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Periodic Trends
• Periodic Table is arranged by:
– Atomic number
– Groups
• Verticle column of the periodic table
• Elements in the same group have similar physical &
chemical properties
• Each group number will give you the number of valance
electrons
Periodic Trends
Periodic Trends
– Periods
• Horizontal row of the periodic table
• Physical and chemical properties change gradually
across a period
• Period will indicate number of energy levels around the
nucleus
Periodic Trends
Periodic Trends
• Atomic Radius
– Distance from the nucleus to the outer most
electron
– Because we don’t know exactly where the
electrons are we say it is ½ the distance between 2
bonded atoms of the same element
Periodic Trends
• Atomic radius will increase as we go down a
group
– Number of energy levels increases
– Outer electron is further from the nucleus which
means radius will increase
• Atomic radius will decrease we go across a period
– Elements will have the same energy level but will have
an increased number of protons (atomic #)
– More protons (+) attracts the electrons (-) in the
energy level bringing it closer to the nucleus and
therefore decreasing the atomic radius
Periodic Trends
Periodic Trends
• Ionic Radius
– Like atomic radius, it will increase in size as we go
down a group because of increase in electron
shells
– However, across a period ionic radius will
decrease (group 1-3) then increase (group 5) and
decreases again (group 6-7)
Periodic Trends
• An example showing the relative ion sizes from Period
3 are shown below:
Na+ Mg2+
Cl-
Al3+
P3-
S2-
• The cations decrease in size because because there
are fewer electrons than protons and therefore a
strong pull towards the nucleus
• The anions are larger in size because there are more
electrons than protons but because the atomic
number increases (# of protons) and the electrons
stay the same the radius will increase
Periodic Trends
• The size of an atom always decreases when
being converted to a positive ion because it
loses an electron and therefore there is less
electron repulsion
• The size of an atom always increases when
being converted to a negative ion because
there is an increase in repulsion between
electrons
Periodic Trends
• Electronegativity
– Measures the attraction an atom has for a shared pair
of electrons when it is covalently bonded
– Electronegativity decreases as you down a group
• Due to shielding effect where electrons in a lower level
(closer to the nucleus) shield the positive charge from the
nucleus and therefore the shells are not bound as tightly
– Electronegativity increases as you move across a
period
• Due to increase in protons (atomic #)
Periodic Trends
• Ionization Energy
– Amount of energy required to remove the outermost
electrons
– Related to electronegativity
– Ionization energy decreases as you go down a group
• Due to shielding effect which makes it easier to remove outer
electrons that have many atoms (near the bottom of the periodic
table)
– Ionization energy increases as you move across a period
• Due to increased nuclear charge which holds the electrons more
strongly