APChem- Chapter 7 Lecture- Periodic Trends

PeriodicProperties

of the Elements

Development of Periodic Table

• Elements in the same group generally have similar chemical properties.

• Properties are not identical, however.

PeriodicProperties

of the Elements


Dmitri Mendeleev and Lothar Meyer independently came to the same conclusion about how elements should be grouped.

PeriodicProperties

of the Elements


Mendeleev, for instance, predicted the discovery of germanium (which he called eka-silicon) as an element with an atomic weight between that of zinc and arsenic, but with chemical properties similar to those of silicon.

PeriodicProperties

of the Elements

QuickTime™ and aSorenson Video decompressorare needed to see this picture.

PeriodicProperties

of the Elements

PeriodicProperties

of the Elements

The atomic number depends on the number of protons in the nucleus, while the atomic weight depends (mainly) on the number of both protons and neutrons in the nucleus.

PeriodicProperties

of the Elements

Periodic Trends

• In this chapter, we will rationalize observed trends inSizes of atoms and ions.Ionization energy.Electron affinity.

PeriodicProperties

of the Elements

Effective Nuclear Charge

• In a many-electron atom, electrons are both attracted to the nucleus and repelled by other electrons.

• The nuclear charge that an electron experiences depends on both factors.

PeriodicProperties

of the Elements

• The attraction of an electron to the nucleus depends onThe charge of the nucleusThe distance the electron is from the nucleus.

In large atoms, the electrons in outer shells do not experience the full charge of the nucleus because inner shells of electrons “shield” them from the nucleus.

PeriodicProperties

of the Elements

Effective Nuclear Charge

The effective nuclear charge, Zeff, is found this way:

Zeff = Z − S

where Z is the atomic number and S is a screening constant, usually close to the number of inner electrons.

PeriodicProperties

of the Elements


PeriodicProperties

of the Elements

The 2p electron of a Ne atom.

PeriodicProperties

of the Elements

1. 2s orbital 2. 2p orbital 3. 3s orbital 4. 3p orbital 5. 3d orbital

In an germanium atom, which electron will experience the greatest effective nuclear charge? An electron in a…

PeriodicProperties

of the Elements

The effective nuclear charge experienced by 3p electrons in phosphorus is

1. +2

2. +3

3. +5

4. +7

PeriodicProperties

of the Elements

1. +2

2. +3

3. +5

4. +7

Correct Answer:

The effective nuclear charge is given by the equation:

Zeff = Z Swhere Z represents number of protons in the nucleus and S represents the average number of electrons between the nucleus and the electron in question. Here:

Zeff = 15 10 = +5

PeriodicProperties

of the Elements

Sizes of Atoms

It is difficult to define atomic radii for single atoms because the outside of the electron cloud is ambiguous.

PeriodicProperties

of the Elements

Sizes of Atoms

The non-bonding atomic radius is defined as one-half of the distance between two atoms when they collide.

PeriodicProperties

of the Elements

Sizes of Atoms

The bonding atomic radius is defined as one-half of the distance between covalently bonded nuclei.

PeriodicProperties

of the Elements

Sizes of Atoms

Adding two bonding radii together gives a useful estimate of bond length.

PeriodicProperties

of the Elements

Sizes of AtomsBonding atomic radius tends to… …decrease from left

to right across a rowdue to increasing Zeff.

…increase from top to bottom of columndue to increasing

value of n

PeriodicProperties

of the Elements


PeriodicProperties

of the Elements

No

PeriodicProperties

of the Elements

Order the following according to increasing atomic radius.

1. Ge < Si < Se < Cl 2. Se < Si < Ge < Cl

3. Si < Cl < Ge < Se

4. Cl < Si < Se < Ge

5. Si < Ge < Se < Cl

Ge Si Se Cl

PeriodicProperties

of the Elements

Based on atomic radii, which of the following bonds would be expected to be the shortest?

1. HH

2. HF

3. FF

4. ClCl

PeriodicProperties

of the Elements

Correct Answer:

The shortest bonding atomic radius belongs to the H atom, so an H2 molecule will have the shortest HX (where X is any atom) bond distance.

1. HH

2. HF

3. FF

4. ClCl

PeriodicProperties

of the Elements

Sizes of Ions• Ionic size depends

upon:Nuclear charge.Number of

electrons.Orbitals in which

electrons reside.

PeriodicProperties

of the Elements

Sizes of Ions• Cations are

smaller than their parent atoms.The outermost

electron is removed and repulsions are reduced.

PeriodicProperties

of the Elements

Sizes of Ions• Anions are larger

than their parent atoms.Electrons are

added and repulsions are increased.

PeriodicProperties

of the Elements

Sizes of Ions

• Ions increase in size as you go down a column.Due to increasing

value of n.

PeriodicProperties

of the Elements

Sizes of Ions

• In an isoelectronic series, ions have the same number of electrons.

• Ionic size decreases with an increasing nuclear charge.

PeriodicProperties

of the Elements


PeriodicProperties

of the Elements

Order the following according to increasing atomic/ionic radius.

1. C < Li+ < O2- < N3-

2. N3- < O2- < C < Li+

3. Li+ < C < N3- < O2-

4. Li+ < C < N3- < O2- 5. Li+ < C < O2- < N3-

N3- Li+ C O2-

PeriodicProperties

of the Elements

Which is larger: Na+ or Na?

1. Na+

2. Na

PeriodicProperties

of the Elements

1. Na+

2. Na

Correct Answer:

Both have the same number of protons, but Na with one more electron will be larger.

PeriodicProperties

of the Elements

Which is larger: Cl or Cl?

1. Cl

2. Cl

PeriodicProperties

of the Elements

1. Cl

2. Cl

Correct Answer:

Both have the same number of protons, but Cl− with one more electron will be larger.

PeriodicProperties

of the Elements

Lecture 7b- Sections 7.4 - 7.5

PeriodicProperties

of the Elements

Ionization Energy

• Amount of energy required to remove an electron from the ground state of a gaseous atom or ion.First ionization energy is that energy

required to remove first electron.Second ionization energy is that energy

required to remove second electron, etc.

PeriodicProperties

of the Elements

PeriodicProperties

of the Elements

• the photoelectric effect

PeriodicProperties

of the Elements

Ionization Energy

• It requires more energy to remove each successive electron.

• When all valence electrons have been removed, the ionization energy shows a HUGE increase.

PeriodicProperties

of the Elements


PeriodicProperties

of the Elements

I2 for the carbon atom is greater.

PeriodicProperties

of the Elements

Trends in First Ionization Energies

• As one goes down a column, less energy is required to remove the first electron.For atoms in the same

group, Zeff is essentially the same, but the valence electrons are farther from the nucleus.

PeriodicProperties

of the Elements


• Generally, as one goes across a row, it gets harder to remove an electron.As you go from left to

right, Zeff increases.

And the electron’s distance from the nucleus decreases.

PeriodicProperties

of the Elements


However, there are two apparent discontinuities in this trend.

PeriodicProperties

of the Elements


• The first occurs between Groups IIA and IIIA.

• Electron removed from p-orbital rather than s-orbitalElectron farther from

nucleusSmall amount of

repulsion by s electrons.

PeriodicProperties

of the Elements


• The second occurs between Groups VA and VIA.Electron removed

comes from doubly occupied orbital.

Repulsion from other electron in orbital helps in its removal.

PeriodicProperties

of the Elements


PeriodicProperties

of the Elements

Which will have the highest ionization energy?

• C

• N

• O

• Al

• Si

PeriodicProperties

of the Elements

Which will be the largest?

• I1 of Na

• I2 of Na

• I1 of Mg

• I2 of Mg

• I3 of Mg

I = ionization energy

PeriodicProperties

of the Elements

PeriodicProperties

of the Elements

They have the same electron configuration: [Ar]3d3.

PeriodicProperties

of the Elements

Electron Affinity

Energy change accompanying addition of electron to gaseous atom:

Cl + e− Cl−

PeriodicProperties

of the Elements


PeriodicProperties

of the Elements

They are equal in magnitude and opposite in sign.

PeriodicProperties

of the Elements

Trends in Electron Affinity

In general, electron affinity becomes more exothermic as you go from left to right across a row.

PeriodicProperties

of the Elements


There are again, however, two discontinuities in this trend.

PeriodicProperties

of the Elements


• The first occurs between Groups IA and IIA.Added electron must

go in p-orbital, not s-orbital.

Electron is farther from nucleus and feels repulsion from s-electrons.

PeriodicProperties

of the Elements


• The second occurs between Groups 4A and 5A.Added electron must

go in an occupied p-orbital.

PeriodicProperties

of the Elements


PeriodicProperties

of the Elements

Based on periodic trends, which of the following elements is expected to have the largest (i.e., most negative) electron affinity?

1. K

2. Na

3. Si

4. S

PeriodicProperties

of the Elements

Correct Answer:

S has most negative electron affinity in this list.

1. K

2. Na

3. Si

4. S

PeriodicProperties

of the Elements

Lecture 7c- Sections 7.6 - 7.8

PeriodicProperties

of the Elements

Properties of Metal, Nonmetals,and Metalloids

PeriodicProperties

of the Elements

Metals versus Nonmetals

Table 7.3 on page 277

Differences between metals and nonmetals tend to revolve around these properties.

PeriodicProperties

of the Elements

Metals versus Nonmetals

• Metals tend to form cations. (Low I.E.)• Nonmetals tend to form anions. ( exo E.A.)

PeriodicProperties

of the Elements

Metals

Tend to be lustrous, malleable, ductile, and good conductors of heat and electricity.

PeriodicProperties

of the Elements

Metals

• Compounds formed between metals and nonmetals tend to be ionic.

• Metal oxides tend to be basic.

PeriodicProperties

of the Elements

Metal oxide + water metal hydroxideMgO(s) + H2O(l) Mg(OH)2(aq) but only if it dissolves a bit in water

Metal oxide (a.k.a. base anhydride) reactions

Metal oxide + acid water + saltMgO(s) + HCl(aq) MgCl2(aq) + H2O(l)

PeriodicProperties

of the Elements

Nonmetals

• Dull, brittle substances that are poor conductors of heat and electricity.

• Tend to gain electrons in reactions with metals to acquire noble gas configuration.

PeriodicProperties

of the Elements

Nonmetals

• Substances containing only nonmetals are molecular compounds.

• Most nonmetal oxides are acidic.

PeriodicProperties

of the Elements

nonmetal oxide + water acidSO3(g) + H2O(l) H2SO4(aq)

nonmetal oxide (acid anhydride) reactions

nonmetal oxide + base water + salt

CO2(g) + 2NaOH(aq) Na2CO3(aq) + H2O(l)

PeriodicProperties

of the Elements

Metalloids

• Have some characteristics of metals, some of nonmetals.

• For instance, silicon looks shiny, but is brittle and fairly poor conductor.

PeriodicProperties

of the Elements

Group Trends

PeriodicProperties

of the Elements

Alkali Metals

• Soft, metallic solids.• Name comes from

Arabic word for ashes.

PeriodicProperties

of the Elements

Alkali Metals

• Found only as compounds in nature.• Have low densities and melting points.• Also have low ionization energies.

PeriodicProperties

of the Elements

Alkali Metals

Their reactions with water are famously exothermic.

2M(s) + H2O(l) 2MOH(aq) + H2(g)

PeriodicProperties

of the Elements

Alkali Metals• Alkali metals (except Li) react with oxygen to

form peroxides.• K, Rb, and Cs also form superoxides:

K + O2 KO2

• Produce bright colors when placed in flame.

PeriodicProperties

of the Elements

Alkaline Earth Metals

• Have higher densities and melting points than alkali metals.

• Have low ionization energies, but not as low as alkali metals.

PeriodicProperties

of the Elements

Alkaline Earth Metals• Be does not react

with water, Mg reacts only with steam, but others react readily with water.

• Reactivity tends to increase down the group.

Mg(s) + 2H2O(g) Mg(OH)2(aq) + H2(g)

PeriodicProperties

of the Elements

Hydrogen

• Hydrogen is a nonmetal.• It has a high ionization energy

because its single electron experiences no nuclear shielding

• When it does lose it’s electron it isn’t really an atom anymore. It is a proton.

PeriodicProperties

of the Elements

Group 6A

• Oxygen, sulfur, and selenium are nonmetals.• Tellurium is a metalloid.• The radioactive polonium is a metal.

PeriodicProperties

of the Elements

Oxygen• Two allotropes:

O2

O3, ozone

• Three anions:O2−, oxideO2

2−, peroxideO2

1−, superoxide

• Tends to take electrons from other elements (oxidation)

PeriodicProperties

of the Elements

Sulfur

• Weaker oxidizing agent than oxygen.

• Most stable allotrope is S8, a ringed molecule.

PeriodicProperties

of the Elements

Group VIIA: Halogens

• Prototypical nonmetals• Name comes from the Greek halos and gennao: “salt formers”

PeriodicProperties

of the Elements

Group VIIA: Halogens

• Large, negative electron affinitiesTherefore, tend to oxidize

other elements easily

• React directly with metals to form metal halides

• Chlorine added to water supplies to serve as disinfectant

PeriodicProperties

of the Elements

Group VIIIA: Noble Gases

• Astronomical ionization energies• Positive electron affinities

Therefore, relatively unreactive

• Monatomic gases

PeriodicProperties

of the Elements

Group VIIIA: Noble Gases

• Xe forms three compounds:XeF2

XeF4 (at right)

XeF6

• Kr forms only one stable compound:KrF2

• The unstable HArF was synthesized in 2000.

APChem- Chapter 7 Lecture- Periodic Trends

Education

Transcript of APChem- Chapter 7 Lecture- Periodic Trends