Structure And Properties

An Introduction to Structure and Properties 1. Stable Octet: an element has a stable octet when 8 electrons occupy the highest energy levels (8 valence electrons). All elements strive for a stable octet, with the exception of “He” – its highest energy level is filled, but it only has 2 electrons (1s2).

Formations of Ions: Na: 1s2 2s2 2p6 3s1  loses 1 electron (Na+) Mg: 1s2 2s2 2p6 3s2  loses 2 electrons (Mg2+) N: 1s2 2s2 2p3  gains 3 electrons (N-3) F: 1s2 2s2 2p5  gains 1 electron (F-)

UNIT

2:

STRUCTURE

AND

PROPERTIES

PAGE

1

OF

6

Electron Dot Diagrams Electron Dot Diagrams consist of an element symbol surrounded by “dots”. The dots represent valence electrons, and 8 electrons is the max.

To draw an Electron Dot Diagram, you must start on the right side of the chemical’s symbol and go clockwise, adding dots one at a time.

UNIT

2:

STRUCTURE

AND

PROPERTIES

PAGE

2

OF

6

Lewis Structures Lewis Structures are diagrams that show the chemical bonds – an electrostatic attraction between two particles, composed of 2 electrons. – between elements, as well as their “lone pairs”.

1. Central Atom  an atom that contains the greatest number of single electrons, and can therefore make the most bonds.

Examples:

Things to Remember: •

Whatever chemical formula can make the most bonds goes in the middle, and is usually known as the “central atom”.

• • • •

UNIT

2:

If you have 3 “lone pairs” left, then you will make a triple bond. If you have 2 “lone pairs” left, then you will make a double bond. Hydrogen can never be a central atom. Carbon will always be a central atom.

STRUCTURE

AND

PROPERTIES

PAGE

3

OF

6

Stereochemistry (Valence-Shell Electron-Pair Repulsion Theory) All molecules have a definite 3-D shape. The number of electron pairs surrounding the central atom(s) can be used to predict the Stereochemistry of a molecule. The electron pairs repel each other and take up positions as far away from one another as possible. This theory of molecular shapes is known as the Valence-Shell Electron-Pair Repulsion Theory (VSEPR). When determining the shape of molecules, the electron pairs of a multiple bond (double or triple) count as only 1 pair of electrons for Stereochemistry purposes.

# of Bonding Electrons

# of Lone Pairs (Central Atom)

Molecular shape

Example

1

3

Diatomic Linear

HCl

2

2

V-Shaped (bent)

H2O

3

1

Pyramidal

NH3

4

0

Tetrahedral

CH4

3

0

Trigonal Planar

BI3

2

0

Triatomic Linear

CS2

1. Covalent bonds between atoms in the same plane are represented with solid lines (– )

2. Covalent bonds between atoms behind the plane of the paper are represented by dotted lines (- - - )

3. A wedge represents covalent bonds between atoms in front of the plane of the paper ()

UNIT

2:

STRUCTURE

AND

PROPERTIES

PAGE

4

OF

6

Examples of Stereochemistry (VSEPR) 1. Diatomic Linear

2. V-Shaped (Bent)

3. Pyramidal

4. Tetrahedral

5. Trigonal Planar

6. Triatomic Linear

UNIT

2:

STRUCTURE

AND

PROPERTIES

PAGE

5

OF

6

UNIT

2:

STRUCTURE

AND

PROPERTIES

PAGE

6

OF

6