Molecules and Covalent Bonding

 

Molecules and Covalent Bonding

Atoms combine to form a larger and more complex entity called a molecule.

Molecule are composed of two or more atoms held together by chemical bonds.

The electrons of an atom contain energy. 

This energy is stored within the charge and movement of electrons and the bonds that atoms make with one another.

 However, this energy is not always stable, depending on the number of electrons within an atom. Atoms are more stable when their electrons orbit in pairs. 

 An atom with an odd number of electrons must have an unpaired electron. In most cases, these unpaired electrons are used to create chemical bonds. A

is the attractive force between atoms and contains energy. By bonding, electrons find pairs, and atoms become part of a molecule.

The most stable situation for an atom is to have its outer shell completely filled with electrons. It is not easy to explain why this is true, but it’s a rule of thumb that predicts how atoms will react with each other. The first electron shell of an atom is considered full (or stable) when it contains two electrons, and the second and third shells are full (stable) with eight electrons. Atoms tend to bond to other atoms in such a way that both atoms have filled outer shells as a result of the interaction. While some elements may be able to hold more electrons in their third shell, most of the important elements in biology (e.g. hydrogen, carbon, nitrogen, and oxygen) are considered stable with eight electrons in this outer shell.

Instead of transferring their electrons completely, atoms typically remain in very close contact and share electrons so that their outer shells are filled. 

In essence, a shared electron is counted “twice” and participates in a larger shell that joins the two atoms. A single pair of shared electrons makes a single covalent bond. Atoms can also share two pairs of electrons (in a double bond). This sharing of electrons is called a

. Covalent bonds are the strongest, most stable types of chemical bonds in the biological world.

One example of covalent bonding to form a molecule is the formation of methane, a colorless and flammable gas that results from burning gasoline or fossil fuels (Fig. 1.4). 

One carbon atom and four hydrogen atoms react to form methane.

 The outer shell of carbon has four electrons, so carbon can share an electron with four other atoms (which will then give carbon a full outer shell of 8 electrons). 

Hydrogen has a single electron in its outermost shell and can share this electron with one other atom. 

The carbon atom forms a covalent bond with four hydrogen atoms to form a molecule of methane.

 In a methane molecule, carbon effectively has a “full” second shell (eight electrons) and each hydrogen has a “full” first shell (two electrons).

 Each hydrogen requires one covalent bond to fill its first shell. Each carbon requires four covalent bonds to fill its second shell.