Transcript Periodic Trends

Periodic Trends



There are many ways to use the periodic table
Trends are based on electronic structure
The trends we will discuss are:








shielding
atomic mass
atomic radius
ionization energy
metallic character
ionic radius
properties of the metals and non-metals
melting point
Valence Shell

Def: the orbitals in the highest energy level

on any one row on the periodic table – s & p

valence electrons are only in these orbitals

core electrons are ALL the electrons between the
valence electrons and the nucleus
Valence Shell (Cont.)


for each row on the periodic table, the outermost
energy levels are always the s and p orbitals (check it
out by comparing/contrasting the electron
configurations for S, Se, Te, and Po)
for each period (row), the highest energy levels only
have s or p as their orbital paths!

exs:
← Highest energy level: 3rd
only orbitals in level: s & p
Se: [Ar] 4s2 3d10 4p4 ← Highest energy level: 4th
only orbitals in level: s & p
Te: [Kr] 5s2 4d10 5p4 ← Highest energy level: 5th
only orbitals in level: s & p
Po: [Xe] 6s2 5d10 4f14 6p4 ←Highest energy level: 6th
only orbitals in level: s & p
S: [Ne] 3s2 3p4
Shielding



There is a rough cancellation of core electrons and
protons
Effective nuclear charge is the positive charge
felt after one-for-one cancellation of core e-s and
protons
For example, Na has an effective nuclear charge of
+1




the electron configuration for Na: [Ne] 3s1
it has 1 valence electron and 10 core electrons
11 protons – 10 core (shielding) electrons
+1 effective nuclear charge
Periodic Trends

Atomic Mass: increases down a group (column)




each time you go down one box in a group, the number
of protons increases
as you add protons to an atom, the mass goes up
you also need to add neutrons to keep the nucleus
stable, increasing the mass even more
Atomic Mass: increases as you go across a row



each time you go across one box in a row, the number
of protons increases
as you add protons to an atom, the mass goes up
you also need to add neutrons to keep the nucleus
stable, increasing the mass even more
Periodic Trends

Atomic Radius: increases as you go down a group



as you go down through each box in a group, you add a
new energy level and more shielding
each new energy level wraps around all the previous
energy levels
each new energy level increase the size of the atom


due to the shielding, the protons do not have an increased
pull on the outer electrons
as the atom grows, so does the radius of the atom
Periodic Trends

Atomic Radius: decreases across each period





as you travel across a row, the maximum number of
energy levels does NOT change, nor does the shielding
as you travel across a row, the number of protons
DOES increase
as the positive charge increases with the same
shielding, it attracts the e–s more strongly
with increased pull from the nucleus, all energy levels
are pulled in closer to the nucleus
with the energy levels closer to the nucleus, the atomic
radius shrinks
Periodic Trends

Atomic Radius comments:



even though the number of protons increases as you go
down a group the atomic radius also increases
the atomic radius increases in this case because the e−s
in the new energy level have to be significantly away
from the electrons in the last energy level
across a row, the shrinking caused by the increase in
protons is substantially smaller then adding a whole
new energy level
Periodic Trends

Ionization Energy: the amount of energy needed
to remove an e– from an atom




even though ions can be made by adding or subtracting
e–s to(from) an atom, we are only considering removal
of e–s here
as you travel across any period, the maximum energy
level does not change (see last slide)
the attraction between the opposite charges in the atom
changes based on the distance between the nucleus and
the electrons
Big distance changes result in even larger energy
changes: going out one energy level results in a big
drop in energy holding electrons in place.
Periodic Trends

Ionization Energy: decreases as you go down a
group




as you add energy levels, the distance between the
nucleus and the outermost electrons increases
this increase in distance decreases the ability of the
nucleus to hold the electrons in the atom
that makes it easier to remove the electrons, decreasing
the amount of energy needed to remove an electron
from the valence shell
that means the ionization energy drops
Periodic Trends

Ionization Energy: increases as you go across a
period




remember, as you go across a row, the atomic radius
shrinks AND the number of protons increases
as the electrons get closer to the nucleus with an
increased positive charge, the attraction between the
electrons and the nucleus increases
this requires more energy to remove any electron
that means the ionization energy increases, as a general
rule…
Periodic Trends


Going from the s orbital to the p orbital, the
ionization energy drops, slightly, due to the
slightly higher energy of the p orbitals (they are a
little further from the nucleus)
The ionization energy drops very slightly after the
fourth electron is added to the p orbital: the first
pair of electrons in one orbital path

The e−- e− repulsion increases b/c there are now 2 e−s
in the orbital. The extra pushing means the electrons
are trying to get away from each other and less energy
is required to remove the electron from the atom
Periodic Trends

Reactivity – the ability of an element to form ionic
compounds




For bonding purposes, metals lose electrons when
bonding to form compounds
The more easily a metal will lose an electron, the more
metallic it is (the more reactive it is)
Reactivity is a measure of how easily an element will
form a compound
from all the other trends it should be clear that Fr is the
most reactive of all the metals:





it is the largest element; it has the lowest ionization energy
so, it will lose a valence electron most easily
meaning it will form ionic bonds easily
this makes it the most reactive metal
Next slide…
Periodic Trends

Reactivity (Continued)



As a trend, the further you get from the steps on the
periodic table on the metals’ side, the more reactive the
element is
This should make sense, b/c the steps are the
metal/non-metal divider
Non-metals next…
Periodic Trends

Reactivity (Continued)




For NONMETALS, there is also a definition for
reactivity
The further you get from the step, the reactivity also
increases
This is because the atoms there can more easily add an
electron because of the small radii and high ENC
making them more reactive…
Periodic Trends

Ionic radius – the radius of an ion as compared to
its neutral atomic radius






As you make positive ions, the size tends to decrease
The number of protons has not changed
With fewer electrons, there is less repulsion pushing
the electrons apart
With the same positive charge but lower repulsive
forces, the protons are able to draw the electrons closer
This shrinks the atomic radius
This is all based on the balance between repulsion
between the e−s and the attraction between the e−s and
the nucleus
Periodic Trends

Ionic radius (continued)






As you make negative ions, the size tends to increase
The number of protons has not changed
With more electrons, there is greater repulsion pushing
the electrons apart
With the same positive charge but higher repulsive
forces, the electrons push away from each other more
This increases the atomic radius
This is again based on the balance between repulsion
between the e−s and the attraction between the e−s and
the nucleus
Properties of the Metals





Metals tend to be more malleable (able to be
pounded or rolled out in to thin sheets)
Metals tend to be shiny
Metals tend to be more lustrous (have a shimmer)
Metals tend to have a heavier feel (have higher
density) for the same volume of material
There are always exceptions to trends:


some metals are dull, feel light, and/or crumble when
you try to make them into thin sheets, but this is not the
majority of metals
Remember the majority of the metals behave as stated
above
Properties of Non-Metals





Non-metals tend to have a duller appearance when
solid
Non-metals tend to crumble when pounded or
rolled out
Non-metals tend to have a lighter feel (lower
density) when compared to the same volume of a
metal
They are better electrical and thermal insulators
They have more variety in their state, as a group