Periodic Trends
Elemental Properties and Patterns
The Periodic Law
Dimitri Mendeleev (1869/1871) was the first scientist to
publish an organized periodic table of the known elements.
He was taking a chemistry course in Russia and tried to find a
way to organize the periodic table.
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.
Periodic Trends
There are several important atomic characteristics that show
predictable trends that you should know.
Atomic properties—
Deal with only single atoms
Atomic Radius
Enables us to gain information on atom’s size
Outer electrons hard to locate
Radius is the distance from the center of the nucleus to the
“edge” of the electron cloud.
Measurement of distance between nuclei of 2 atoms
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.
1. Covalent Radius
Half the distance between the nuclei of 2 bonded atoms.
Radius of nonmetallic atoms
Ex. Br
2.86 Å
1.43 Å
1.43 Å
2. Metallic Radius
Half the distance between nuclei of adjacent atoms in a metal
Radius of metallic atoms
Atomic Radius Trend
Decreases
Increases
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.
Identify the atom with the largest
radii
1) Ca, F
2) Cl, O
3) N, P
Ionization Energy
If an electron is given enough energy (in the form of a
photon) to overcome the electrical forces holding it, the
electron can be removed from the atom completely.
Amount of energy needed to remove ONE electron from a
neutral atom
Removal from ground state in neutral atom of gaseous state
to form positive ion
Ionization Energy (cont.)
The energy required to remove an electron from an atom is
ionization energy.
The larger the atom is, the easier its electrons are to remove.
Ionization energy and atomic radius are inversely
proportional.
Ionization Energy (cont.)
1st ionization energy
Energy required to remove 1st electron from atom
Taken from highest energy level
Easiest to remove
Energy increases as more electrons are removed
Ionization Energy Trend
Increases
Decreases
Ionization Energy
Identify the highest ionization
energy
1)
F, Mg
2)
Na, Rb
3)
P, O
Classwork: Ionization Energy
Circle the atom with the highest first ionization energy.
1)
Al, B
5) Fr, Li
9) Se, Cl
2)
Mg, Na
6) Mg, Al
10) Rb, Mg
3)
P, As
7) C, F
4)
I, At
8) K, Sc
Homework
Read pp. 150-156
p. 152 #1-3 atomic radii problems
Ionization Energy problems
Study for Nuclear Chemistry Quiz
Start Periodic Trends Chart
What does affinity mean?
Electron Affinity
energy change that occurs when electrons are added to a neutral
atom
Some atoms will release energy, others will absorb energy to
“force” electron into atom
Ex. Hot Potato
Greater value with smaller atoms
Energy released when electron added
Exothermic, negative value
An atom’s “desire/affinity” for more electrons, wants to get more
electrons ! ! !
Metals—decrease electron affinity.
Nonmetals—increase electron affinity, more reactive
Stable atoms—full octet
Electron Affinity Trend
Increases
Decreases
Electron Shielding
Repulsive forces exist between electrons.
Electrons are found at various distances from the nucleus
Electrons closer to the nucleus (at lower energy levels) can
“screen/shield” outer electrons
Outer electrons do not feel the attractive force from the
nucleus as a result
Affects periodic trends such as ionization energy
Explains why ionization decreases down a column
Electronegativity
trait or characteristic of an atom
Ability of an atom to pull electrons toward it, how tightly
an atom wants to hold onto its electrons
Atom’s ability to attract electrons from another atom
Periodic Trend:
increases
decreases
Practice: Which atom is more
electronegative?
1)
Lithium or fluorine
2)
Chlorine or iodine
3)
Magnesium or oxygen
4)
Calcium or barium
5)
Rubidium or Strontium
Metallic Character
This is simply a relative measure of how easily atoms
lose or give up electrons.
Tendency to LOSE electrons
Related to atomic radius/ionization energy, how easy
to remove electrons
Metallic Character
Atoms with a tendency to LOSE electrons
Decreases
Increases
Nonmetallic Character
Atoms with a tendency to GAIN electrons
Increases
Decreases
October
th
9
Homework
Finish Periodic Trends Chart
Finish Periodic Trends Worksheet