Chapter 4
Formation of Compounds
Every macroscopic change in matter has a
submicroscopic explanation…….there is a microscopic
change at the atom level.
Scientist’s job is to observe and explain reactions on
submicroscopic level.
WHAT IS A COMPOUND?
Chemical combination of two or more different
elements chemically joined in a fixed ratio.
IMPORTANT FAMILIAR COMPOUNDS
SALT (NaCl)
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made up of Na and Cl
NaCl
one of the most abundant natural cpds on earth
found in large solid underground deposits
dissolved in oceans
- essential nutrient in living things
Physical properties
• white, solid at RT
• cube like crystals
• hard
• brittle: shatters with pressure
• very water soluble
• conducts electricity in solution
Chemical Properties
• very stable crystal
• not highly reactive with other substances
• can last for years in hard crystal form and remain
same
Properties of NaCl’s Individual Elements
Sodium (Na)
Chlorine (Cl)
• Solid metal: silvery, white,
soft
• reacts violently with water
• Must be stored under oil
otherwise explodes
• never found in environment
freely – always combined
with another element
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gas: pale green , poisonous
soluble in water
used as disenfectant
kills living cells
one of most reactive elements
Do compounds have the same properties as the
elements they are made of?
NO
Salt: solid, white, hard, stable, non metal
Sodium: silver, soft, very unstable and reactive solid
metal
Chlorine: green, unstable, reactive gas
CARBON DIOXIDE (CO2)
Physical Properties
• colorless gas
• contained in small amounts in air
• major compound in cellular respiration
Chemical properties
• relatively stable
• does not support burning (used in fire ext.)
• major product of photosynthesis
• change of state: solid to gas (sublimation)
ex: dry ice
Properties of CO2 Components
Carbon
• non metal
• fairly unreactive at RT
• most abundant compound
in living things
• clear, grey, black in color
Oxygen
• non metal
• colorless, odorless, tasteless gas
• makes up about 21% of air
• supports burning gas
• slightly soluble in water
(fish breathe)
WATER (H2O)
Physical properties
• occurs in all three states of matter
• 70% Of earth surface and humans mass
• boiling point 100o C
• freezing point 0o C
• does not conduct electricity in pure state
• universal solvent
Chemical Properties
• stable, not highly reactive
• doesn’t break down in normal conditions
• main component of all chemical reactions on
earth and in body
• main compound needed for life
Properties of Water’s Components
Hydrogen
• lightest, most abundant
element in universe
Oxygen
• non metal
•colorless, odorless, tasteless gas
• gas: odorless, tasteless,
colorless
•makes up about 21% of air
• does not conduct electricity
• slightly soluble in water
• very reactive: usually found
in cpds.
•nonmetal
•supports burning gas
•slightly soluble in water
WE HAVE SEEN HOW COMPOUNDS CAN HAVE
GREATLY DIFFERENT PROPERTIES THAN THE
PROPERTIES OF ELEMENTS THAT COMPOSE THEM.
IN THE NEXT SECTION WE WILL EXAMINE
SUBMICROSCOPICALLY WHY THIS IS THE CASE BY
EXAMINING HOW THEIR ATOMS COMBINE
THE MANY DIFFERENT COMBINATIONS OF ELEMENTS
DETERMINE THE CHARACTERISTICS OF THE
COMPOUNDS THEY COMPRISE.
HOW ELEMENTS FORM COMPOUNDS
Compounds form when electrons collide with each other.
Which electrons are involved in reactions?
All atoms want to be stable by completing their valence
shells.
Formula
• combination of chemical symbols that show what
elements make up a compound and the number of
atoms of each element.
• It also shows the ratio of each element to each other
Ex: NaCl (1 atom of each, always in same ratio)
1:1
H20 (2 atoms H, 1 atoms O, always in same
ratio)
2:1
How many atoms of each element are present in these
compounds?
What is the ratio of elements to each other?
H2O2
CO2
C6 H12 O6
HCO3
K2S
FeO2
Al2 O3
CHEMICAL BONDS
Chemical bond: process if joining atoms in a compound
Electrons are subatomic particles involved in bond
Goal of bond: to complete outer shell and become
stable
To form a compound electrons:
- gain
- lose
- share
Bonding Basics
•Atoms try to fill valence shell
(orbital) to become stable
•H and He: need 2 valence
electrons
•All other atoms: need 8
valence electrons
- Metals lose electrons
- Non metals gain electrons
Octet Rule
• Elements gain, lose, or share electrons to reach a full
octet (8 valence electrons in the outer shell)
•H and He (2 valence electrons)
Noble gas configuration
• The state of an atom achieved by having the same
valence electron configuration as the noble gases
• Most stable electron configuration
• All atoms try to achieve this configuration in forming
compounds
TYPES OF BONDS
1. Ionic:
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transfer of electron between metal
and non metal
metal gives electrons to non metal
- ions formed
strong magnetic attraction
keeps compound together
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ION: charged atom
IONIC COMPOUND:
compound composed of ions
of opposite charges
Ionic compounds are neutral
(+ and – charge cancel each
other)
Ionic Bond and Ion Formation
Na has 11 protons and electrons
(electronically neutral)
Na gives away its valence electron
Na now has: 11 P (+)
10 e (-)
----------1 more positive charge
Na is now an ion: Na +
Cl has 17 protons & electrons
(electronically neutral)
Cl accepts one electron
Cl now has: 17 P (+)
18 e (-)
-----------1 more negative charge
Cl is now an ion: Cl-
Since the new ionic compound has the same number of positive and
negative charges, the compound is electronically neutral
Drawing Ionic Compounds
Steps
1. Find number of valence electrons for each atom
(use periodic table)
2. Draw Lewis dot diagram for each atom (use X’s and O’s)
3. Determine which atoms lose or gain electrons
4. Draw Lewis diagram of compound
5. Check octet rule for each atom in compound
Draw Lewis Dot Structures for the following
Ionic Compounds
MgF2
RbI
Li2S
CsI
GaF3
CaBr2
KBr
K2S
BaCl2
TYPES OF BONDS
2. Covalent:
• two non metals share electrons
• forms covalent compounds or
molecules
• interparticular forces keep atoms
together
• resulting molecule is neutral
• very strong bond
Covalent Bonds and Molecule Formation
H2
•H is not found as a single atom naturally, it is
usually found as H2 (diatomic molecule)
WHY?
•H: group 1on per table
- How many valence electrons?
- Is that stable?
IF ONE H ATOM GAVE ITS ELECTRON TO
THE OTHER WOULD IT BE A STABLE?
HOW DOES MOLECULE BECOME STABLE?
Difference between Ionic and Covalent Bonds
Multiple Covalent Bonds
MULTIPLE COVALENT BONDS
bonds with more than one shared pair of electrons
between two individual atoms
Multiple Covalent Bonds
Double Bond
•two shared pairs of electrons
between two atoms
•4 total electrons
•stronger than single bond
Drawing Covalent Compounds
Steps
1. Determine number of valence electrons
2. Write the symbols to show neighboring atoms
3. Use electron pairs to form bonds
4. Complete octets for each atom
Make Lewis dot structures for the following compounds:
O2
H2Se
H2S
HCN
Multiple Covalent Bonds
Triple Bond
•three shared pairs of electrons
between two atoms
•6 total electrons
•stronger than double bond
Review: Types of Covalent Bonds
TYPES OF BONDS
3. Hydrogen:
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weak bond between polar
molecules
–polar molecule: has
opposite charges on
opposite ends
IONIC COMPOUNDS VS COVELANT COMPOUNDS
IONIC
well organized ions
held together by magnetic
attraction
very strong bond
neutral
solid, 3D cyrstals at RT
electrolytes in H2O
non conductive as solid
high temperature needed to
melt compound
COVALENT
composed of molecules
held together by interparticular forces
weaker than ionic bond
unless multiple
neutral
most liquid or gas at RT
hard, rough, brittle
many non water soluble
less than ionic cpds.
non-conductive when
dissolved in H2O
low temperature needed to
melt compound (wax, fat)
Study for the test !