Knowing electron configurations is important because the number of valence electrons largely determines the chemical properties of an element.  Valence electrons are the electrons in the highest occupied energy level of an element's atom.  You may recall that when Mendeleev organized his periodic table, he did so with the properties of the elements in mind.  Scientists later learned that all the elements in a particular group of the periodic table have the same number of valence electrons.  For example, the elements in Group 1A (hydrogen, lithium, sodium, and so forth) all have one valence electron.  For the representative elements an atom's number of valence electrons can be determined by looking up the group number of that element. Carbon and silicon, in Group 4A, have four valence electrons.  The noble gases are the one exception to this rule.   Helium has two valence electrons, and all the others have eight.  Valence electrons are usually the only electrons used in the formation of chemical bonds.   Thus it is customary to show only the valence electrons in electron dot structures.   Electron dot structures depict valence electrons as dots. The inner electrons and the atomic nuclei are represented by the symbol for the element being considered. 

Why are some elements found mainly as ions?  It is because the nature of thing is to adjust to achieve the lowest possible energy. Noble gas atoms are stable.  They are of low energy and low chemical reactivity because they have stable electron configurations.  The atoms of all other elements are less stable.  They are of higher energy and higher chemical reactivity because they do not have stable electron configurations.  In forming compounds, atoms make adjustments to achieve the lowest possible energy.

In 1916 Gilbert Lewis provided an explanation for why atoms tend to form certain types of ions and molecules.  He proposed the octet rule:  Atoms react by changing the number of their electrons so as to acquire the stable electron structure of a noble gas.   

Recall that each noble gas, except helium, has eight electrons in its highest energy level. The octet rule takes its name form this fact. Atoms of the metallic elements obey the octet rule by losing electrons. Atoms of some nonmetallic elements obey the rule by gaining electrons. The loss of valence electrons from an atom produces a cation, or positively charged ion. The gain of valence electrons produces an anion, or negatively charged ion.

The most common cations are those produced by the loss of electrons from metal atoms. These atoms usually have up three valence electrons that are easily removed. When forming a compound, a sodium atom loses its 1 valence electron. It then has the same electron configuration as neon, a noble gas. The sodium ion has an octet in its highest energy level. Because the number of protons in the sodium nucleus is still 11, the lack of one unit of negative charge produces a sodium ion with a charge of 1+.

An anion is an atom or a grouping of atoms with a negative charge. Atoms of nonmetallic elements attain stable electron configurations more easily by gaining electrons than by losing them. For example, chlorine belongs to Group 7A, the halogen family, of the periodic table. A gain of one electron converts a chlorine atom into a chloride ion. It is an anion with a single negative charge.

Anions and cations have opposite charges. They attract one another by electrostatic forces. The forces of attraction that bind oppositely charged ions together are called ionic bonds. Ionic compounds are electrically neutral groups of ions joined by electrostatic forces.

Another part of the Lewis theory holds that some atoms share electrons to attain noble-gas electron configurations. Covalent bonds are the result of electron-sharing between atoms. Key word-valence.  Atoms of hydrogen and the nonmetallic elements in Groups 4A, 5A, 6A, and 7A of the periodic table are particularly prone to form covalent bonds. This tendency is summarized in the octet rule for covalent bonding: the sharing of electrons occurs when the atoms involved can thus acquire particularly stable electron configurations. Often the configurations contain eight valence electrons. The notable exception is hydrogen. When hydrogen shares two electrons it acquires the stable electron configuration of helium.

There is a maximum of four different types of chemical bonds.

The electronegativity of an element is the tendency for an atom to attract electrons to itself when it is chemically combined with another element. Electronegativities have been calculated for the elements. They are expressed in arbitrary units on the Pauling electronegativity scale. This scale is based on a number of factors including the electron affinity and ionization potential of the atoms.

As we go across a period from left to right, the electronegativity of the representative elements increases. The metallic elements at the far left of the periodic table have low electronegativities. By contrast, the nonmetallic elements at the far right (excluding the noble gases) have high electronegativities. Ordinarily, electronegativity decreases as we move down a given group.

Anions and cations have opposite charges. They attract one another by electrostatic forces. The forces of attraction that bind oppositely charged ions together are called ionic bonds. Ionic compounds are electrically neutral groups of ions joined by electrostatic forces. They are also known as salts. In any sample of an ionic compound, the positive charges of the cations must equal the negative charges of the anions.

Ionic bonds are bonds with differences in electronegativity that are 1.8 or greater. Ionic bonds do not share electrons, rather one element gives up electrons and the other takes them. In forming a compound, sodium has a single valence electron and an electronegativity of 0.9 which, makes it easy to lose one electron. Chlorine has seven valence electrons and an electronegativity of 3.0 which, makes it easy to gain an electron. When sodium and chlorine react to from a compound, they must do so in 1:1 ratio. The sodium atom gives its one valence electron to a chlorine atom. The electronegativity difference is 2.1 which, is greater than 1.8 indicating that it is an ionic bond.

Covalent bonds are the result of electron-sharing between atoms. Atoms of hydrogen and the nonmetallic elements in Groups 4A, 5A, 6A, and 7A of the periodic table are particularly prone to form covalent bonds. The tendency is summarized in the octet rule for covalent bonding: The sharing of electrons occurs when the atoms involved can thus acquire particularly stable electron configurations. Often the configurations contain eight valence electrons (an octet). Covalent bonds are bonds with differences in electronegativity that are less than 1.8. Covalent bonds share electrons more equally than ionic bonds. There can be a sharing of one, two or three pairs of electrons in a covalent bond. A single covalent bond is formed when one pair of electrons is shared between two atoms. Double covalent bonds involve two shared pairs of electrons. Triple covalent bonds include three shared pairs of electrons. Oxygen is an example of a molecule that should have a double covalent bond according to the octet rule. An oxygen atom, with six valence electrons, could share two of these electrons with another oxygen atom to form the double bond.

The nitrogen molecule contains a triple covalent bond. Each nitrogen atom has five valence electrons. They each need three more electrons to attain the electron configuration of neon. In the nitrogen molecule each nitrogen has one unshared pair of electrons.

Covalent bonds are formed by electron-sharing between atoms.  Not all covalent bonds are the same.  The character of these bonds in a given molecule depends on the kind and number of atoms joined together.   These features in turn determine the properties of the molecules. 

The bonding pairs of electrons in covalent bonds are pulled, as in a tug of war, between the nuclei of the atoms sharing the electrons. When Two different atoms are joined by a covalent bond, and the bonding electrons are shared unequally, the bond is a polar covalent bond, or simply a polar bond. The atom with stronger electron attraction (the more electronegative atom) in a polar bond acquires a slightly negative charge. The less electronegative atom acquires a slightly positive charge. Polar bonds have electronegativity differences of 0.6 or greater, and less than 1.8.

Non-polar bonds are covalent bonds with electronegativity differences less than 0.6. When the atoms in a molecule are the same, the bonding electrons are shared equally, and the bond is a non-polar bond. Because the difference in electronegativity in non-polar bonds is so small, the electron attractive forces of each atom are almost equal. This results in a molecule with a relatively neutral charge.

Scientists believe that a piece of pure metal such as copper or iron consists not of metal atoms but of closely packed cations. The cations are surrounded by mobile valence electrons that are free to drift from one part of the metal to another. Metallic bonds consist of the attraction of the free-floating valence electrons for the positively charged metal ions. These are the forces of attraction that hold metals together.

Scientists have accounted for many physical properties of metals on the basis of this picture of metallic bonding. Metals are good conductors of electrical current (flow of electrons) because as electrons enter one end of a bar of metal, an equal number leave the other end. Metals are also malleable in that they can be hammered into different shapes. The electron mobility theory says that the metal cations are insulated form one another by electrons. When a metal is subjected to pressure, the metal cations easily slide past one another like ball bearings immersed in oil. In contrast, if an ionic crystal is struck with a hammer, the blow tends to push ions of like charge into contact. They repel, and the crystal shatters.

Hydrogen bonds are attractive forces in which a hydrogen that is covalently bonded to a very electronegative atom is also weakly bonded to an unshared electron pair of an electronegative atom in the same molecule or in a nearby molecule. Hydrogen bonding always involves hydrogen. It is the only chemically reactive element whose valence electrons are not shielded form the nucleus by a layer of underlying electrons. A very polar covalent bond is formed when hydrogen bonds to an electronegative atom like oxygen, nitrogen, or fluorine. This leaves the hydrogen nucleus quite electron-deficient. The hydrogen makes up for its deficiency by sharing a nonbonding electron pair on a nearby electronegative atom. The resulting hydrogen bond has about 5% of the strength of an average covalent bond. Hydrogen bonds are the strongest of the intermolecular forces. They are extremely important in determining the properties of water and biological molecules like proteins.