Covalent Bonds

Gases, liquids, or solids (made of molecules) Low melting and boiling points Poor electrical conductors in all phases Many soluble in nonpolar liquids but not in water

Covalent Bonds Covalent compounds do not give up electrons or accept electrons as readily as sodium does in combining with chlorine (ionic bond). Instead a “tug of war” for electrons takes place between the atoms, ending more or less in a standoff.

Covalent Bonds A hydrogen atom has a single valence electron. A pair of hydrogen atoms shares electrons to form a diatomic hydrogen molecule In this molecule, each hydrogen achieves the electron configuration of the noble gas helium, which has two valence electrons. The hydrogen atoms form a single covalent bond

Single covalent bond single covalent bond: a bond in which two atoms share a pair of electrons When writing the formula for a covalent bond, the pair of electrons is represented by a dash, this is called a structural formula and is never used to represent an ionic bond

Single covalent bond Notice that the two fluorine atoms share only one pair of valence electrons. Pairs of valence electrons that are not shared between atoms are called unshared pairs unshared pairs: also known as lone pairs or nonbonding pairs; a pair of electrons that is not involved in bonding All halogens form single covalent bonds in their diatomic molecules, fluorine is one example

Some examples of covalent bonding…

H 2 O

How would you put together N and H to form a compound? (ammonia)

How about C and H? (methane - natural gas)

Draw the electron dot structure for HCl

Double and Triple Covalent Bonds Atoms sometimes share more than one pair of electrons to attain stable, noble-gas electron configurations Double covalent bonds: bonds that involve two shared pairs of electrons Triple covalent bonds: bonds that involve three shared pairs of electrons

Examples… Carbon dioxide

Examples… O 2 octet rule not satisfied - 7 electrons on each oxygen octet rule not satisfied on both oxygen atoms - 6 electrons on one and 8 electrons on the other Multiple bonds - have two sets of shared electrons. The octet rule is satisfied on both oxygen atoms. Again, the lone pairs are often omitted, but if you understand electron configuration you know that they must be there!

Examples… N 2 Triple bond

Coordinate Covalent Bonds Carbon monoxide (CO) is an example of a type of covalent-bonding different from that seen in water, ammonia, methane, and carbon dioxide. A carbon atom needs to gain four electrons to attain the electron configuration of neon (octet). An oxygen atom needs two electrons. It is possible for both atoms to achieve an octet by a type of bonding called coordinate covalent bonding

Coordinate Covalent Bonds With the double bond in place, the oxygen has a stable configuration but the carbon does not The dilemma is solved if the oxygen also donates one of its unshared pairs of electrons for bonding

coordinate covalent bond: a covalent bond formed when one atom contributes both bonding electrons The polyatomic ammonium ion (NH 4 + ) has a coordinate covalent bond

Covalent Bonds Most polyatomic cations and anions contain covalent and coordinate covalent bonds. The atoms in polyatomic ions are covalently bonded. You can write electron dot structures for these ions. The negative charge of a polyatomic ion shows the number of electrons in addition to the valence electrons of the atoms present. Because a polyatomic ion is part of an ionic compound, the positive charge of the cation of the compound balances these additional electrons

Example Draw the electron dot structure of the sulfite ion (SO 3 2- ), in which sulfur is the central atom Draw the atoms, their valence electrons, and the two extra electrons

Answer Join two of the oxygens to sulfur by single covalent bonds Join the remaining oxygen by a coordinate covalent bond and add the two extra electrons. Put brackets around the structure and indicate the 2- charge

Problem The polyatomic hydronium ion (H 3 O + ) contains a coordinate covalent bond. It forms when a hydrogen ion is attracted to an unshared electron pair in a water molecule. Write the electron dot structure of the hydronium ion

Answer

Exceptions to the Octet Rule Resonance resonance structures: one of two or more equally valid electron dot structures for a molecule or polyatomic ion; the actual bonding is a hybrid, or mixture, of the resonance structures There is sometimes an ambiguity in the location of double bonds. This ambiguity is illustrated by the Lewis structure for ozone (O 3 ). The following are two possible structures:

Resonance In such cases, the actual Lewis structure is regarded as a blend of these contributions and is written:

Resonance For some molecules or ions, it is impossible to write structures that satisfy the octet rule. This occurs whenever the total number of valence electrons is an odd number. The NO 2 molecule, for example, contains a total of seventeen valence electrons. Each oxygen contributes six electrons and the nitrogen contributes five

More examples

1) sulfur dioxide (SO 2 ) 2) nitric acid (HNO 3 )