Figure 1: Periodic table divided into metal and nonmetal regions of elements

Bonds form from atoms when electrons feel nearby atoms.  Outer electrons rearrange to possess lower energy. This happens three ways. 

An element is either a metal or nonmetal. Recall the periodic table has two general regions: metals and nonmetals, Figure 1.

There Are Three Kinds of Bonds

Figure 2: The three ways atoms form bonds.

The kind of bond depends on the identity of which atoms form bonds:

• • Ionic Bonds

   Attraction between positive and negative charges

   Between metal and nonmetal

• • Covalent Bonds

   Electrons between two atoms

   Between nonmetal and nonmetal

• • Metallic Bonds

    Electrons shared with community of atoms

    Between metal and metal

The drive to bond comes from each element’s need to have filled outer electron shells. Outer electron shells are called valence shells. 

Ionic Bonds

Figure 3: Metals lose electrons to nonmetals to form ionic bonds

When a metal with a weak electron affinity encounters a nonmetal with a high electron affinity, the electron(s) transfers from one to the other.

Ionic bonds form from metals and nonmetals. A metal surrenders electrons. Nonmetals absorb electrons. These two events must happen together, Figure 3.

Figure 4: Cations from loss of electrons, anions from gain of electrons

In this way, metals adopt a positive charge and become cations. Nonmetals form negative charges, anions, (Figure 4).

Because a cation has a postive charge and an anion has a negative charge, the two opposite charges attract each other.  Metals  lose electrons from unfilled outer shells. Nonmetals use electrons from metals to completely fill outer electron shells.

Figure 5: aleternating arrangement of cations and anions

Anions and cations alternate positive and negative charges. The example in Figure 5 shows sodium chloride. The larger green spheres show the anion with a negative charge. Smaller gray spheres represent postive sodium cations.

Sodium chloride forms cubic lattice. It extends in exactly the same way in all directions.  Under a microscope, you can see clear cubes.

Covalent Bonds

When nonmetal elements interact with nonmetals, they share electrons. Unlike ionic bonds, electrons do not transfer from one atom to another. The relative ability of each element to attract electrons means electrons cannot transfer completely.

Figure 6: Two simple atoms become a covalent molecule

Both atoms remain neutral. The over all charge of the bond stays neutral. In order for each element to fill their valence electron shell, both elements count shared electrons as part of their own valence shell. 

The simplest scenario occurs when two hydrogen atoms, each with one electron, share one electron. This allows both hydrogen atoms to have a full valence shell.

Recall that hydrogen requires two electrons in order to have a full valence shell, Electron Shells and Orbitals.

Figure 7: Shared electrons form water, when shared between hydrogen and oxygen

In a slightly more involved example, when hydrogen requires two electrons for a complete valence shell, and oxygen must have six valence electrons, the atoms share electrons to form the molecule of water, Figure 7.

Each separate hydrogen has one electron, while oxygen atoms possess six valence electrons, Lewis structure.

In order for each atoms to complete their valence shells, they share electrons. Together they form molecular water.

Metallic Bonds

Figure 8: Metal metal bonds share electrons with the bulk material. Positive nuclei in blue and electrons red arrows

Metals interact with one another in a specific way. When a pure metal combines with others of the same kind, they share electrons also. However unlike nonmetals, metals do not share electrons with a specific adjacent atom. Instead, the electrons belong to all of the nearest neighbors.

This makes the metal atoms behave like a collection of atoms with positive charges held together with a negative “electron gas”. These negative charges act as a glue which keeps the metal atoms joined to each other, Figure 8.

Blue spheres represent the positive cationic nuclie of each metal. Red arrows show electrons shared among any given metal and its nearest neighbor. The arrows appear in various directions to display how easily electrons wander from one metal center to another,