In Chemical Reactions,
Elements combine to
form compounds
Reactions that affect an Atom
• Nuclear reactions – these type
reactions change the composition of
an atom’s nucleus.
• Chemical reactions – the most
common one where atoms interact
only through their valence electrons;
the nuclei remain unchanged.
Lewis Dot Diagrams
• G.N. Lewis used ‘dots’ to represent the valence
electrons around an atom in his teaching of chemical
bonding.
• An electron dot diagram is a model of an atom in
which each dot represents a valence electron. The
chemical symbol in the middle represents the nucleus
and all the other electrons in the atom.
• It is those valence electrons that are lost, gained or
shared to in a chemical reaction between atoms.
Remember this?
•
A compound is a substance made from
two or more different atoms chemically
bonded together.
•
For example, water(H2O) and salt
(NaCl) are compounds.
•
A compound always contains two or
more elements joined in fixed
amounts.
Just like the Alphabet
• How many words can be formed from the 26
letters of the alphabet?
• The elements in the periodic table can
likewise form millions of compounds.
• So when you think of all the substances
around you, they can’t all be elements can
they?
• Most substances, therefore, are compounds.
More on Compounds
• The properties of a compound depend
on:
– Which atoms the compound contains
– How the atoms are arranged
• Atoms are not changed by chemical
reactions, just rearranged into
different compounds.
• Atoms are neither created nor
destroyed by chemical reactions.
Chemical Formulas
• A chemical formula represents the
atoms of elements and their ratios (fixed
amounts) in a chemical compound.
• Example:
Carbon Dioxide is a compound of 1 atom of
carbon bonded to 2 atoms of oxygen.
To indicate the chemical formula we use C and
O with a subscript of 2 beside oxygen to
show there are 2 atoms.
This is how we get CO2
More Chemical Formulas
Compound
Name
Atoms
Chemical
Formula
Hydrogen
Chloride
H & Cl
HCl
Water
H&O
H 2O
Hydrogen
Peroxide
H&O
H 2 O2
Ammonia
N&H
NH3
Methane
C&H
CH4
Propane
C&H
C 3H 8
Chemical bonds
•
Chemical bonds are the ‘glue’ that
holds the atoms of elements together in
compounds.
•
Chemical bonds form when the valence
electrons in the electron cloud around
the atoms interact.
•
When atoms exchange their electrons
to form compounds, the number of
electrons given must equal the
number taken. Therefore, all
compounds are neutral.
Types of Chemical Bonds
• We will learn about two types of chemical bonds – Ionic
and Covalent bonds.
• Ions are formed when atoms lose or gain electrons:
– Gaining e- makes a negative ion or anion
– Losing e- makes a positive ion or cation
• An element’s location on the periodic table gives clues
about the type of bond an element might form with another
atom.
Ionic bonds
• Always involve metals
• Metals bonding with non-metals or metalloids.
Ionic Bonds
•
What happens when an atom of an element from Group 1, like Na,
meets an atom of an element from Group 17, like Cl?
•
Na loses an electron and forms a positive ion and Cl gains that
electron to form a negative ion.
•
What happens to particles with opposite charges?
•
They are drawn toward each other by electrical attraction.
•
This force of attraction between positive and negative ions is called
an ionic bond.
• The overall charge on the compound formed is neutral
or zero; the number of electrons lost by one atom must
equal the number of electrons gained by the other atom.
Also, the outer shells of each atom will be complete.
Naming Ionic Compounds
•
The name of an ionic compound is based on the names of the ions
that made it.
•
The positive ion uses the name of the atom which formed it.
•
The name of the negative ion is formed by dropping the last
part of the name of the atom and adding the suffix – ide.
•
To name an ionic compound, the name of the positive ion is
placed first, followed by the name of the negative ion.
•
Therefore, in our example the ionic compound is NaCl – Sodium
Chloride.
•
So, how would you name the compound with the chemical formula
of BaI2 - KBr - MgF2?
Covalent Bonds
•
Another way in which atoms can bond together is by sharing
electrons.
•
A pair of shared electrons between two atoms is called a covalent
bond.
•
‘co’ means partner and ‘valent’ refers to valence.
•
In a covalent bond, neither atom gains or loses an electron,
therefore, no ions are formed.
•
They share in pairs Each atom in the compound formed has a
share in the number of electrons required to complete the outer
valence shell.
•
Compounds that contain only covalent bonds are called molecular
compounds or molecules.
Covalent Bonds
• Nonmetals reacting with another
nonmetal will usually form a covalent
bond.
• A metalloid reacting with a nonmetal
will usually form a covalent bond.
Covalent Bonds
Covalent Bonds
Chemical Formula
When writing the chemical formula for
a covalent bond, the elements are
written in order of increasing group
number. (NO for nitric oxide; not ON)
Summary Ionic vs. Covalent
•
Ionic Bonds – atoms lose or gain electrons to fill their
outermost energy level. (form ions)
–
–
–
–
–
–
•
Usually metals reacting with nonmetals will form an ionic bond
Ionic bonds are very strong
They are usually solids at room temperature.
Ionic bonds are good conductors of electricity
Ionic bonds have high melting points
Ionic bonds dissolve in water
Covalent Bonds – atoms share electrons to fill their
outermost energy level.
– Nonmetals reacting with another nonmetal will usually form a covalent
bond
– They tend to be gases or liquids at room temperature
– They are very poor conductors of electricity
– They have low melting points and low boiling points.
– Covalent bonds are weaker than ionic bonds.