-6 g/L.

This is a measurable concentration, 2.9 µg/L.

Clinical laboratories use both mg/dL and mM to express concentrations of substances in blood and urine. The results of testing your fluids should state the units of concentration. Do not pay for diagnostic tests unless the results  are stated properly.