5.3 Covalent Bonds• Key Concepts:• What holds covalently bonded atoms together?• What are the properties of molecular

compounds?• How does unequal sharing of electrons occur, and

how does it affect molecules?• Key terms:– Covalent bond, molecule, double bond, triple bond,

molecular compound, polar bond, nonpolar bond

How Covalent Bonds form

• Covalent bond – the chemical bond created when two atoms share electrons

• Electron sharing– The force that holds atoms together in a covalent

bond is the attraction of each atom’s nucleus for the shared pair of electrons.

– (molecule- neutral group of atoms joined by covalent bonds)

Wait

• What’s the diff. between a covalent and an ionic bond?

• In an ionic bond, atoms are attracted because of ions’ opposite charges. Ions formed because atoms lost or gained electrons.

• In a covalent bond, atoms are held together because of shared electrons.

How many bonds?

• With few exceptions (like Hydrogen and Helium), the number of covalent bonds that nonmetal atoms can form equals the number of electrons needed to make a total of 8. (Hydrogen and Helium only need 2)

• Just like before, but now they’re sharing. Like friends in kindergarten.

It’s like dis…

• Oxygen has six valence electrons.

• How do we know that? Well, it’s in group 16.

• So, it looks like….

It’s like dis…

• Hydrogen needs 2 (as we mentioned before) valence electrons.

• It’s in group 1, so how many does it have? It has…. Drum roll.. 1 electron.

• It looks like a dis….

So

• Oxygen would like 8 to be stable, and Hydrogen has 1 and would like 2 to be stable.

• How would they come together in nature, when both are present?

• Oxygen needs 2 more valence electrons to be stable, so it bonds with two hydrogen atoms. The hydrogen atoms are happy with 2 shared electrons, and the oxygen atoms are happy with 8 electrons. Everyone’s happy.

Happy because his electrons are happy

What about other stuff?

• Well, Carbon, for example, is in group 14.

• So how many valence electrons does it have?

• Oh well uh, 4. It’s in group 14 so it has 4. Tada.

• Nitrogen is in group 15 so it has… 5. Boom.

• Look at Nitrogen. It has 5 valence electrons. How many covalent bonds can it form?

• Look at Boron, it has 3 valence electrons. It is in group 13. How many covalent bonds can it form?

Wait a minute!• What’s that weird stuff in ethylene and

acetylene?

• That weird double set of electrons?

• It’s called a “double bond” – when atoms share two pairs of electrons.

• A “triple bond” is when atoms share 3 pairs of electrons.

Example of double bonds

Molecular compounds

• A molecular compound is a compound that is composed of molecules. (LOL – this is a definition from your book, you’re welcome)

• The atoms in a molecular compound are covalently bonded (meaning they share electrons). Molecular compounds usually have low melting points and boiling points and they do not conduct electricity when dissolved in water.

Why?

• Because the forces between molecules are much weatker than the forces between ions in an ionic solid. So less heat must be added to molecular solids to separate the molecules.

Also, molecular solids

• Are poor conductors. • Most molecular compounds do not conduct

electrical current

Unequal Sharing of electrons

• Atoms of some elements pull more strongly on shared electrons than do atoms of other elements. As a result, the electrons are pulled more toward one atom, causing the bonded atoms to have slight electrical charges.

Polar Bonds and nonpolar bonds

• The unequal sharing of electrons is enough to make the atom with the stronger pull slightly negative and the atom with the weaker pull slightly positive.

• A covalent bond in which electrons are shared unequally is called a polar bond.

• If they are pulled equally, it is a nonpolar bond.

Polar Bonds in molecules

• A molecule may contain polar bonds and still be nonpolar overall.

Attractions

• There is little attraction between nonpolar molecules.

• The properties of polar and nonpolar compounds differ because of their differences in attractions.