Chapter 10
Periodic Trends
Density
Atomic and Ionic Radii
Density
Density
• Iridium (Z = 77) has the highest density.
• Why not Meitnerium (Z = 109?)
• We can predict that an element is more
dense if it is closer to Iridium.
Density
Effective Nuclear Charge (Zeff)
• The effective nuclear charge (Zeff) of an
atom is basically how well it is able to hold
on to its most loosely held electron.
Atomic radius (radii)
• The atomic radius is essentially the size
of an atom.
• The largest atom is Francium (Z = 87).
Atomic Radius
Xylophone monkey
Rank the atoms from smallest to largest
Na, K, O, N
O < N < Na < K
Atomic Radius and Effective
Nuclear Charge
• The atomic radius decreases as effective
nuclear charge increases.
Effective Nuclear Charge (Zeff)
•
The effective nuclear charge of an atom
is primarily determined by:
1. The nuclear charge
2. The shielding effect
The Nuclear Charge (Z)
• Based on the number of protons in the
nucleus.
– Example: Carbon vs. Nitrogen
The Nuclear Charge (Z)
Carbon
Nitrogen
The Nuclear Charge (Z)
• The greater the number of protons in the nucleus
the greater the effective nuclear charge.
Carbon
Nitrogen
Conclusion: Nitrogen is a smaller atom than carbon because
nitrogen has 7 protons to pull in its electrons whereas carbon
only has 6.
Carbon
Nitrogen
Nuclear charge explains why atoms get smaller across a period
Atomic Radius
Xylophone monkey
Shielding Effect.
• The shielding effect is when electrons
between the nucleus and the outermost
electrons in an atom shield or lessen the
hold of the nucleus on the outermost
electrons.
Shielding Effect.
He
Ne
Conclusion: Neon is larger than helium because it has an
increased shielding effect from having two energy levels
whereas helium only has one energy level.
He
Ne
Shielding Effect explains why atoms get larger down a group
Atomic Radius
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Chemical Reactivity
Chemical Reactivity
Chemical Reactivity
• Metals tend to lose electrons when reacting.
– Large metal atoms are more reactive.
• Nonmetals tend to gain electrons when
reacting.
– Small nonmetal atoms are more reactive.
Chemical Reactivity
• Metals increase
in reactivity left
and down.
• Nonmetals
become more
reactive up and
to the right.
• Most reactive
metal is? Fr
• Most reactive
nonmetal is? F
Ionic Radius
• Ionic Radius is the size of an ion.
Size Change in Ion Formation
Rules for Ionic Radius
• Anions (negative ions) are “always” larger
than cations (positive ions).
• Ionic Radius goes by the same rules as
atomic radius (ions get larger as we move
down and to the left).
• However it is necessary to treat anions
and cations separately. (Anions are
bigger).
Rank the ions from smallest to largest
K+, N3-, Na+, O2-
+
Na
+
<K <
2O
<
3N
Homework
• Worksheet: Density, Atomic and Ionic Radii