Transcript Periodic Trends

Periodic Trends
Objectives:
AHSGE Reading 4.3 Discern organizational
patterns.
Other AOD C.3.2 Recognize periodic trends of
elements, including the number of valence
electrons, atomic size, and reactivity.
Atomic Radius
• Electron clouds surrounding the nucleus
(do, do not) have a clearly defined
boundary.
• Atomic size is defined by how closely an
atom lies to a neighboring atom, so the
size of the atom varies some from one
interaction to another.
Atomic Radius (continued)
• Trends across periods: Atomic radius
tends to DECREASE as you move across
a period.
• Reason: Increase in positive charge of
the nucleus, while adding electrons (e- s)
to the same principal energy level.
• This means no increasing distance b/t
e- s and p+s, so p+s pull e- s closer.
Atomic Radius (continued)
• Trends within groups: Atomic radius
tends to INCREASE as you move down a
group.
• Reason: Nuclear charge still increases as
p+ s are added, but electrons are added to
higher energy levels.
• Because these e- s are located further
from the nucleus, with more e- s b/t it and
the nucleus, the positive charge of the
nucleus has less “pull” on them.
Ionic Radius
• Define ion.
• Def: an atom, or bonded group, with a
positive or negative charge.
• This happens with the gain or loss of an e-.
• What is the charge on an electron?
• What will the charge be if an atom GAINS
an e-?
• What will the charge be if an atom LOSES
an e-?
Ionic Radius(continued)
• Look at the diagram of a Na atom on the
board? What would happen to its size if
we remove the outermost valence e-?
• 2 Reasons for its decrease in size:
1. It lost its complete outermost orbital.
2. Electrostatic repulsion b/t the remaining e- s
decreases, allowing them to be pulled
closer together and closer to the nucleus.
Ionic Radius(continued)
• Look at the diagram of a Cl atom on the
board? What would happen to its size if
we add another outermost valence e-?
• Reason for its increase in size:
– Electrostatic repulsion b/t the remaining e- s
increases, making them move further apart
from each other.
Ionic Radius(continued)
• Trends within periods:
1. Groups 1A-4A, and group B elements form
smaller positive ions.
2. Groups 3A-8A form larger negative ions.
3. Positive ions get smaller going left-to-right.
4. Negative ions start out larger, but get
smaller going from group 5A-8A.
Ionic Radius(continued)
• Trends within groups: Higher energy
levels = larger ionic radius