chembond.ppt

Atom – the smallest unit of matter “indivisible”

electron shells

a) Atomic number = number of Electrons

b) Electrons vary in the amount of energy they possess, and they occur at certain energy levels or electron shells.

Electrons are placed in shells according

to rules:

Octet Rule = atoms tend to gain, lose or share electrons so as to have 8 electrons

_{C would like to} N would like to

_{O would like to}

Why are electrons important?

1) Elements have different electron configurations

 _{different electron configurations mean different}

Electron Dot Structures

Symbols of atoms with dots to represent the valence-shell electrons

1 2 13 14 15 16 17 18

H He:

_{         }

Li _Be  _B   _C   _N   _O 

:

:

:

_{          }

Chemical bonds: an attempt to fill electron shells

1. Ionic bonds –

• _{The octet rule is a simple chemical}

rule of thumb

• _{Octet Rule says atoms with 8 atoms with 8}

electrons in their outer shell are stable

electrons in their outer shell are stable

• _{Atoms tend to combine in such a way}

that they each have eight electrons in their valence shells, giving them the same electron configuration as a

noble gas.

•

The rule applies to the

main-group elements, especially

carbon, nitrogen, oxygen, the

halogens, and also to metals

such as sodium or magnesium.

•

In simple terms, molecules or

ions tend to be most stable

when the outermost electron

Noble Gases

• _{group of elements were discovered at the} end of the 18th century

• _{They were un-reactive so they were called} the “Noble GasesNoble Gases”

• _{Later they were called “Inert Gases” as they} did not react and it sounded more scientific. • Now called “Noble” again because they do

react under extreme conditions

Limitations

Doesn’t allow for

•

H, He or Li

[stable with 2 e

in their outer shells]

- Duet Rule

•

Transition elements - 18 electron

rule

•

BF

which only has 6 e

in its outer

How can atoms get a full outer

shell?

•

Give away electrons

•

Take in electrons

•

Share electrons

•

Don’t bother getting a full outer shell

IONIC BOND

bond formed between

two ions by the

Formation of Ions from Metals

 _{Ionic compounds result when metals react with}

nonmetals

 _{Metals lose electrons to match the number of valence}

electrons of their nearest noble gas

 _{Positive ions form when the number of electrons are}

less than the number of protons

Group 1 metals  ion 1+

Formation of Sodium Ion

Sodium atom Sodium ion

Na  – e  Na +

2-8-1 _{2-8 ( = Ne)}

11 p+ _{11 p}+ _{11 e}- _{10 e}

Formation of Magnesium Ion

Magnesium atom Magnesium ion



Mg  – 2e  Mg2+

2-8-2 _{2-8 (=Ne)}

12 p+ _{12 p}+

12 e- 10 e

Some Typical Ions with Positive

Charges (Cations)

Group 1 Group 2 Group 13

H+ _Mg2+ _Al3+

Li+ _Ca2+

Na+ _Sr2+

Ions from Nonmetal Ions







1-Fluoride Ion

unpaired electron octet

_{   } 1 -

:

:

:

   

2-7 2-8 (= Ne)

9 p+ 9 p+

9 e- 10

0 1 -

Ionic Bond

• _{Between atoms of metals and nonmetals with very} different electronegativity

• _{Bond formed by transfer of electrons}

• _{Produce charged ions all states. Conductors and have} high melting point.

• An electronegativity difference of 2 is essential for a compound to be ionic.

• _{Ionic compounds are solids at room temperature and} are hard and brittle.

1). Ionic bond – electron from Na is transferred to Cl, this causes a charge imbalance in each atom. The Na becomes (Na+) and the Cl becomes (Cl-), charged

Size of Na

ion is smaller

than Cl

ion.

Why Ionic Compounds exist as

Crystals?

• _{Positive and negative ions attract each other}

with electrostatic force that extends in all directions. This means that ions will be

Why Ionic Compounds are hard

and brittle?

• _{Crystals are made of alternate positive and}

Why Ionic Compounds have high

melting points?

• _{Since in crystal lattice the positive and}

negative ions are tightly held in their positions in the lattice, only at high temperature do the ions acquire sufficient kinetic energy to

Solubility of Ionic compounds in

Water

COVALENT BOND

bond formed by the

Covalent Bond

• _{Between nonmetallic elements of similar}

electronegativity.

• _{Formed by sharing electron pairs}

• _{Stable non-ionizing particles, they are not}

conductors at any state

when electrons are

shared

equally

NONPOLAR

COVALENT BONDS

2. Covalent bonds- Two atoms share one or more pairs of outer-shell electrons.

Oxygen Atom

Oxygen Atom Oxygen AtomOxygen Atom

Oxygen Molecule (O

when electrons are

shared but shared

unequally

POLAR COVALENT

BONDS

- water is a polar molecule because oxygen is more

METALLIC BOND

bond found in

metals; holds metal

atoms together

Metallic Bond

• _{Formed between atoms of metallic elements} • _{Electron cloud around atoms}

• _{Good conductors at all states, lustrous, very}

high melting points

Metals Form Alloys