Covalent Bonds

Triple bond

A chemical bond formed when atoms share three pairs of electrons.

Double bond

A chemical bond formed when atoms share two pairs of electrons.

Molecular compound

A compound that is made up of molecules. Water, oxygen, and sucrose (table sugar) are molecular compounds. They contain atoms that are covalently bonded, unlike ionic compounds, which are made up of ions and do not form molecules.

Nonpolar bond

A covalent bond in which electrons are shared equally.

Polar bond

A covalent bond in which electrons are shared unequally.

Describe polar bonds and overall charges of molecules.

A molecule is polar if it has a positively charged end and a negatively charged end. However, some molecules with polar bonds are not polar overall. In carbon dioxide, the oxygen atoms pull electrons more strongly than the carbon atom. The bond between the oxygen and carbon atoms is polar. However, the straight-line shape of a carbon dioxide molecule means that the two oxygen atoms pull equally in opposite directions. This cancels out and causes the molecule to be nonpolar overall. A water molecule, however, has polar bonds *and* is polar overall. The bent shape of the molecule causes the hydrogen atoms to be on one end of the molecule and the oxygen atom to be on the other end. The oxygen atom pulls more strongly than hydrogen, so the end with oxygen is slightly negative and the end with hydrogen is slightly positive.

Molecule

A neutral group of atoms joined by covalent bonds.

Why do polar and nonpolar compounds have different properties?

Because of differences in attractions between their molecules. The melting and boiling point of water is much higher than that of oxygen. The attractions between the polar water molecules require more energy to overcome than the attractions between the nonpolar oxygen molecules.

🔑Unequal sharing of electrons causes . . .

Covalently bonded atoms to have slight electric charges.

What do the electron dot diagrams of single, double, and triple bonds look like?

Diagram on page 140

Why do molecular compounds have low melting and boiling points?

Forces hold the molecules close to one another in a molecular solid. However, the forces between molecules are much weaker than those between ions. Compared with an ionic solid, less heat must be added to a molecular solid to separate the molecules and change the solid to a liquid. Table salt (ionic) melts at 801 C, but table sugar (molecular) melts at a mere 190 C.

🔑The attractions between the shared electrons and the protons in the nucleus of each atom . . .

Hold the atoms together in a covalent bond.

Describe the covalent bonds in water.

Hydrogen has 1 valence electron. Oxygen has 6 valence electrons. In a water molecule, oxygen forms one covalent bond with each of two hydrogen atoms. As a result, the oxygen atom has a stable arrangement of 8 valence electrons. Each hydrogen atom forms only one bond because it needs only two electrons to be stable.

Atoms of some elements pull {more // less} strongly on the shared electrons of a covalent bond than do atoms of other elements.

More. This causes electrons to be shared unequally.

What happens if two atoms pull equally on shared electrons?

Neither atom is charged. This happens when identical atoms bond.

What atoms usually form covalent bonds?

Nonmetal atoms. Ionic bonds usually form between metal and nonmetal atoms.

What holds polar molecules together?

Opposite charges attract. The slight positive and negative charges in molecules are weakly attracted to each other, connecting the polar bond. These attractions are van der Waals forces. The negatively charged oxygen ends of water molecules are attracted to positively charged hydrogen ends of nearby water molecules. Van der Waals forces pull the molecules together. They are also the reason why gecko feet can grip smooth surfaces.

What happens if electrons in a covalent bond are shared unequally?

The atom with the stronger pull gains a slightly negative charge. The atom with the weaker pull gains a slightly positive charge.

Covalent bond

The chemical bond formed when two atoms share electrons

Describe how nonmetals bond.

They bond by sharing electrons. Atoms of some nonmetal elements can bond with each other. Two fluorine atoms can react by sharing a pair of electrons. By sharing electrons, each fluorine atom has 8 valence electrons.

Why are molecular compounds poor conductors?

They do not contain charged particles that are available to move, so there is no current. Some wires are insulated by plastic or rubber because these are molecular compounds that do not conduct electricity from inside the wire. Even as liquids, they are poor conductors. Pure water does not conduct the electric current. Table sugar melted or dissolved in pure water is also not a conductor.

🔑What are some properties of molecular conpounds?

Unlike ionic compounds, molecular compounds usually do not conduct electric current when melted or dissolved in water. Also, compared to ionic compounds, molecular compounds generally have lower melting points and boiling points.

van der Waals forces

Weak attractions between the slight charges in molecules