Periodic trends

Periodic Trends
Elemental Properties and Patterns

The Periodic Law
 Dimitri Mendeleev was the first scientist to publish an
organized periodic table of the known elements.
 He was perpetually in trouble with the Russian government
and the Russian Orthodox Church, but he was brilliant never-
the-less.

The Periodic Law
 Mendeleev even went out on a limb and predicted the
properties of 2 at the time undiscovered elements.
 He was very accurate in his predictions, which led the world
to accept his ideas about periodicity and a logical periodic
table.

The Periodic Law
 Mendeleev understood the „Periodic Law‟ which states:
 When arranged by increasing atomic number, the chemical
elements display a regular and repeating pattern of chemical
and physical properties.

The Periodic Law
 Atoms with similar properties appear in groups or families
(vertical columns) on the periodic table.
 They are similar because they all have the same number of
valence (outer shell) electrons, which governs their chemical
behavior.

Valence Electrons
 Do you remember how to tell the number of valence
electrons for elements in the s- and p-blocks?
 How many valence electrons will the atoms in the d-block
(transition metals) and the f-block (inner transition metals)
have?
 Most have 2 valence e-, some only have 1.

A Different Type of Grouping
 Besides the 4 blocks of the table, there is another way of
classifying element:
 Metals
 Nonmetals
 Metalloids or Semi-metals.
 The following slide shows where each group is found.

Metals, Nonmetals, Metalloids
There is a zig-zag or
staircase line that
divides the table.
Metals are on the left
of the line, in blue.
Nonmetals are on
the right of the line,
in orange.

Elements that border
the stair case, shown
in purple are the
metalloids or semi-
metals.
There is one
important exception.
Aluminum is more
metallic than not.

 How can you identify a metal?
 What are its properties?
 What about the less common nonmetals?
 What are their properties?
 And what the heck is a metalloid?

Metals
Metals are lustrous
(shiny), malleable,
ductile, and are good
conductors of heat
and electricity.
They are mostly
solids at room temp.
What is one
exception?

Nonmetals
Nonmetals are the
opposite.
They are dull, brittle,
nonconductors
(insulators).
Some are solid, but
many are gases, and
Bromine is a liquid.

Metalloids
Metalloids, aka semi-
metals are just that.
They have characteristics
of both metals and
nonmetals.
They are shiny but brittle.
And they are
semiconductors.
What is our most important
semiconductor?

Periodic Trends
 There are several important atomic characteristics that show
predictable trends that you should know.
 The first and most important is atomic radius.
 Radius is the distance from the center of the nucleus to the
“edge” of the electron cloud.

Atomic Radius
 Since a cloud‟s edge is difficult to define, scientists use
define covalent radius, or half the distance between the
nuclei of 2 bonded atoms.
 Atomic radii are usually measured in picometers (pm) or
angstroms (Å). An angstrom is 1 x 10-10 m.

Covalent Radius
 Two Br atoms bonded together are 2.86 angstroms apart.
So, the radius of each atom is 1.43 Å.
2.86Å
1.43 Å 1.43 Å

Atomic Radius
 The trend for atomic radius in a vertical column is to go from
smaller at the top to larger at the bottom of the family.
 Why?
 With each step down the family, we add an entirely new PEL
to the electron cloud, making the atoms larger with each
step.

Atomic Radius
 The trend across a horizontal period is less obvious.
 What happens to atomic structure as we step from left to
right?
 Each step adds a proton and an electron (and 1 or 2
neutrons).
 Electrons are added to existing PELs or sublevels.

Atomic Radius
 The effect is that the more positive nucleus has a greater pull
on the electron cloud.
 The nucleus is more positive and the electron cloud is more
negative.
 The increased attraction pulls the cloud in, making atoms
smaller as we move from left to right across a period.

Effective Nuclear Charge
 What keeps electrons from simply flying off into space?
 Effective nuclear charge is the pull that an electron “feels” from
the nucleus.
 The closer an electron is to the nucleus, the more pull it feels.
 As effective nuclear charge increases, the electron cloud is
pulled in tighter.

Atomic Radius
 The overall trend in atomic radius looks like this.

Atomic Radius
 Here is an animation to explain the trend.
 On your help sheet, draw arrows like this:

Shielding
 As more PELs are added to atoms, the inner layers of
electrons shield the outer electrons from the nucleus.
 The effective nuclear charge (enc) on those outer electrons is
less, and so the outer electrons are less tightly held.

Ionization Energy
 This is the second important periodic trend.
 If an electron is given enough energy (in the form of a photon) to
overcome the effective nuclear charge holding the electron in the
cloud, it can leave the atom completely.
 The atom has been “ionized” or charged.
 The number of protons and electrons is no longer equal.

Ionization Energy
 The energy required to remove an electron from an atom is
ionization energy. (measured in kilojoules, kJ)
 The larger the atom is, the easier its electrons are to remove.
 Ionization energy and atomic radius are inversely proportional.
 Ionization energy is always endothermic, that is energy is added to
the atom to remove the electron.

Ionization Energy (Potential)
 Draw arrows on your help sheet like this:

Electron Affinity
 What does the word „affinity‟ mean?
 Electron affinity is the energy change that occurs when an
atom gains an electron (also measured in kJ).
 Where ionization energy is always endothermic, electron
affinity is usually exothermic, but not always.

Electron Affinity
 Electron affinity is exothermic if there is an empty or partially
empty orbital for an electron to occupy.
 If there are no empty spaces, a new orbital or PEL must be
created, making the process endothermic.
 This is true for the alkaline earth metals and the noble gases.

Electron Affinity
 Your help sheet should look like this:
+
+

Metallic Character
 This is simple a relative measure of how easily atoms lose or
give up electrons.

Electronegativity
 Electronegativity is a measure of an atom‟s attraction for another
atom‟s electrons.
 It is an arbitrary scale that ranges from 0 to 4.
 The units of electronegativity are Paulings.
 Generally, metals are electron givers and have low
electronegativities.
 Nonmetals are are electron takers and have high electronegativities.
 What about the noble gases?

Electronegativity
0

Overall Reactivity
 This ties all the previous trends together in one package.
 However, we must treat metals and nonmetals separately.
 The most reactive metals are the largest since they are the best
electron givers.
 The most reactive nonmetals are the smallest ones, the best
electron takers.

Overall Reactivity
 Your help sheet will look like this:
0

The Octet Rule
 The “goal” of most atoms (except H, Li and Be) is to have an
octet or group of 8 electrons in their valence energy level.
 They may accomplish this by either giving electrons away or
taking them.
 Metals generally give electrons, nonmetals take them from other
atoms.
 Atoms that have gained or lost electrons are called ions.

Ions
 When an atom gains an electron, it becomes negatively
charged (more electrons than protons ) and is called an
anion.
 In the same way that nonmetal atoms can gain electrons,
metal atoms can lose electrons.
 They become positively charged cations.

Ions
 Here is a simple way to remember which is the cation and
which the anion:
This is a cat-ion.This is Ann Ion.
He‟s a “plussy”
cat!
She‟s unhappy
and negative.
+ +

Ionic Radius
 Cations are always smaller than the original atom.
 The entire outer PEL is removed during ionization.
 Conversely, anions are always larger than the original atom.
 Electrons are added to the outer PEL.

Cation Formation
11p
+
Na atom
1 valence electron
Valence e-
lost in ion
formation
Effective nuclear charge
on remaining electrons
increases.
Remaining e- are pulled
in closer to the nucleus.
Ionic size decreases.
Result: a smaller sodium
cation, Na+

Anion Formation
17p
+
Chlorine atom
with 7 valence
e-
One e- is added to
the outer shell.
Effective nuclear charge is
reduced and the e- cloud
expands.
A chloride ion is
produced. It is larger
than the original atom.

Activities
1.Which of these elements has the largest atomic radius?
 aluminum
 calcium
 fluorine
 potassium
 sulfur

2.Which of these elements has the
smallest atomic radius?
 potassium
 iron
 Arsenic
 Bromine
 Krypton

highest first ionization energy?
 oxygen
 beryllium
 fluorine
 carbon
 boron

lowest first ionization energy?
 sodium
 aluminum
 phosphorus
 sulfur
 chlorine

highest electronegativity?
 lithium
 nitrogen
 potassium
 arsenic
 beryllium

lowest electronegativity?
 Sodium
 Aluminum
 Phosphorus
 Sulfur
 chlorine

highest electron affinity?
 Astatine
 Iodine
 Bromine
 chlorine

8.Which of these elements has
the lowest electron affinity?
 Polonium
 Tellurium
 Selenium
 Sulfur

9.As you move up and to the right
on the periodic table:
 atomic radius increases and electronegativity increases
 atomic radius decreases and electronegativity increases
 atomic radius increases and electronegativity decreases
 atomic radius decreases and electronegativity decreases

10.As you move from the top to the
bottom of the periodic table:
 ionization energy increases and electronegativity increases
 ionization energy decreases and electronegativity increases
 ionization energy increases and electronegativity decreases
 ionization energy decreases and electronegativity decreases

Periodic trends

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Periodic trends