Quick ReviewCovalent bond – two atoms held together by
sharing electrons -- Usually occurs between nonmetals.
Octet Rule – chemical compounds tend to form so that each atom, by gaining, losing, or sharing electrons, has an octet of electrons in its highest energy level.
Valence electrons – the electrons in an atom’s outer energy level
New TermsMolecule – a neutral group of atoms held
together by covalent bonds
Molecular formula – shows the types and numbers of atoms combined in a single molecule (ex: H2O, CO2, H2SO4)
Diatomic molecule – molecule containing two atoms
(Examples: H2, N2, O2, F2, Cl2, Br2, I2, HCl, NO)
Using Electron Dot Notation diagramsH2 molecule (nonpolar covalent = EQUAL sharing)
H· ·H H : H Structural formula: H-H (Use single line
for e- pair)
the hydrogen orbitals
overlap
· · · ·
F2 molecule
·· ·· ·· ·· : F· ·F : : F : F :
·· ·· ·· ··
Structural formula: F-FEach fluorine atom has 8 electrons in the
outer energy level.
Usually there will be more than two atoms bonding together.Water H2O Each hydrogen atom is bonded to oxygen
atom
H O H O
H H Structural formula H O H
Try these molecules: NH3, ClI, H2O2
NH3 moleculeEach hydrogen atom is bonded to the
nitrogen atom.
H N H
H
Bond Length and Energy
Bond length – the average distance between bonded atoms
Bond energy – energy required to break a chemical bond and form neutral isolated atoms.
Single BondsSingle bonds – a covalent bond produced by
the sharing of one pair of electrons between two atoms.
All of the examples so far have been of single
bonds.
Double BondsDouble bonds – a covalent bond produced by
the sharing of two pairs of electrons between two atoms.
O2 molecule
O O Structural formula O=O
Triple BondsTriple bonds – a covalent bond produced by
the sharing of three pairs of electrons between two atoms.
N2 molecule
N N Structural formula N≡N
RelationshipsBetween bond length and # of bonds:
As the number of bonds INCREASES, the bond length DECREASES
Between bond energy and # of bonds:As the number of bonds INCREASES, the
energy required to break those bonds INCREASES
How to draw Lewis Structures (AKA electron dot diagrams)
Two Types of Electron Pairs:Shared Pair – a bonded pair; exists between 2
atoms in the same molecule. Represented by a straight line between the bonded atoms.
Unshared Pair – a lone pair; belongs entirely to one atom. Represented by a pair of dots on that atom.
Rules for Drawing Lewis Structures:
1) Draw a skeleton structure for the compound by joining the atoms with single bonds (single straight lines). The central atom is usually:
a) The one with the highest number of valence electrons
b) The largest atom
c) The least electronegative atom
Rules for Drawing Lewis Structures:
d) Hydrogen will NEVER be a central atom
e) Oxygen will only be central if bonded to H or F
Rules for Drawing Lewis Structures:2) Count/Tally the number of total valence
electrons
3) Determine the number of electron pairs by dividing the total number of valence electrons (step #2) by 2
4) Determine the number of available electron pairs by subtracting the number of pairs already used in the skeleton structure
Rules for Drawing Lewis Structures:5) Place LONE PAIRS around the terminal/end
atoms. If any pairs are left, put them as lone pairs on the central atom
6) Check the OCTET RULE. If the central atom is not yet surrounded by 8 electrons, form multiple bonds.
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