Transcript 1.3 Trends in the Periodic Table File

Periodicity:
Trends in the
Periodic Table
Chemical reactivity is related to:
•
•
•
•
Atomic radius
Ionic radius
Ionization energy
Electronegativity
ATOMIC RADIUS
Definition: Atomic radius is one-half the
distance between the nuclei of two
atoms of the same element
Refers to the size of the atom
Trends in Atomic Radius
1) Atomic radius decreases moving from left to
right across a period of elements in the PT
WHY?
• With each successive element in a period, the
# of protons increases.
• With the increasing positive charge on the
nucleus, there is a greater attraction between
the valence shell and the nucleus  The
valence electrons are pulled closer to the
nucleus, and the atom decreases in size.
2) Atomic radius increases moving down a
family of elements (from top to bottom).
WHY?
•
The number of shells
increases with each
successive element
moving down a group.
•
The outer electron shell
is further away from the
nucleus, so the
valence. electrons are
not as strongly
attracted
•
Thus, the atom
increases in size.
Ions
Ion: A charged atom resulting from the
addition or removal of one or more
electrons from a neutral atom
Cation: an ion with a positive charge (Na+)
Anion: an ion with a negative charge (F-)
Trends in Ionic Radius
1) For positive ions, the ionic radius is
smaller than the atomic radius of the
neutral atom from which it was formed.
WHY?
• Less electrons  less electron-electron
repulsions + more attraction to the
nucleus = smaller ionic radius
2) For negative ions, the ionic radius is
larger than the atomic radius of the
neutral atom from which it was formed.
WHY?
• More electrons  more electron-electron
repulsion + less attraction to the nucleus
= bigger ionic radius
Trends in Ionic Radius
Positive ion
Negative ion
3) Regardless if the atom is a cation or
anion, the ionic radius decreases from
left to right across a period, and
increases when going from the top of a
family to the bottom
WHY?
• Same reasons listed in “Atomic radius”
IONIZATION ENERGY
First ionization energy: The amount of energy required to
remove an electron from a neutral atom (create a cation)
in the gaseous state (e.g., Na → Na+ + 1e-)
Second ionization energy: The amount of energy required
to remove an electron from an ion in the gaseous state
(e.g., Mg1+ → Mg2+ + 1e-)
There exists 3rd, 4th, 5th, etc. Ionization energy
Trends in Ionization Energy
1) First ionization energy increases moving
from left to right across a period of
elements on the PT.
WHY?
• Works inversely to the atomic radius: The
bigger the radius, the less the valence
electron is attracted to the nucleus  the
lower the ionization energy
2) First ionization energy decreases moving down a group
of elements (from top to bottom) on the PT.
Li 2.1 ==> Li+ or [2]+
Na 2.8.1 ==> Na+ or [2.8]+
K 2.8.8.1 ==> K+ or [2.8.8]+
WHY?
• As more shells of
electrons are added, the
distance between the
nucleus and the outer
shell of electrons
increases.
• The valence electrons are
not as strongly attracted
to the nucleus.
• Less energy is required to
remove the most weakly
held electron.
3) Second ionization energy is greater than
first ionization energy
WHY?
• When the first electron is removed, the
atomic radius decreases. The 2nd
electron is now held to the nucleus more
strongly, requiring more energy to
remove.
ELECTRONEGATIVITY
Electronegativity is the relative ability of an
atom involved in a covalent bond to attract
the electrons in that bond.
Measured on the Pauling scale, with fluorine
being the most electronegative (4.0) and
cesium and francium being the least
electronegative (0.7).
Trends in Electronegativity
1) Electronegativity increases moving from
left to right across a period of elements.
WHY?
• Moving across a period, the atomic
radius decreases
• Valence electrons are more strongly
attracted to the nucleus, and there is a
greater tendency to gain electrons.
2) Electronegativity decreases moving
down a group of elements (from top to
bottom).
WHY?
• Moving down a group, the atomic radius
increases, and the valence electrons are
not as strongly attracted to the nucleus.
• There is less tendency to gain electrons.
Reactivity
• Reactivity is the ability of a substance to
react chemically. Usually involves the
gain/loss of electrons
• The lower the ionization energy, the easier
it is for an atom to lose an electron 
higher reactivity
• The higher the electronegativity, the easier
it is for an atom to gain an electron 
higher reactivity