Periodic Trends Homework
1)
Why does atomic radius decrease as you move across the periodic table?
As you move from left to right on the periodic table the number of core
electrons stays the same but the number of protons increases. This
increase in effective nuclear charge pulls the valence electrons closer
to the nucleus, resulting in a smaller radius.
2)
How are the trends of atomic radius and ionization energy related? Explain in both groups and periods.
These trends are inverses of each other. As the size of the atom
increases, the ionization energy (energy required to remove the
electron) decreases because the electron being removed is farther from
the nucleus in a larger atom. Less energy is required to remove an
electron further from the nucleus. [AR increases down a group while IE
decreases; AR decreases across a period while IE increases.]
3)
What’s the difference between electronegativity and ionization energy?
Electronegativity is related to the pull of a nucleus on an electron being
shared in a bond. Ionization energy is the energy required to remove an
electron from an unbonded atom. Electronegativity is a ranking and has
no units. Ionization energy is an amount of energy and has units such
as kilojoules.
4)
What does it mean to be isoelectronic? Identify the largest ion and smallest ion of the group: Se2-, Br1-, Kr, Rb1+,
Sr2+. Explain why.
Isoelectronic means a group of atoms/ions having the same number of
electrons. All of the atoms in this group have 36 electrons; each
atom/ion has the same number of core electrons and the same amount
of electron shielding. What they don’t have in common (and what
accounts for the difference in size) is the number of protons. The
atom/ion with the most protons will be smallest (Sr2+) while the atom/ion
with the lowest number of protons will be largest (Se2-).
5)
What does the octet rule have to do with the periodic trends?
A trend like ionization energy is very closely related to the octet rule.
The halogens are the smallest of atoms and the closest to the having a
full octet. Since gaining an electron would give these atoms a full octet,
the amount of energy required to remove an electron (IE) would be very
high. When determining the charge an atom will have (as in number 4),
the number of electrons gained or lost is the number required to have a
full octet of electrons.
6)
The first ionization energy of beryllium is 9.322 eV, the second ionization energy is 18.211 eV, and the third
ionization energy is 153.893 eV. Explain why the third ionization energy of beryllium so much higher than the first two.
[eV is a electronVolt and is a unit of energy] When Be has lost two
electrons to become Be2+ it has the electron configuration of a noble
gas (He). This electron configuration is very stable, so a much larger
amount of energy is required to remove the third electron.
1. Rank the following elements from smallest to largest: N, Sb, Ar. N, Ar, Sb
2. Given any two elements within a group, is the element with the larger atomic number likely to have a larger or smaller
radius? larger, more protons means more electrons and a larger atomic radius
3. Explain why is it harder to remove an inner shell electron than a valence electron from an atom. An inner shell
electron will be closer to the nucleus. The greater attraction from being closer to the nucleus means that more energy
will be required to remove the electron.
4. An element forms a negative ion when ionized. On what side of the periodic table is the element located? right
(nonmetals form anions)
5. Of the elements magnesium, calcium, and barium, which forms the ion with the largest radius? The smallest? What
periodic trend explains this? Barium will have the largest radius while magnesium will have the smallest. These
elements are within a group. As you proceed down a group each element gains an energy level. Having additional
energy levels means more core electrons and more electron shielding, making the valence electrons farther from the
nucleus.
6. Define an ion: an ion is a charged particle; it becomes charged when electrons are gained (anion, negative) or lost
(cation, positive)
7. How does the ionic radius of a nonmetal compare with its atomic radius? Explain why the change in radius occurs.
Nonmetals form anions, meaning they gain electrons. Since the number of protons remains constant but the number
of electrons increases, the increase in electron repulsion causes the ion to have a larger radius.
8. Explain why atomic radii decrease as you move left-to-right across a period. The number of protons increases,
increasing the effectice nuclear charge. Since the valence electrons feel a greater pull from the nucleus, the radius
decreases.
9. Arrange the elements oxygen, sulfur, tellurium, and selenium in order of increasing atomic radii. Is your order an
example of a group trend or a period trend? O, S, Te, group trend
10. Arrange the elements in order of increasing atomic radii and explain: Mg, N, P, Sr.
N, P, Mg, Sr
Nitrogen will be the smallest since it is the only atom with just two energy levels. Phosphorus and magnesium both have
three energy levels, but phosphorus has more protons and a greater effective nuclear charge, so it will be smaller
than magnesium. Strontium will be the largest with five energy levels.