Chemical BondingForces that Ties

Behind the Ties

The term “Valence” has various meaning.

• It came from a latin word “Valentia” meaning “power” or “capacity”

• It is also known as valency or valency number, which is the number of chemical bonds formed by atoms.

• Valence electrons refer to the electrons in the outer most energy level that are involved in the chemical bonding or reaction.

Behind the TiesLewis Dot Structure• The Lewis structure was named

after Gilbert Newton Lewis, who introduced it in 1916.

• Dots and dashes were used in representing valence electrons, lone pairs and bonds between electrons of elements.

• In the diagrams, only the valence electrons are shown.

• A dot represents an electron and a dash represents a bond of 2 electrons .

Behind the Ties

Lewis Dot Structure

Behind the Ties

Lewis Dot Structure (Group A Elements)

Behind the Ties

Oxidation States – hypothetical charge of an ion.

**a neutral (free) atom (or element) has an oxidation number of zero.

• Group IA = +1 (Alkali Metals)• Group IIA = +2 (Alkali Earth Metals)**These groups are metals and are

highly reactive. They have the highest metallic property (Group IA).

Behind the Ties

Group IIIA = +3 (Boron Group: Earth Metals) except Tl +1

Group IVA = +4 (Carbon Group)Group VA = -3 for 3 elements, +3 for

Sb and Bi (Nitrogen Group)

Group VIA = -2 (Chalcogen Group) except for Po

+4Group VII = -1 (Halogen Group)

The Objective: Quest for Stability

• Elements would try to have 8 electrons in their outermost energy level to become stable to have the same configuration with the noble gases.

• Octet Rule – is a rule of thumb that states the atoms (or ions) combine to have 8 electrons in their valence shell (outer most).

** this is mostly applicable to Group A elements. There are exemptions to the rule.

The Objective: Quest for Stability

• Duet rule – where they follow He as their noble gas structure. Applicable for H, Li, Be, B and sometimes C.

• 18 electron rule – applicable to transition elements (d block elements)

**in this quest, atoms (ions) may either give (Ionic Bonding) or share (Covalent Bonding) their electrons to be stable and become noble (as to noble gases)

Ionic or Covalent

Electronegativities can predict the nature of bond in a compound.

**subtract the electronegativity values of the involved elements.

Electronegativities and Bond Polarity

0 – less than 0.4 = non-polar covalent bond

0.4 – 1.78 = polar covalent bondGreater than 1.78 = ionic bond

Ionic or Covalent

Examples:F – Na: 4.0 – 0.9 = 3.1 Ionic BondBr – Fr: 2.8 – 0.7 = 2.1 Ionic BondN – C: 3.0 – 2.5 = 0.5 Polar

CovalentS – Se: 2.5 – 2.4 = 0.1 Non-polar

CovalentBr – O: 3.5 – 2.8 = 0.7 Polar

Covalent

Ionic or Covalent

Ionic Bonding: Giving of Electrons

Ionic bond – is the giving or donation of electrons.

** being observed by metals (Group IA – IIIA and non-metals (Some of Group VA – VIIA).

Ionic Bonding: Giving of Electrons

Atoms donate or accept their electrons giving them a net charge.

The one that donates an electron becomes a positively charged ion and the one that receives becomes a negatively charged ion.

This would cause an attraction that will form the IONIC COMPOUND.

Ionic Bonding: Giving of Electrons

Ionic Bonding: Giving of Electrons

Properties of Ionic Compounds:They form a Crystal Lattice Structure.

Because of the opposite charges, they arrange themselves alternately.

Ionic Bonding: Giving of Electrons

Properties of Ionic Compounds:• High melting and boiling pointsIt takes high amount of thermal energy to

separate the strong electrical forces.

• Electrical ConductivitySolid ionic compounds do not conduct

electricity because once attracted, they become neutral. There are no free charges to carry the electricity. Electrons are bonded tightly

Ionic Bonding: Giving of Electrons

Properties of Ionic Compounds:Once dissolved or molten, the structure

breaks thus opening the charges. Electrons now move freely and can carry electricity. They become electrolytes. There are also solid electrolytes.

• HardnessMost ionic compounds are hard because

ions are strongly bound to the lattice and are not easily displaced.

Ionic Bonding: Giving of Electrons

Properties of Ionic Compounds:

• Brittleness Most ionic compounds are brittle, they

shatter when distorted. Once distorted, the structures is ruined, they do not stick together anymore for the charges will not be placed as they were before. Broken ends will end up repelling each other.

Covalent Bonding:Sharing of Electrons

Covalent Bonding – the sharing of electrons. This happens commonly to non-metals (Groups IVA – VIIA ) where both attract electrons. The energy levels overlap to share the electron.

Covalent Bonding:Sharing of Electrons

2 Subtypes of Covalent Bonding:Nonpolar Bond – happens to atoms with

similar attraction (affinity). Commonly observed in diatomic molecules. (H2, N2, O2) The difference in their E.N. is less 0.4. The electrons are evenly shared.

Polar Bond – happens when one atom has a greater affinity. The electrons spend most of the time with the atom of greater affinity. This will cause an uneven sharing of electrons and will form partial charges.

Covalent Bonding:Sharing of Electrons

Non-Polar Covalent Bond

Polar Covalent Bond

Covalent Bonding:Sharing of Electrons

Covalent Bonding:Sharing of Electrons

Covalent Bonding:Sharing of Electrons

Properties of Covalent Compounds• Generally are weak compounds, they are

usually liquid or gases. But there are also hard and solid covalently bonded compounds like granite and diamonds.

• They are known as network solids or covalent network solids. They are bonded continuously in a network leaving no free molecule thus making a solid structure.

Covalent Bonding:Sharing of Electrons

Properties of Covalent Compounds• The more bonds the compound has, the stronger

their bonds.

• Limited amount of electricity is conducted, sometimes none. There are no open electrons for they are shared by atoms. Polar bonds has the tendency to conduct electricity because of the partial charges.

• They are soft because the attraction is quite weak (compared to ionic compounds) and could easily be displaced.

Covalent Bonding:Sharing of Electrons

Properties of Covalent Compounds• They have low melting and boiling

points. The weak attraction requires a little amount of thermal energy to separate these compounds.

A snowflake’s symmetry reflects the ordering of water molecules when frozen.