CHEMICAL BONDING
Covalent compounds Covalent bonds are formed by atoms sharing electrons to form molecules. This type of bond usually formed between two or more non-metallic elements. The molecules might be that of an element i.e. one type of atom only OR from different elements chemically combined to form a compound.
Forming covalent compounds Covalent bonding a bond formed when atoms share one or more pairs of electrons. Single Covalent Bond sharing of 1 pair of electrons Double Covalent Bond sharing 2 pairs of electrons between the same two atoms Triple Covalent Bond sharing 3 pairs of electrons between the same two atoms Quadruple Covalent Bond? Not possible!!!
Properties of Covalent Compounds Can be solids, liquids or gases at room temperature Molecular solids are softer than ionic solids Have low melting points Have low boiling points Most are poor conductors of electricity in all phases Many are soluble in non-polar solvents Most are not soluble in water
Diatomic molecules: A special type of covalent bond between two of the same atom. It is the only covalent bond that is 100% covalent There are seven atoms that exist normally as diatomic molecules. Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine Br-I-N-Cl-H-O-F Or the rule of 7: Start with 7, make the number 7, and the seventh is the exception (hydrogen)
Covalent Crystals Always solids Composed of one large molecule Very stable Diamond and graphite are examples Diamond Graphite Quartz (SiO 2 )
Different Types of Covalent Bonds? Non-Polar: Electrons are shared about equally between two atoms. Polar: Electrons are shared unevenly, causing an imbalance of charge in the molecule. Because of this, the molecule behaves a little like an ionic compound.
Polar Bonds Polar Covalent Bonds are those in which the electrons are not shared equally One end of the bond has a slightly negative charge (d-) The other end has a slightly positive charge (d+) This is determined by the difference in electronegativity of the two atoms joined by the bond.
Covalent Bonds Polar Covalent unequal sharing of the electrons causes the oxygen to have a slight negative charge and the hydrogens to have slight positive charges. Covalent 2 hydrogen atoms share electrons with an oxygen atom.
The type of bond can usually be calculated by finding the difference in electronegativity of the two atoms that are going together.
Electronegativity
Electronegativity Difference If the difference in electronegativities is between: 0.0 to 0.4: Non-Polar Covalent 0.4 to 1.7: Polar Covalent 1.7 to 4.0: Ionic Example: NaCl Na = 0.8, Cl = 3.0 Difference is 2.2, -so this is an ionic bond
Covalent Bond Review http://media.pearsoncmg.com/bc/bc_c ampbell_biology_7/media/interactive media/activities/load.html?2&d
Representations of Covalent Bonds
Covalent nomenclature Covalent compounds are formed from the bonding between non-metallic elements. Covalent Nomenclature Rules: 1. The more positive atom is written first (the atom which is the furthest to the left and to the bottom of the periodic table) 2. The more negative second atom has an "-ide" ending. 3. Each prefix indicates the number of each atom present in the compound.
Covalent prefixes Number of Atoms 1 2 3 4 5 Prefix Number of Atoms Prefix mono 6 hexa di 7 hepta tri 8 octa tetra 9 nona penta 10 deca
How do covalent bonds differ from ionic bonds?
Bond Type Ionic Composed of a cation and an anion. Electrons are transferred A metal loses one or more electrons to form the cation A nonmetal gains one or more electrons to form the anion Ionic compounds are called salts Covalent Composed of two or more atoms Electrons are shared among two or more nonmetals. Covalent compounds are called molecules Seven elements exist as molecules: H 2, N 2, O 2, F 2, Cl 2, Br 2 and I 2
Sodium Chloride (Table salt) - Ionic Cl -1 Na +1 Cl -1 Na +1 Cl -1 Na +1 Cl -1 Na +1 Cl -1 Na +1 Cl -1 Na +1 Cl -1 Na +1 Cl -1 Na +1 Cl -1 Na +1 Cl -1 Na +1
Sodium Chloride - Rotatable http://www.cem.msu.edu/~reusch/vir tualtext/intro2.htm#strc2
Chlorine Gas - Covalent Two chlorine atoms One chlorine molecule Cl 2
Physical Properties Differ Property NaCl Cl 2 Phase at room temperature Density Solid 2.165 g/cm 3 Gas 0.003214 g/cm 3 Melting point 801 o C -100.98 o C Boiling point 1413 o C -34.6 o C Ability of aqueous solution to conduct electricity Conducts Does not conduct
Review of Chemical Bonds Most bonds are somewhere in between ionic and covalent. There are 3 forms of bonding: Ionic complete transfer of 1 or more electrons from one atom to another (one loses, the other gains) forming oppositely charged ions that attract one another Covalent some valence electrons shared between atoms Metallic holds atoms of a metal together