Bonding

Ionic Bonds

Recall that ions are atoms/molecules with a net electrical charge. Cations form when neutral atoms lose electrons while anions form when neutral atoms gain electrons. Compounds formed by ionic bonds are simply called ionic compounds or salts.

Most ionic solids have a lattice structure representing the individual ions, like shown above with sodium chloride(NaCl), also known as table salt. You can see in the diagram that the chloride anions(green) are connected with the sodium cations(purple) through electrostatic attraction. Notice that no two ions of the same type are next to each other(no chloride ions are next to each other and no sodium ions are, either). This is because this arrangement makes it so that the each ion experiences maximum attraction from the other ion and minimal repulsion.

While more is gone into depth on the lattice structure and its implications on this page, the basic idea is that electrostatic attractions between oppositely charged ions cause ionic compounds, shaped like unit cells, to form.

In the case of solutions, ionic compounds are usually soluble in water because the strong ion-dipole attractions between the strong water dipoles and the ions are usually stronger than the lattice energy keeping the ionic compound together.

Covalent Bonds

Covalent bonds are formed between two nonmetals, usually, whereas ionic bonds form between a metal and nonmetal usually. Covalent bonds arise from the attractions between valence electrons in one atom and atomic nuclei in the other. To explain, take a diatomic hydrogen molecule(H₂), shown at the top of the image above. Both hydrogens have only 1 valence electron. Each electron is attracted to the atomic nucleus of the other hydrogen atom. Essentially, the electron for the hydrogen on the right is attracted to the nucleus of the hydrogen on the left and the electron for the hydrogen on the left is attracted to the nucleus of the hydrogen on the right. These bond between the nuclei and electrons are the basis of the covalent bond. This why covalent bonds are seen as a sharing of electrons as both electrons experience force from the other atom.

However, covalent bonds have a "sweet spot" as shown by the potential energy diagram on the image above. Basically, the bond energy is minimal when the two hydrogen atoms are 74 picometers from one another. If the bond energy is minimal here, that means the intramolecular force created by the two hydrogen atoms(the force each nucleus exerts on the other electron) is maxed out. However, if the distance increases beyond this, the atoms will repel each other and move away, causing the force to decrease and the bond energy to then increase.

There's two types of covalent bonds and they're entirely distinguished through the property of electronegativity. Remember that electronegativity is a measure of the tendency of an atom to attract shared electrons to itself in a bond. If two identical atoms form a covalent bond, then the two atoms will have equal electronegativity(since it's the same element) and so the electrons between the two atoms will be roughly spaced out. The molecules that are like this are usually diatomic molecules, like O₂, I₂, Cl₂, Br₂, and H₂. This is known as a pure covalent bond and a lot of other molecules with atoms that have fairly equal electronegativities are like this, too. However, there are no pure rules in place, because there are quite a few exceptions to most traditional rules. Thus, unfortunately, it's best to memorize them as they're given to you.

A polar covalent bond arises from the fact that the atoms involved in the molecule have very big differences in electronegativity. This causes a majority of the electrons being shared in the bond to shift over to the atom with the higher electronegativity.

Metallic Bonds