Bonding Basics

Two general types of bonds:

Covalent – sharing of electrons between two atoms

Ionic – electrostatic forces that exist between ions of opposite charge

Molecule – two or more atoms covalently bound together

Diatomic molecule – two of the same atom bound together

Binary Compounds consist of 2 elements

Binary covalent compounds can be recognized by containing 2 nonmetals

This is different from ionic compounds that contain a metal & nonmetal, metal & a polyatomic ion, or 2 polyatomic ions

A covalent bond is a chemical bond in which two or more electrons are shared by two atoms. Why should two atoms share electrons?

Form because the shared electrons in the bond are attracted to two different nuclei

Attraction to another nuclei overrides the added electron-electron repulsion

Majority of chemical substances are covalently bonded

Can be represented using Lewis structures

Shared electrons typically represented by a line

• Single bond represents one shared pair
• Double bond represents two shared pairs
• Triple bond represents three shared pairs

Unshared electrons typically represented by a dot (or x)

Lewis Structures – shows how the valence electrons are arranged among the atoms of a molecule

There are rules for Lewis Structures that are based on the formation of a stable compound

Atoms want to achieve a noble gas configuration

Octet Rule – atoms ‘want’ to have 8 valence electrons

Duet Rule – H is the exception. It ‘wants’ to be like He & is stable with only 2 valence electrons

1. Sketch a simple structure with a central atom and all attached atoms

2. Add up all of the valence electrons for each individual atom

If you are drawing a Lewis structure for a negative ion add that many electrons to create the charge

If you are drawing a Lewis structure for a positive ion subtract that many electrons to create the charge

3. Subtract 2 electrons (or 1 pair) for each bond drawn

4. Complete the octet on the central atom & subtract those electrons

5. Complete the octet on the surrounding atoms & subtract those electrons

6. Get your final number

If 0 you are done!

If + add that many electrons to the central atom

If - need to form multiple bonds to take away that many electrons

Often it is possible to write more than one correct Lewis structure for a molecule or ion

When two or more correct Lewis structures differ only by placement of electrons, they are called resonance structures

As the number of bonds increases, the bond length decreases

The shorter the bond, the stronger the bond

• Pure covalent bond – atoms having identical or nearly identical electronegativities resulting in an equal sharing of electrons

• Polar covalent bond – atoms having differing electronegativities resulting in an unequal sharing of electrons & a partial charge

Valence shell electron-pair repulsion model

Predict the geometry of molecules formed from nonmetals

IDEA: structure determined by the minimalization of electron-pair repulsions

Bonding and non-bonding pairs will be positioned as far apart as possible around a given atom

Draw the Lewis structure for the molecule

1. When a molecule exhibits resonance, any one of the resonance structures can be used to predict molecular structure
2. Count the electron pairs and put the pairs as far apart as possible (minimize electron pair repulsion)
3. Multiple bonds count as one effective pair
4. A 120º angle provides lone pairs with enough space so that distortions do not occur
5. Determine the positions of the atoms from the way the electron pairs are shared
6. Determine the name of the molecular structure from the position of the atoms

1. Draw the Lewis structure for the molecule
   
2. Count the total number of ‘things’ that are around the central atom to determine the electron pair geometry.
   
3. Imagine that the lone pairs of electrons are invisible and describe the molecular shape.

If there are 2 electron pairs attached to the central atom, the shape is linear.

• If there are 3 electron pairs the shape will be trigonal planar.

• Bond angle = 120°

• Now imagine that you have 3 electron pairs, but one is just a lone pair (invisible). What would it look like then?

If there are 4 electron pairs, the shape will be tetrahedral.

What if 1 of the electron pairs is a lone pair (invisible)? What would it look like then?

Trigonal Pyramidal

Bent!

If there are 5 electron pairs the shape will be trigonal bipyramidal.

Bond angles = 90º & 120º

Seesaw

What if there are 2 lone pairs (invisible)?

T-shaped

Bond angle = 90°

Octahedral

Square pyramidal

What if there are 2 lone pairs (invisible)?

square planar

Here’s a shortcut for how to determine the hybridization of an atom in a molecule. This will save you a lot of time.

–BEGIN SHORTCUT–

Here’s what you do:

1. Look at the atom.
2. Count the number of atoms connected to it (atoms – not bonds!)
3. Count the number of lone pairs attached to it.
4. Add these two numbers together.

• If it’s 4, your atom is sp3.
• If it’s 3, your atom is sp2.
• If it’s 2, your atom is sp.

(If it’s 1, it’s probably hydrogen!)

Complete transfer of electrons from one atom to another to form ions

• Metal is OXIDIZED (lose electrons to form cations)
• Non-metal is REDUCED (gains electrons to form anion)

OIL RIG