Ch 2.1
Elements combine to
form compounds
Compounds have different properties
from elements
• Elements have individual properties
that help us identify them.
• Although, when these elements
combine to form compounds a whole
new set of properties are created.
• Example: carbon (C) and oxygen (O)
alone are harmless, but carbon
monoxide (CO) is deadly.
Atoms combine in predictable numbers
•Ammonia (NH3) will always have 3
hydrogen atoms for every nitrogen;
1:3
•3 hydrogen atoms with 1 electron in
the last energy level and 1 nitrogen
atom with 5 electrons in the last
energy level. Sharing these will give
nitrogen 8 electrons & hydrogen 2
electrons, filling the last energy levels!
•Water (H2O) will always have 2
hydrogen atoms for every oxygen.
•2 to 1 ratio of hydrogen to oxygen
2:1
•2 hydrogen atoms with 1 electron in
the last energy level and 1 oxygen
atom with 6 electrons in the last
energy level. Sharing these will give
oxygen 8 electrons & hydrogen 2
electrons, filling the last energy levels!
Forming compounds
•Atoms can be
described in one
of two ways:
•They tend to gain or share
electrons to become stable.
•They tend to lose or share
electrons to become stable.
Example: Hydrogen
•Hydrogen has only 1 electron- it
needs two to fill its energy level.
•Hydrogen atoms tend to gain or
share 1 electron to become stable.
•If two hydrogen atoms come
together they will become stable
by sharing their electrons. This will
give each atom 2 electrons.
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Ionic Bonds
• An ion is a charged atom caused by
the loss or gain of electrons
• A positively charged ion has lost
electrons; mainly metals.
• A negatively charged ion has gained
electrons; mainly non-metals.
• An ionic bond forms from the
attraction of positive and negative
ions.
Example of ionic bonding
Na
Na
+
+
Cl
NaCl
Cl
salt
• Sodium, with one outer electron will
become a positive ion when it gives
its one electron to chlorine.
• Chlorine will become a negative ion.
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Covalent Bonds
• A pair of shared electrons between
two atoms.
• Non-metals usually form bonds with
each other by sharing electrons.
• Covalent atoms are neutral because
they do not gain or loss electrons
• These electrons spend time in the
outer energy levels of both atoms.
Covalent Bonds
• When atoms share electrons, the
shared electrons move around the
nuclei of the atoms that are involved.
Step by step:
1. The atoms approach each other
2. The shared electrons begin to
move around the nuclei.
3. To form a hydrogen molecule in
which each hydrogen atom shares
its electrons.
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CHEMICAL BONDS
• When two atoms share their
electrons, they join to form a
chemical bond.
• Atoms are held together by chemical
bonds when one or more of their
electrons are attracted to, and move
around the nuclei of both atoms.
CHEMICAL FORMULAS
A chemical formula of a
compound tells how much of
each element is present.
Examples: CO₂ shows one
atom of carbon and two atoms
of oxygen.
SUBSCRIPTS
• Subscripts are small numbers written
slightly below the element symbol to
show how many atoms of that
element are in the compound.
• When there is no subscript the
number of atoms is understood to be
one.
SUBSCRIPTS
Examples of subscripts:
• MgCl₂ is the formula for magnesium
chloride. It is made up of one atom of
magnesium and two atoms of
chlorine.
• H₂O is the formula for water. It is
made up of two atoms of hydrogen
and one atom of oxygen.
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