3.2 Periodic trends

• Atomic Radius-one half the distance between the nuclei of two atoms of the same element when the atoms are joined

Trends in Atomic Size

Group Trend: Atomic radii of atoms increase as you move from TOP to BOTTOM within a group.

• As you move from top to bottom on the PT, the principal QN (n) increases. As n increases, the size of the electron cloud increases.

Trends in Atomic Size

Periodic Trend: Atomic radii of atoms decrease slightly as you move from LEFT to RIGHT on the periodic table.

• As you move from left to right on the PT, n remains constant, but the positive charge on the nucleus increases by one proton for each element in the period. This results in the electron cloud being pulled in a little tighter.

• We call the positive charge that an electron experiences from the nucleus the nuclear charge.

Trends in Atomic Size

SUMMARY OF TREND: The size of atomic radii increase top to bottom within a group and decrease left to right across a period.

Trends in Atomic Size

PRACTICE: From each of the following pairs of atoms, select the atom that is larger in radius.

1. B - less nuclear charge

2. P - larger “n”

3. Cl - larger “n”

4. Ca - less nuclear charge

5. Ar - larger “n”

1. B, C

2. N, P

3. Cl, O

4. Ca, Sc

5. Ar, Ne

Metals: Metallic ions (cations), on the left and in the center of the table, are formed by loss of e-. When an electron is lost, electron-electron repulsion becomes less and the positively charged nucleus is now attracting fewer electrons. Metallic ions are therefore smaller than the atoms from which they are formed.

Trends in Ionic Size

Nonmetals: Nonmetallic ions (anions), located on the right side of the table, are formed by gain of e-. When an electron is gained, electron-electron repulsion is greater resulting in anionic radius larger than the atomic radius. Nonmetallic ions are therefore larger than the atoms from which they are formed.

Trends in Ionic Size

Group Trend: Ions increase in size as

you move from TOP to BOTTOM

within a group.

• Due to increasing value of n.

Trends in Ionic Size

Periodic Trend: Ions decrease in size as you move from LEFT to RIGHT amongst metals, and LEFT to RIGHT amongst non-metals on the periodic table.

• In an isoelectronic series, ions have the same number of electrons, however there is increasing nuclear charge.

Trends in Ionic Size

SUMMARY OF TRENDS: Metallic ions are smaller than their respective atoms, whereas nonmetallic ions are larger than their respective atoms. The size of ions increase as you move from top to bottom within a group and decrease as you move from left to right (among metals and among non-metals) across a period.

Trends in Ionic Size

PRACTICE: From each of the following pairs of atoms, select the atom that is larger in radius.

1. S2-

Non-metals: anions are larger than atoms due to greater e- repulsion

2. Ca

Metals: cations are smaller than atoms due to stronger pull on less e-

1. S or S2-

2. Ca or Ca2+

• When an atom forms an ion, the stability of an octet of electrons enables you to predict the number of electrons to be gained or lost. You can then predict the charge on the ion formed.

• When a single atom takes on a charge, it is called a monatomic ion.

• The charge on a monatomic ion is known as the oxidation state (oxidation number) of the atom.

Oxidation state (number): the apparent charge on an

atom if the electrons in a compound are assigned according to established rules

Predicting Oxidation States

1+ 2+ 3+ 2+ 3- 2- 1- 0

or

4+

3+

3+ or 4+

Tend to have more than one

oxidation state

Predicting Oxidation States

• Ionization energy-the energy required to remove the highest energy e- from a neutral atom

– First ionization energy is that energy required to remove first electron

– Second ionization energy is that energy required to remove second electron, etc.

• It requires more energy to remove each successive electron.

Trends in Ionization Energy

• When all valence electrons have been removed, the ionization energy increases enormously.

Trends in Ionization Energy

Group Trend: Ionization energy of atoms decrease as you move from TOP to BOTTOM within a group.

• As you move from top to bottom on the PT, the distance of outermost electrons from the nucleus increases, as does the shielding effect (inner electrons block the attraction of the nucleus for outer electrons) due to increase in n.

• Less energy is required to remove first e-.

Trends in Ionization Energy

Periodic Trend: Ionization energy of atoms increases as you move from LEFT to RIGHT across the periodic table.

• As you move from left to right on the PT, shielding effect remains constant, but nuclear charge increases.

• More energy is required to remove first e-.

Trends in Ionization Energy

• SUMMARY OF TREND: The ionization energy of atoms decrease top to bottom within a group and increase left to right across a period.

Trends in Ionization Energy

Two deviations from this trend exist.

Trends in First Ionization Energy

The first occurs between Groups IIA and IIIA.

• A IIIA element (ns2 np1) has a smaller IE than the preceding IIA element (ns2).

• Electron more easily removed

from p-orbital rather than

s-orbital

– e- n farther from nucleus

– Small amount of

repulsion by s e-

Trends in First Ionization Energy

The second occurs between Groups VA and VIA.

• A VIA element (ns2 np4) has a smaller IE than the preceding VA element (ns2 np3).

• e- more easily removed

from a doubly occupied np

orbital than a singly occupied

np orbital.

– Repulsion from other

e- in orbital helps in its

removal.

Trends in First Ionization Energy

• Nuclear Charge- the larger the nuclear charge, the higher the IE

• Shielding Effect- the greater the shielding effect, the lower the IE

• Radius- the greater the distance between the nucleus and the outer electrons of an atom, the lower the ionization energy

• Sublevel- an e- from a full or half-full sublevel requires more energy to be removed

Factors Affecting Ionization Energy

PRACTICE: From each of the following pairs of atoms, select the atom that has a lower first ionization energy.

1. Mg-greater shielding, larger radius

2. Al-smaller nuclear charge

2. B-less energy required than removing e- from full 2s

1. Be, Mg

2. Al, P

3. Be, B

• Electronegativity-the measure of the ability of atom to attract electrons when the atom is in a compound

– Can be used to determine the type of bond that will form during a reaction

– Can be represented in a table, however the noble gases do not appear because they are not very reactive

– The values are expressed in units called Paulings

The most electronegative element is Fluorine at 4.0 Atoms that tend to gain e- easily (nonmetals) have a high electronegativity

Atoms that have little tendency to gain e- (metals) have a low electronegativity The least electronegative elements are Cesium and Francium at 0.7

Trends in Electronegativity

• Influenced by the same factors that affect ionization energy

Group Trend: Electronegativity values decrease as you move from top to bottom within a group.

Periodic Trend: For representative elements, the values tend to increase from left to right across a period. Electronegativity values among the transition elements are not regular.

Trends in Electronegativity

PRACTICE: Arrange the following elements in order of increasing attraction for electrons in a bond.

1. In < Sb < Se < F

2. Fr < Zn < Ga < Ge < P

1. S, F, In, Se

2. Fr, Ga, Ge, P, Zn

Group Trend: Metals become more reactive as we move from TOP to BOTTOM on the PT. Nonmetals become more reactive as we move from BOTTOM to TOP on the PT.

As we move “down” the PT, each successive element

has one greater EL filled with e-.

This increases the shielding of the nucleus and therefore e- are more easily “lost” (i.e., less pull from nucleus) from larger atom metals.

A “smaller” atom (fewer EL’s) will have a greater attraction for e- due to less shielding and therefore e- are more easily “gained” by smaller atom nonmetals.

Trends in Reactions of Metals & Nonmetals