CHAPTER 1

STRUCTURE DETERMINES PROPERTIES

ATOMIC STRUCTURE

nucleus(contains protons (+) and neutrons, most of the atomic mass)

cloud of electrons(negatively charged,very small mass)

number of protons determines the element

combinations are possible, with a constant number of protons but varying number of neutrons - ISOTOPES

1H - 1 proton, 0 neutrons2H - 1 proton, 1 neutron (deuterium)3H - 1 proton, 2 neutrons (tritium)

Chapter 1: Introduction and Review

ORBITALS AND SYMMETRY

x

y

z

1s-orbital(spherical symmetry, highest density at nucleus)

x

y

z

2p-orbital(axial symmetry, nodal plane through nucleus)

nodal plane

x

y

z

three 2p-orbitals(equal energy - degenerate orbitals)

Chapter 1: Introduction and Review

x

y

z

2s-orbital(spherical symmetry, has a nodal region )

THE BUILDING OF THE ELECTRONIC CONFIGURATION OF ATOMS

1. A maximum of two (2) electrons can be placed in any single orbital (The Pauli’s exclusion principle)

2. Electronic states are occupied in order of increasing energy.

3. Degenerate states (of equal energy!) are first all single occupied (The Hund’s rule).

Lewis’ Octet Hypothesis

•Atoms will enter into chemical reactions and form bonds with the principle goal to achieve octet configurations of their valence shells

ELECTRONEGATIVITY – PAULING’S SCALE

H 2.2

Li 1.0

Be 1.6

B 1.8

C 2.5

N 3.0

O 3.4

F 4.0

Na 0.9

Mg 1.3

Al 1.6

Si 1.9

P 2.2

S 2.6

Cl 3.2

K 0.8

Br 3.0

I 2.7

Chapter 1: Introduction and Review

RULES FOR LEWIS STRUCTURES

Chapter 1: Introduction and Review

• Count the total number of electrons available (A)• Calculate the total number of necessary electrons (B), in order for each atom to achieve an octet (Hydrogen needs only two electrons!!)• Subtract and divide: (B – A)/2 = number of bonds• Construct a meaningful structure with the above calculated number of bonds• If electrons remain, first assign lone pairs to the terminal atoms, then to the central atoms, wherever necessary

• Do not forget: Hydrogens are ALWAYS terminal!

RESONANCE STRUCTURESA resonance structure is acceptable if:

• it is a meaningful Lewis structure (Please refer to rules for writing Lewis structures!)• electrons are moved but the skeleton of the molecule is retained

A resonance structure has a greater importance if:

• it contains a greater number of bonds and atom octets• it has minimum charge separation• the negative charge resides on a more electronegative atom

Chapter 1: Introduction and Review

COMMON BONDING PATTERNS

Atom ValenceElectrons

PositivelyCharged Neutral

NegativelyCharged

B

C

N

O

Halogen(Cl, Br or I)

3

4

5

6

7

C

N

O

B

C

N

O

B

C

N

O

ClCl Cl

Chapter 1: Introduction and Review