Chemical bonding

The name’s Bond…….chemical bond.

Chemical

Ionic Bonding• In ionic bonding, electrons are completely

transferred from one atom to another. • In the process of either losing or gaining

negatively charged electrons, the reacting atoms form ions.

• The oppositely charged ions are attracted to each other by electrostatic forces, which are the basis of the ionic bond.

For example, during the reaction of sodium with chlorine: sodium (on the left) loses its one valence electron to chlorine (on the right), resulting in a positively charged sodium ion (left) and a negatively charged chlorine ion (right).

+1 charge-1 charge

Covalent Bonding • The second major type of atomic bonding occurs when

atoms share electrons. As opposed to ionic bonding in which a complete transfer of electrons occurs, covalent bonding occurs when two (or more) elements share electrons.

• Covalent bonding occurs because the atoms in the compound have a similar tendency for electrons (generally to gain electrons).

• This most commonly occurs when two nonmetals bond together. Because both of the nonmetals will want to gain electrons, the elements involved will share electrons in an effort to fill their valence shells.

Lewis structures (aka: electron dot structures).

•The diagrams are named for Gilbert N. Lewis, who described them in his 1916 article entitled The Atom and the Molecule. •Lewis structures depict the bonds between atoms of a molecule as well as any unbonded electron pairs. •You can draw a Lewis dot structure for any covalent molecule or coordination compound.

Lewis Structure Basics •A Lewis structure is a type of shorthand notation.•Atoms are written using their element symbols.•Lines are drawn between atoms to indicate chemical bonds. •Single lines are single bonds. •Double lines are double bonds. •Triple lines are triple bonds. •Dots are drawn next to atoms to show unbonded electrons. A pair of dots is a pair of excess electrons.

Steps to Drawing a Lewis Structure

• Pick a Central Atom Start your structure by picking a central atom and writing its element symbol. This atom will be the one with the lowest electronegativity. Sometimes it's difficult to know which atom is the least electronegative, but you can use the periodic table trends to help you out.

• Electronegativity typically increases as you move from left to right across the periodic table and decreases as you move down the table, from top to bottom.

• For the compounds we will encounter in this class, O, N, C, P and S are the best candidates to start with as the central atom.

• Once you have selected the central atom, write it down and connect the other atoms to it with a single bond.

• You may change these bonds to become double or triple bonds as you progress.

Count Electrons

• Lewis electron dot structures show the valence electrons for each atom. You don't need to worry about the total number of electrons, only those in the outer shells.

• The number indicated over each family (vertical columns) indicates the number of valence electrons.

Practice

How many valence electrons do atoms (neutral) of the following elements have?

CarbonNitrogenOxygenFluorineHelium SodiumNeon

How many electrons do atoms (neutral) of the following elements have?

Carbon = 4Nitrogen = 5Oxygen = 6Fluorine = 7Helium = 2Sodium = 1Neon = 8

Practice

The octet rule

• The octet rule states that atoms with 8 electrons in their outer shell are stable.

• This rule applies well up to period 4, when it takes 8 electrons to fill the outer orbitals.

Place Electrons around Atoms

• Once you have determined how many electrons to draw around each atom, start placing them on the structure.

• Start by placing one pair of dots for each pair of valence electrons. Once the lone pairs are placed, you may find some atoms, particularly the central atom, don't have a complete octet of electrons.

• This indicates there are double or possibly triple bonds. Remember, it takes a pair of electrons to form a bond.

Practice

Write the Lewis structure for the following compound

• CHCl3 (chloroform)

Practice: homeworkWrite the Lewis structure for the following molecules:1. CHCl3 (chloroform)

2. H2

3. O2

4. H2O5. CH4 (methane)6. C2H6 (ethane)7. HBrO (Hypobromous Acid)8. C5H12 (pentane)

2 points each = 18 pts.1attempted 1correct