In the first article on chemical reactions, we looked at the fundamental rule for chemical bonding, namely, that elements bond so that by sharing their electrons, each atom completes its outer electron shell and so looks like a noble gas. The noble gases, which can be found in the far right hand column of the periodic table, have outer shell electron counts of 2, 8, or 18 electrons. For now, we are ignoring the 18-electron case - complete shells have either two or eight electrons in their outer shell.

The examples considered in the previous article included two bonding concepts: no bond (lone pair) and single bonds with two electrons per bond. Lone pairs of electrons arise when the central atom has five or more electrons in the outer shell. The exact configuration of electrons around an atom depends largely on the number of electrons in the outer shell. Columns of the periodic table contain elements with the same number of electrons and are called groups. We will now look at several groups of the periodic table and study their bonding characteristics.

Group I elements all have exactly one electron in their outer shell. There is one exception: hydrogen has exactly one electron, period, is not a metal, and is in this group sort of by default. We’ll pretend hydrogen is excluded from the following discussion. The group one elements (except hydrogen) are all soft, highly reactive metals that are never found naturally in pure form. All of them readily give up their single outer shell electron when reacting with another atom that can accept an electron. Thus, by losing an electron, they look like the noble gas that immediately precedes them on the periodic table. For example, lithium loses an electron to look like helium, sodium loses one to look like neon, and potassium loses one to look like krypton, and so on. Losing an electron during a reaction is a bit different than forming a chemical bond. The process is called ionization. The alkali metal becomes a charged ion (in fact it takes a +1 charge) while the other atom involved takes on a negative charge.

Lithium reacts with hydrogen to form lithium hydride. Show the electron transfer that makes this reaction happen.

There are actually two processes happening simultaneously here. We’ll show them as separate reactions,

These elements possess three electrons in their outer shell. The first such element, boron, is rather rare and principally found naturally in borax, used as a detergency-enhancing agent. Boron is used industrially as a semiconductor dopant; certain boron compounds, such as sodium borohydride, NaBH₄ and the boranes, B_nH_m are useful in organic chemistry as reactants. A much more common Group IIIA element is aluminum, which we will discuss later in our section on metals.

The first two elements of Group IVA each define large industries exclusively dedicated to producing economically beneficial products from the special properties of one atom. Carbon is the basic building block for all organic compounds, including all petroleum products, and all biologically interesting compounds including living systems. Silicon is the basic building block of the semiconductor industry, the rock upon which high technology has been built. For now, we’ll just take a look at carbon and how it typically bonds.

The property that makes carbon different from most other elements is its ability to bond with itself – carbon forms chains which can be thousands of atoms long or more. We shall consider this property in detail when we begin to discuss organic chemistry. For now, we’ll just look at a simple example.

Nitrogen and like atoms in Group VA have five electrons in the outer shell, and so typically form three bonds and one lone pair when combined in molecules. We will have more to say about the way nitrogen bonds when we discuss covalent bonding in detail.

These elements, typified by oxygen, all have six electrons in their outer shell. During reactions, they usually form two bonds and two lone pairs of electrons, as in:

These elements, typified by chlorine, all have seven electrons in their outer shell. During reactions, they normally form one bond and three pairs of lone pairs. They are called halogens (salt-forming) elements because they form salts so frequently. These elements really like electrons, and will steal one from another element to complete their outer shell. They often bond in this fashion (ionization) when combining with metals, but when combining with non-metals, particularly carbon and oxygen, they are more likely to share electrons.

Please see the articles on ionic bonding, covalent bonding, organic chemistry, quantum mechanics and atomic orbitals for more detailed information on the topics covered in this article.