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The Atom
• The atom is the basic unit of matter,
everything is made up of atoms.
• Atoms are very small, and cannot even be
seen by a regular microscope.
• Atoms have no charge (neutral) and are
made up of several sub-atomic particles.
Sub – atomic particles:
Charge
ELECTRON
Negative
(-)
Location
Relative
Mass
Orbiting
outside
nucleus
Light
PROTON
Positive
(+)
Nucleus
Heavy
NEUTRON
Neutral
Nucleus
Heavy
Diagram of the atom:
What makes atoms different from
one another?
• The number of protons, neutrons and electrons
an atom has determines what kind of atom it is.
• Example: The oxygen atom has 8 electrons and
protons therefore we know it is an oxygen atom.
• You can determine the number of electrons and
protons an atom has by looking at its atomic
number found on the periodic table.
Atomic Number
• The atomic number of an element is found on
the periodic table. They are arranged in
increasing order going from left to right. So
Hydrogen has an atomic number of 1, Helium is
2 and so on.
• The atomic number tells us how many protons
an atom has. This also tells us how many
electrons an atom has, because the number of
protons and electrons are balanced. So an
atom has the same number of protons as
electrons.
To calculate the number of protons and
electrons an atom has we must look at the
atomic number.
Examples:
1. Si
14 electrons, 14 protons
2. Rb
37 electrons, 37 protons
3. Ag
47 electrons, 47 protons
4. N
7 electrons, 7 protons
Isotopes
• All atoms of the same type would have the
same number of protons and electrons,
but they don’t have to have the same
number of neutrons.
• Isotope = atoms of the same type, but
having different numbers of neutrons,
changing the atomic mass.
Atomic Mass
• The atomic mass on the periodic table a
weighted average of the isotopes
Atomic mass = #protons + # neutrons
# Neutrons = atomic mass - # protons
Example:
1. Na has a mass # of 22.98. How many e-,
p+ and neutrons does Na have?
e-, p+ = 11
22.98 – 11 = 11.98  12 neutrons
Electron Shells
• Electrons are found orbiting in shells
outside the nucleus
• Bohr Diagrams are a way to represent
where the electrons in an atom are located
• 1st shell holds 2 electron
• 2nd shell holds 8 electrons
• 3rd shell holds 8 electrons
• 4th shell holds 18 electrons
Drawing Bohr Diagrams
1. Put the symbol of the element in the
centre
2. Start placing electrons in the innermost
shells and work outwards
3. Place electrons at N, S, E and W
4. GO back and double if necessary
(electrons are usually found in pairs)
Example: Carbon
•Carbon has an atomic number of 6
• Therefore C has 6 electrons
• 2 electrons in 1st shell
• 4 electrons in 2nd shell
Valence Electrons
• The outer most electron shell in any atom is
called the valence shell
• The electrons in the valence shell are called
Valence Electrons
• By looking at the number of valence electrons an
element has we can predict its reactivity.
• THE OCTET RULE: Atoms will try to lose, gain
or share electrons to obtain a filled outer level
(to form a stable octet).
• Stable octet = 8 electrons
You can tell the number of valence electron
that an elements has by looking at the group
number
Group 1 has 1 valence electron
Group 2 has 2
Group 13 has 3
Group 14 has 4
Group 15 has 5
Group 16 has 6
Group 17 has 7
Group 18 has 8
Electron Dot Diagrams
•
•
An easier way to represent the number
of valence electrons an atom has is to
use electron dot diagrams.
Electron dot diagrams show only the
valence electrons
Predicting Chemical reactivity:
• By looking at the number of valence
electrons an element has we can predict
its reactivity.
• Atoms will try to lose, gain or share
electrons to obtain a filled outer level, or to
form a stable octet (8 electrons)
Ions
• When atoms gain or lose electrons they
form something called IONS. Energy is
released when this happens.
• Ions are electrically charged atoms.
(positive or negative)
Cations (+)
• CATIONS: are positively charged
ions. Cations are formed when an
atom loses electrons.
• Metals and Hydrogen form cations
• Ex: Na lose 1 electron
 Na1+
Anions (-)
• ANIONS: are negatively charged
ions. Anions are formed when atoms
gain electrons.
• Most non-metals form anions.
• Ex: Cl gains one electron
 Cl1-
Trend 4: Ions
Charges of Ions
Group 1  loses 1 electron = +1
Group 2  loses 2 electrons = +2
Group 13  loses 3 electrons = +3
Group 15  gains 3 electrons = -3
Group 16  gains 2 electrons = -2
Group 17  gains 1 electron = -1
Group18  has a full outer shell
= very stable 
Noble Gases
•
•
•
•
Group 8A (He, Ne, Ar, etc.)
Have a full valence shell (complete octet)
They neither want to gain or lose electrons
Therefore, DO NOT form IONS
What kind of ions do the following
elements make? (either + or -)
a) O
e) Mg
b) Br
c) Ca
d) Se
f) B
g) Cl
h) C
Answers:
a)
b)
c)
d)
e)
f)
g)
O (–)
Br (-)
Ca (+)
Se (+)
Mg (+)
B (+)
Cl (-)
h) C can be both (+) (-)
Reactivity
• The Periodic Table
Reactivity
• What is the most reactive of each pair?
– Na or Li
– Ba or Mg
– Li or Be
– Rb or In
– H or He
– F or He
– O or F
– I or Cl
Compounds
• Compound = a pure substance made up of
two or more elements that are chemically
combined, are neutral
• Ionic Compounds = substances
composed of cations and anions
• Ionic Bond = the attraction between
positive and negative
Making “Ionic Compounds”
Activity
• Groups of 2 or 3
• Gather a handful of paper squares
• Write the symbols for the ions on the
squares Positive in one color and Negative
in another
Label each of the following on
squares:
•
•
•
•
•
•
•
H+
Li+
Na+
K+
Be2+
Mg2+
Ca2+
• Fe2+
• Co3+
• Cr2+
• Cu2+
• Al3+
• Ni2+
•
•
•
•
•
•
•
•
FClBrIO2S2Se2N3-
• P3• As3-
The challenge:
(only one copy per group needed)
• By using your paper squares, see how
many different combinations your group
can make that sum to zero
• Always write the cation (+) then anion (-)
• Use subscripts to represent the # of ions
Examples:
• H+ Cl-  HCl
• H+ H+ O2- H2O
IONs Summary:
•
•
•
•
•
•
•
Alkali metals form 1+ cations
Alkaline earth metals form 2+ cations
Noble gases don’t form ions
Halogens form 1- anions
Group 16 form 2- anions to staircase
Group 15 form 3- anions to staircase
Transition metals, and metals under staircase,
form more than one cation so look at #2 periodic
table. (table of ions)
ex: Copper can form 2+ and 1+ cations