UNIT VIII The Periodic Table and Chemical Bonding Lesson 4.

UNIT VIIIThe Periodic Table and Chemical Bonding

Lesson 4

Review

Going DOWN a FAMILY ----- size of atom increases and distance between valence electrons and nuclei increase Going ACROSS a PERIOD ----- properties are affected by differing valence, nuclear charge and charge on the species

One more piece of info on trends…Electronegativity!

Electronegativity Electronegativity – the tendency of an atom to attract electrons from a neighbouring atom.

Hey! I find your electrons attractive!

Get lost, loser!

VIII.3 CHEMICAL BONDING

ELECTRONEGATIVITY --- tendency of the atom to attract electrons from a neighbouring atom Going from left to right across the periodic table the electronegativity of the atoms will __________________

Going down a family of the periodic table the electronegativity of the atoms will _____________________

Electronegativity increases as you move from left to right.

Electron

egativity decreases as you move dow

n each column

.

Ex: Which is more electronegative: K or Br? Pb or S?

Electronegativity & Ionization Energy(IE)

If an atom has high electronegativity, it strongly attracts e- from a neighbouring atom (also difficult to remove its own e- so it has high IE)

If an atom has low electronegativity, it has little attraction to the electrons of neighbouring atoms (easy to remove e- so low IE)

Electronegativity and IE have the same trend on PT!

SUMMARY OF PERIODIC TABLE TRENDS:

SIZE OF IONS VS SIZE OF A NEUTRAL ATOM

SIZE OF IONS vs NEUTRAL atoms

 Li → e- + Li+

1 valence e- closed shell

Positive ions are smaller than the corresponding neutral atom

Why??

(fewer electrons, so less repulsion between them).

SIZE OF IONS vs NEUTRAL atoms

Cl + e- → Cl-

7 valence e- closed shell

Negative ions are larger than the corresponding neutral atom

Why??

(more electrons, so more repulsion between them).

Types of chemical bonds

Ionic and covalent

Types of Chemical Bonds

A. Ionic Bonding:  Ionic Bond –

VIII.3 CHEMICAL BONDING

Ex: Na + Cl

Ex: Ca + O

*The greater the distance between the charges, the smaller the attraction!

NaCl

Crystal Lattice

Li F

Li F

A Li Atom An F Atom

A Li+ Ion

-+

An F- Ion

The melting points of some Ionic Compounds are as follows:

NaF 993 oCKCl 770 oCLiCl 605 oC

These high melting points are experimental evidence that Ionic Bonds are VERY STRONG. (Hard to break just by heating).

Ionic Bonds

IONIC BONDS are VERY STRONG, so compounds held together by ionic bonds have HIGH MELTING TEMPERATURES

Think about it…*When there is a smaller radius, there is a stronger/weaker bond*When there is a larger charge, there is a stronger/weaker bond

Covalent Bonds

Covalent Bonds

b. Covalent Bonding:

 Covalent Bond:

Octet Rule:

In Covalent bonds, electrons are Shared

VIII.3 CHEMICAL BONDING

Ex: Cl – Cl

 Non-mental bonds to a nonmetal (SHARE e-)

Covalent bonds are STRONG, but the bonds between molecules are WEAK

ex: Cl2 molecules bonded together 

Covalent Bonds

Polar Covalent

HCl

Non-polar Covalent

Cl2

diamond structure

Diamonds are “forever”!

Some melting points of Network Solids:

Diamond (Carbon) 3550 oCSilicon Carbide (SiC) 2700 oCBoron Nitride (BN) 3000 oC

Covalent bonds are also very strong!

HOWEVER LOOK AT THESE MELTING POINTS…

CH4 -182 oC

O2 -218 oC

F2 -220 oC

How come they are so low??

Third type of chemical bonding…

“Van der waals Forces”

The sum of the attractive or repulsive forces between molecules

other than those due to covalent bonds or the electrostatic interaction of ions with one another.

Third type of chemical bonding…

“Van der waals Forces”

INTRAmolecular INTERmolecular (within/inside the molecule) (between)

COVALENT BONDS Van Der Waals

I

I

The covalent intramolecular bond in I2 is very strong.

I

I

I

I

I

I

I

I

I

IThere are weaker intermolecular forces which hold covalent molecules together in a molecular solid.

Intra Vs Inter molecular forces

2 TYPES OF VAN DER WAALS

London forces London Forces are weak attractive forces which

exist between atoms as a result of temporary dipoles

Dipole-dipole forces A permanent dipole

So what are dipoles??????

What are dipoles?

Dipole –

Polarization causes dipoles!

Polarization –

LONDON FORCES

London Forces – 

London Forces

London Forces: The WEAKEST TYPE of bonding force

ALWAYS present, even in species which have covalent or ionic bonding

IMPORTANT??? when they are the only force of attraction....adjacent noble gas atomsadjacent covalently-bonded molecules

The greater the atomic number, the stronger the London forces it experiences

HOMEWORK

P172 #57 P173 #59,60 P 175 #62,63,64

P177 #68, 69 P 179 #72 from a to k P 181 #74,76,78

Read summary on page 182 and try to solve the summary exercises if you have time.