Download Atomic Radius and Ionization Energy

Periodic Trends
Section 6.3
The Periodic Law states...
Periodic Law
• The pattern of properties within a
period repeats as you move across a
period from left to right…
When elements are arranged in
order of increasing atomic number,
there is a periodic repetition of their
physical and chemical properties
Periodic Trends
• Trend – a predictable change
• Our focus will be on the main
block elements
• How do electron configurations
help us explain many of the
trends in properties observed?
Trend in Atomic Radius
• Measure the molecule that forms when two
atoms of the same element combine
• Atomic radius = half of the distance between
the nuclei of the two atoms
• Measured in picometers (1 pm = 10-12 m)
Why does this trend occur?
• 1. Changes in n- As the principal quantum
number (n) increases, the outer electrons are
farther from the nucleus, so the atoms are
larger.
• 2. Changes in Zeff – As the effective nuclear
charge increases, outer electrons are pulled
closer to the nucleus, so the atoms are smaller.
• (Zeff = the positive charge felt by an electron)
1. Down a group, n dominates
• Elements have one more energy
level of core electrons
• These SHIELD the outer electrons
• Atomic radius generally
INCREASES in a group from top to
bottom
2. Across a period, Zeff dominates
• Moving across a period, electrons
are added to the same outer level
• Shielding by inner electrons does not
change
• Zeff increases and outer electrons are
pulled closer
• Atomic radius generally DECREASES
in a period from left to right
Try the following…
• Put in order of decreasing atomic size
• Ca, Mg, Sr
• K, Ga, Ca
• Br, Rb, Kr
• Sr, Ca, Rb
• Sr > Ca > Mg
• K > Ca > Ga
• Rb > Br > Kr
• Rb > Sr > Ca
Ions
• Atom or group of atoms that has a
positive or negative charge
• Form when electrons are transferred
between atoms
• Metals tend to lose electrons,
forming cations
• Nonmetals gain electrons, forming
anions
Trends in Ionization Energy
• Energy needed to remove an electron
from an atom
• Energy needed to remove the first
electron from an atom is the FIRST
IONIZATION ENERGY
• Produces cation with 1+ charge
• First ionization energy
tends to decrease from top
to bottom in a group and
increases from left to right
across a period
Why does this trend occur?
• As atomic size increases down a
group, Zeff has a smaller effect on the
electrons in highest level
• Shielding Effect
• Less energy is needed to remove
electron from the energy level
making first ionization energy lower
Across a period…
• Zeff increases and shielding effect
remains constant
• Increase in the attraction the nucleus
has for an electron
• More energy is needed to remove an
electron from the atom and first
ionization energy is higher
Try these…
• Put the following in order of decreasing IE1
• Kr, He, Ar
• Sb, Te, Sn
• K, Ca, Rb
• I, Xe, Cs
• He > Ar > Kr
• Te > Sb > Sn
• Ca > K > Rb
• Xe > I > Cs