04 periodic trends and effective nuclear charge supplement

Errors in Predicting Atomic Radius
Rearrange the following atoms in the order of
decreasing atomic radius:
Na, Mg, Sr, Cl, S, O, F, Br, Ga, Ti, Rn, Kr
Answer: Ti , Rn, Sr, Ga, Br, Kr, Na, Mg, S, Cl, O, F

But according to actual atomic radii data, the order should be this:
Ti , Rn, Sr, Ga, Br, Kr, Na, Mg, S, Cl, O, F
Sr, Na, Ti , Mg, Rn, Ga, Br, Kr, S, Cl, O, F
Sr (2.15 Å), Na (1.86 Å), Ti (1.71 Å), Mg (1.60 Å), Rn (1.4 Å), Ga (1.22 Å), Br (1.14 Å), Kr (1.09 Å), S (1.04 Å), Cl (0.99 Å), O (0.66 Å), F (0.64 Å)

When comparing periods, it is not true that all atoms in a lower period
are larger than the atoms in the period above it.
Alkali and alkali
earth metals
are usually
significantly
larger than
even the
largest non-
metals

To predict more accurately, consider that alkali and alkali earth metals
tend to be much larger than even the largest non-metal atoms.

Predicting Ion Radius
You can also predict the radius of elements in their ionic form.
How do metals and non-metals differ when it comes to comparing their
ionic radius to their atomic radius?

Metals lose electrons to form ions, so their ions have smaller radii
Non-metals gain electrons to form ions, so their ions have larger radii

Increases
General Trend: Look at metals and non-metals separately
Increases
Increases
Increases

Periodic Trends and Effective Nuclear Charge (Zeff)
Atomic radius can also be explained by effective nuclear charge.
Effective Nuclear Charge (Zeff) - The net force of attraction between
the outer electrons and the nucleus they surround.
Zeff = # of protons– inner electrons*
* Inner electrons: electrons that are on all the shells between the nucleus and the outer shell.
Example: Li
Zeff = # of protons + inner electrons
= 3 – 2
= 1
The inner electrons have a shielding effect on the outer electrons,
blocking some of the positive charge of the nucleus. As a result, the
outer electrons can only see a +1 charge in the nucleus.

P
P
NN
N
P
e
e
e
P
e
hydrogen lithium
No inner
electrons to
shield nucleus
Inner electrons
shield nucleus
Only hydrogen’s single electron experiences the full positive charge of
its nucleus. For all other atoms, the nuclear charge that any given
electron experiences is offset to some degree by other electrons.

When Zeff increases  Attraction of outer electrons to nucleus is stronger
When Zeff decreases  Attraction of outer electrons to nucleus is weaker
General trend:
The higher up
a group, the
fewer the inner
electrons to
shield the
nucleus
Across a period,
the number of
protons increases
without an
increase to the
inner electrons
NOTE:
- A change in Zeff going down a group is greater than a similar change in Zeff across a period
- Transition elements do not display the same general trend as the main group elements.

Examples: Which element has the larger Zeff value?
a) Li or K b) C or O c) C or P d) C or Se e) Li or Br

Relating Zeff with atomic radius:
Atomic radii is indirectly related to Zeff. As Zeff increases,
atomic radii decreases

Across a period:
e
e e
e
P P
P
N
N
N
N P
e
e e
e
P P
P
N
N
N
N P
P
P
P
N
N
N
N
P
e
ee
e
Beryllium Oxygen
The valence electrons in beryllium experience lower net attraction to
the nucleus compared to those of oxygen, since oxygen has more
protons (both atoms share the same number of inner electrons)

Down a group:
g2 7 g2 8 7
A greater net force of attraction is exerted on the outer electrons
of fluorine than those of chlorine, since chlorine has many more
inner electrons

04 periodic trends and effective nuclear charge supplement

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04 periodic trends and effective nuclear charge supplement