A Little Chemistry Background

In order to gain a better understanding of what happens inside living things, it helps to have a basic understanding of chemistry. If you find that you enjoy learning about chemistry, the good news is that you'll be covering chemistry in greater detail in 8th grade. For now, though, here is some basic information.

Atoms

Everything in the universe is composed of one or more atoms of the 92 natural elements. There are actually more than 92 elements, but they are man-made. Most of the man-made elements exist for very brief periods of time. You will find information that indicates that there are more than 92 natural elements. That's because science is changing all the time. There is some controversy about whether there are 94, or 95, or even 98 natural elements. All scientists agree, however, that there are at least 92, so for the purposes of this brief introduction to chemistry, lets use 92.

Each of the 92 natural elements represents an atom, so that means that there are 92 different kinds of atoms. The difference between atoms is the number of protons, neutrons, and electrons in each atom. Protons and neutrons are found in the nucleus, or center, of an atom. Electrons orbit around the nucleus. If you can imagine a picture of our solar system, the sun would represent the nucleus and the planets would represent the electrons.

Atomic Numbers & Atomic Weights

Each element is identified in 4 ways. First, each has a name. Second, each has a symbol. Third, each has an atomic number. Finally, each has an atomic weight. As an example, the symbol for oxygen is O. Oxygen carries the atomic number of 8 and has an atomic weight of 16.

The atomic number for any element tells you how many electrons and protons an atom of that element has. Using oxygen again as the example, we know that it has 8 protons and 8 electrons. By the way, all atoms have the same number of electrons as protons. What we cannot tell from the atomic number is how many neutrons an atom has. But, if we know the atomic weight, we can get the number of neutrons. Just subtract the number of protons from the atomic weight and you will get the number of neutrons. What should you do when the atomic weight is not a whole number? Most of the time, you should round down to the nearest whole number, but not always. You'll have plenty of time in 8th grade and in high school to learn why the atomic mass might not be a whole number. When you take a test in my class, you will only have to deal with elements that have whole number atomic weights.

Compounds

Even though there are only 92 natural elements, there are way more than 92 different kinds of things in the universe. That's because some things are made of more than one kind of atom. When two different kinds of atoms combine, we call it a compound. One compound that is important in biology is carbon dioxide, or CO₂. Carbon dioxide is composed of one carbon (C) atom and two oxygen (O) atoms. Another compound that is extremely important in biology is water, or H₂O. Water is composed of two hydrogen (H) atoms and one oxygen (O) atom. It is important to understand that when two different types of atoms combine to make a compound, the properties of that compound might be totally different from the properties of either element it is composed of. Water is a great example. Both hydrogen and oxygen are extremely flammable. If you have ever been to a hospital, then you have probably noticed all of the "No open flame - Oxygen in use" signs. That's because oxygen will burn. Hydrogen, too, will burn. A long time ago, blimps were commonly used as air transportation. That era basically came to an end when a famous blimp, the Hindenberg Zeppelin, caught fire and exploded. You can see newsreel coverage of the disaster by clicking on this link.

Compounds are composed of two or more different kinds of atoms. It is possible to break compounds apart into their component atoms and then put the atoms back together to make a new compound. This goes on all the time in living things. Plants take carbon dioxide from the air (CO₂) and water (H₂O) and, using energy from the sun, produce oxygen (O₂) and sugar (C₆H₁₂O₆).

Not every element will form a compound with every other element. There are even some elements that will not form compounds at all. These elements are called "inert". A common example of an inert element is neon. Neon is a gas that glows when an electrical current runs through it. If it weren't for neon, there would be no neon signs.

To get a complete list of elements, including their atomic symbols, atomic numbers, and atomic weights, you need to look at the Periodic Table of the Elements. You can find a good one online by following this link.

Isotopes

In addition to the 92 elements, there are variations that are called isotopes. For example, carbon has an atomic number of 6 and an atomic weight of 12. We know from the atomic number that carbon has 6 protons and 6 electrons. We know from the atomic weight that it has 6 neutrons. But there is another form of carbon, called an isotope, that has an atomic weight of 14. This is known as carbon 14 and is written ¹⁴C. The difference between C and ¹⁴C is that ¹⁴C has two extra neutrons. You do not really need to have a deep understanding of isotopes other than to know that over time, they decay. In other words, given enough time, ¹⁴C will turn into something else. ¹⁴C turns into Nitrogen, but that's not what is important. What is important is that because ¹⁴C decays over time at a known and predictable rate, scientists can determine how old something is by measuring the amount of ¹⁴C that is still in the object.

You can watch the Bozeman Science video about Carbon 14 dating by clicking here.

Chemicals in Living Things

There are about 25 elements that are found in organisms. Four of these -- oxygen, nitrogen, hydrogen, and carbon -- represent over 90% of the chemicals found in organisms. Other important elements include calcium, phosphorus, and sulfur. Some elements, like zinc, are only found in trace amounts.

The Importance of Water

You probably know that scientists get excited when they find water (or evidence of water) on other planets. Just because there is water on other planets does not mean that there is life there. But we're pretty sure that if there is no water, there is also no life. Based on everything we know about living things, life cannot exist without water. Cells are made mostly of water. All of the chemicals in cells, like DNA, are surrounded by water. Water interacts with everything in a cell, so it is important to understand just how special water is.

Water is made of 2 hydrogen (H) atoms joined to an oxygen (O) atom. The shape of the water molecule is important. This shape makes water a "polar" molecule. Think about a magnet. You know that a magnet has a North pole and a South pole. You know that if you put two magnets near each other, the North pole of one magnet will be attracted to the South pole of the other. You also know that the North pole of one magnet will be repelled by the North pole of another. This is called polarity. The same is true of a battery. Batteries have one end marked with a minus (-) sign and one end marked with a plus (+) sign. When you put batteries into an electronic device, you have to put them in the right way or the device will not work. Batteries are another example of polarity. Well, a water molecule is also polar. The end of the water molecule where the oxygen atom is has a slight negative (-) charge and the end of the water molecule where the hydrogen atoms are has a slight positive (+) charge.

You can see in this illustration of a water molecule that the hydrogen atoms have a positive (+) charge and the oxygen atom has a negative (-) charge. Because water is a polar molecule, molecules of water stick together in interesting ways. They also stick to other compounds that are found in cells.