What is a chemical bond, or just plain bond?

• The generally accepted definition of a bond is a force (some type of interaction) which holds atoms together so the bonded atoms can function as a unit as described by the chemical formula of the substance.

We usually describe a bond as a force because we know it takes energy to separated atoms that are bonded together. The energy required to separate the atoms depends on the atoms that are bonded together.

So what is the force which holds the atoms together? It is a very fundamental force, but it manifests itself in slightly different ways. Fundamentally the force is attractive and the attraction results from the opposite charge of the electron and the nucleus. How the attraction of opposite charges manifest itself in a chemical bond depends on whether the electrons are transferred or shared by the two atoms forming the bond.

How do we know whether an atom wants to transfer or share its electrons?

• Metals love to lose electrons!

So when a metallic atom interacts with a nonmetallic atom electron transfer occurs. But when two nonmetallic atoms interact electrons are shared. It is pretty much that simple!

How many electrons does a metal want to lose?

• A metal in Group IA, IIA or IIIA will lose as many electrons as are required to attain the electron configuration of the nearest noble gas element (Group VIIIA).

For example, sodium has an electron configuration of 1s²2s²2p⁶3s¹. Notice sodium has one valence electron because of the single electron in its outer most shell. By losing that valence electron the electron configuration becomes 1s²2s²2p⁶, which is the same electron configuration for neon (the nearest noble gas). Sodium is in Group IA. All of the metallic elements in Group IA lose one electron. Metals in Group IIA lose two electrons, and metals in Group IIIA lose three electrons. When a metal loses electron (negatively charged) the species left has a positive charge.

An important point about the transition metals: in general they like to lose two or three electrons.

Who gains the electrons lost by the metallic element?

• Nonmetallic elements do!

How many electrons will a nonmetallic element gain?

• A nonmetal in Group VIIA, VIA or VA will gain as many electrons as are required to attain the electron configuration of the nearest noble gas element (Group VIIIA).

For example, chlorine has an electron configuration of 1s²2s²2p⁶3s²3p⁵. Notice chlorine has seven valence electron because of the seven electrons in its outer most shell. By gaining one valence electron the electron configuration becomes 1s²2s²2p⁶3s²3p⁶, which is the same electron configuration for argon (the nearest noble gas). Chlorine is in Group VIIA. All of the nonmetallic elements in Group VIIA gain one electron. Nonmetals in Group VIA gain two electrons, and nonmetals in Group VA gain three electrons. When a nonmetal gains an electron (negatively charged) the species has a negative charge.

When a cation (positively charged metallic ion) interacts with an anion (negatively charged ion) an ionic bond is formed. An ionic bond occurs when the force is electrostatic (due to oppositely charged ions) in nature. Ions are formed when electrons are transferred between a metallic element and a nonmetallic element.

Predict the formula for some ionic compounds. In class we discussed two examples;

Determine the formula of the compound formed from Na and Cl₂. To solve this problem we recognized one of the elements is a metal. Metals like to lose electrons. How many electrons will sodium lose? Since it is in Group IA, it has one valence electron and it will lose that one electron, forming Na⁺. Chlorine, on the other hand, is a nonmetal. In the presence of a metal, nonmetals gain electrons. How many electrons will chlorine gain? It is in Group VIIA so it will gain one electron, forming, Cl^-. To write the formula we determine how many of each ion is required to balance the charge, or have a total charge of zero. In this case the formula must be NaCl. (Note: Even though the standard state of chlorine is a diatomic molecule, Cl₂, when we write formulas and consider the number of electron gained or lost by an element we consider the element in terms of a single atom. When write a chemical equation to describe the reaction between sodium and chlorine and the product formed balancing the equation resolves the total number of atoms issue.)

Determine the formula of the compound formed from Na and O₂. To solve this problem we recognized one of the elements is a metal. Metals like to lose electrons. How many electrons will sodium lose? Since it is in Group IA, it has one valence electron and it will lose that one electron, forming Na⁺. Oxygen, on the other hand, is a nonmetal. In the presence of a metal, nonmetals gain electrons. How many electrons will oxygen gain? It is in Group VIA so it will gain two electron, forming, O^2-. To write the formula we determine how many of each ion is required to balance the charge, or have a total charge of zero. In this case the formula must be Na₂O. (Note: Even though the standard state of chlorine is a diatomic molecule, O₂, when we write formulas and consider the number of electron gained or lost by an element we consider the element in terms of a single atom. When write a chemical equation to describe the reaction between sodium and chlorine and the product formed balancing the equation resolves the total number of atoms issue.)

So now we need to spend a minute thinking about what an ionic compound 'looks like' when viewed at the atomic level. Since ionic compounds are composed of ions, and when we think of an ion the best shape to imagine is a sphere. Anions are usually larger spheres than cations, but both are spheres. The sphere is the best shape because ions have filled electronic shells. So the best way to think about the arrangement of the ions at the atomic level is in three dimensional arrays where each ion is surrounded by the ion of opposite charge. Here is an example of a small section of the arrangement common for alkali halides like NaCl.

Discuss covalent bonding.