- Chemistry

The columns of a periodic table of the elements each contain several elements whose chemistry is very similar. Each row of a periodic chart is called a period. The elements listed in the same column are called a group.

The elements important to Biology are underlined below ( H, C, N, O, F, Na, Mg, P, S, Cl, and Ca). As you can see there aren't very many of them. Moving your mouse over these elements will give you additional information about each.

Although atoms are the smallest unique unit of a particular element, in nature only the noble gases (the last column) are found as isolated atoms. Most matter is in the form of ions, or as compounds (molecules). A molecule is comprised of two or more chemically bonded atoms. The atoms may be of the same type of element, or they may be different.

Many elements in nature are found in the molecular form - two or more atoms (of the same type of element) bonded together. Oxygen, for example, is most commonly found in its molecular form as "O2" (two oxygen atoms chemically bonded together).



The octet rule - The octet rule states that atoms are most stable when they have a full shell of electrons in the outside electron shell. The first shell can hold only two electrons. Helium has two electrons, a full shell, so it is a stable, inert element. Hydrogen, though, has only one electron. It can lose an electron to become H+, a hydrogen ion or it can gain an electron to become H-, a hydride ion. All the other shells have at least eight electrons on the outside.

On the Periodic Table you can easily see the octet rule. All of the elements in the first column have one electron in the outside shell and they all have a valence of plus one. Group I elements will lose one and only one electron, that single outside electron to become a single positive ion with a full electron shell of eight electrons (an octet). Elements in this column have the lowest electronegativity.

Elements in the second column all have two electrons in the outer shell and all have a valence of plus two. They do not lose only one electron, but two or none. Group III elements have a valence of plus three. The small Group IV elements, carbon and silicon, rarely lose the entire four electrons in the outer shell. These elements tend to make only covalent bonds, sharing electrons.

Elements in the fifth column (V), like nitrogen and phosphorus, tend to either gain three electrons to make an octet or bond covalently. The Group VI elements, oxygen and sulfur, tend to either gain two electrons or bond covalently. All elements in the VII column have seven electrons in the outer shell and either gain one electron to become a -1 ion or they make one covalent bond. The Group (column) VII elements are diatomic gases due to the strong tendency to bond to each other with a covalent bond. Elements in this column have the highest electronegativity.

All of the elements in the last column, the noble gases, have a full octet in the outside shell (or two in the first shell) and so do not naturally combine chemically with other elements. Since they are essentially inert, we pretty much ignore them in this class.

Thus, the number of electrons needed to fill each shell is 2 for the first, 8 for the second, and 8 for the third. In biology, we don't have to worry too much about the more distant shells. Also worth noting is that the more distant an electron is from the nucleus, the more Energy it has.

Now its time to make some chemical bonds; bonds combine atoms into molecules