ch. 8 periodic properties of the elements

Ch. 8 Periodic Properties of the Elements

Multielectron Atoms

•“Hydrogen-like” orbitals are used for all atoms•Energy levels are affected by other electrons

– Coulomb’s Law—electrostatic repulsion of like charges is proportional to the amount of charge, and inversely proportional to the distance between them (see text for eqn)

– Shielding—screening of one electron from the nuclear charge by other electrons around the same atom

– Penetration—probability of the electron to be close to the nucleus

– Effective nuclear charge (Zeff)—the amount of nuclear charge an electron experiences after taking shielding into account

– Degenerate—of equal energy

Order of Filling Subshells

Electron Spin and the Pauli Exclusion Principle

• Electrons have intrinisic angular momentum -- “spin” -- ms

– Possible values: ms = +1/2 and -1/2 (only two possible values)

• Pauli Exclusion Principle:– No two electrons in an atom can have identical values of all 4

quantum numbers -- maximum of 2 electrons per orbital!– A single orbital can hold a “pair” of electrons with opposite “spins”– e.g. the 3rd shell (n = 3) can hold a maximum of 18 electrons: n = 3 l = 0 1 2

subshell 3s 3p 3d # orbitals 1 3 5 # electrons 2 6 10 = 18 total

• A single electron in an orbital is called “unpaired”

• Atoms with 1 or more unpaired electrons are paramagnetic, otherwise they are diamagnetic

Electronic Configurations• The Aufbau Principle -- Order of Filling Subshells

– Atomic # = # of protons = # electrons (in neutral atom)– Add electrons to atomic orbitals, two per orbital, in the

general order of increasing principle quantum number n, for example:

# Atom Configuration

1 H 1s1

2 He 1s2

3 Li 1s22s1

4 Be 1s22s2

5 B 1s22s22p1

6 C 1s22s22p2

7 N 1s22s22p3

8 O 1s22s22p4

9 F 1s22s22p5

10 Ne 1s22s22p6

11 Na 1s22s22p63s1

Hund’s Rule• Maximum number of unpaired electrons in orbitals of

equal energy

Orbital diagrams:

C __ __ __ __ __

N __ __ __ __ __

O __ __ __ __ __

1s

1s

1s

2s

2s

2s

2p

2p

2p

Relationship to Periodic Table

e.g. complete electronic configuration of Ge (#32, group IV)Ge 1s22s22p63s23p64s23d104p2

or, Ge 1s22s22p63s23p63d104s24p2 (by values of n)

• Short-hand notation -- show preceding inert gas config.– Ge [Ar]4s23d104p2 where [Ar] = 1s22s22p63s23p6

Valence Shell Configurations• valence shell -- largest value of n (e.g. for Ge, n = 4)

plus any partially filled subshells

Ge 4s24p2 (valence shell electron configuration)

Ge __ __ __ __ (valence shell orbital diagram)

Elements in same group have same valence shell e– configurations

e.g. group V: N 2s22p3

P 3s23p3

As 4s24p3

Sb 5s25p3

Bi 6s26p3

4s 4p

Sample Questions• Write the complete electron configuration of gallium.Answer:

• Write the short-hand electron configuration for zirconium.Answer:

• Write the orbital diagram for the valence shell of tellurium.

Answer:

Sample Questions• Write the complete electron configuration of gallium.Answer:

Ga 1s22s22p63s23p64s23d104p1

• Write the short-hand electron configuration for zirconium.Answer:

Zr [Kr]5s24d2

• Write the orbital diagram for the valence shell of tellurium.

Answer:

Te ___ ___ ___ ___5s 5p

Sample QuestionHow many unpaired electrons does a ruthenium(II) ion,

Ru2+, have?Show an appropriate orbital diagram to explain your

answer. Is the atom paramagnetic or diamagnetic?

Sample QuestionHow many unpaired electrons does a ruthenium(II) ion,

Ru2+, have?Show an appropriate, valence-shell orbital diagram to

explain your answer. Is the atom paramagnetic or diamagnetic?

Answer:4 unpaired electrons, so paramagneticOrbital diagram:

Ru2+ ___ ___ ___ ___ ___

4d

Variation of Atomic PropertiesAtomic Size (atomic radius, expressed in pm -- picometers)

e.g. group 1 metals:

e.g. some elements in 2nd period:

Atom Radius in pm Valence Shell

Li 152 2s1

Na 186 3s1

K 227 4s1

Cs 248 5s1

Atom B C N O F

radius 88 77 70 66 64

e– config 2p1 2p2 2p3 2p4 2p5

(10–12 m!)

General Trend in Atomic Size

Relative sizes of ionscations are smaller than parent atomse.g. Na 186 pm 2s22p63s1

Na+ 95 pm 2s22p6

anions are larger than parent atomse.g. Cl 99 pm 3s23p5

Cl– 181 pm 3s23p6

Ionization EnergyI.E. = energy required to remove an electron from an atom

or ion (always endothermic, positive values)

e.g. Li(g) --> Li+(g) + e– I.E. = 520 kJ/mole

Exceptions: special stability of filled subshells, and of half-filled subshells

Electron Affinity• E. A. = energy released when an electron is added to an

atom or ion (usually exothermic, negative EA values)e.g. Cl(g) + e– --> Cl–(g) E. A. = -348 kJ/mol

• The general trends in all these properties indicate that there is a special stability associated with filled-shell configurations.

• Atoms tend to gain or lose an electron or two in order to achieve a stable “inert gas configuration” -- many important consequences of this in chemical bonding.

Types of Elements

Metals:Shiny, malleable, ductile solids with

high mp and bpGood electrical conductorsMetal character increases to lower left of periodic table

Nonmetals:Gases, liquids, or low-melting solidsNon-conductors of electricity

Metalloids:Intermediate properties, often semiconductors

Diatomic elements: H2, O2, N2, F2, Cl2, Br2, I2

Sample QuestionsOf the following atoms, circle the one with the highest

electron affinity.K Cl P Br Na

Write a balanced chemical equation that corresponds to the electron affinity of the element that you selected above.

Sample QuestionOf the following atoms, circle the one with the highest

electron affinity.K Cl P Br Na

Write a balanced chemical equation that corresponds to the electron affinity of the element that you selected above.

Answer: Cl(g) + e– --> Cl–(g)

Alkali Metals• They want to be +1!• Easily oxidized, low EA, low IE.• Density increases moving down the group. (mass rises

faster than atomic radius)• Reactions

– With halogens to form salts, e.g. 2 Na(s) + Cl2(g) 2 NaCl(s)

– With water to make base + hydrogen, e.g.2 K(s) + 2 H2O(l) 2 K+

(aq) + 2 OH–(aq) + H2(g)

• Reactions are more vigorous as you get lower in the group (why?)

http://www.youtube.com/watch?v=9bAhCHedVB4&feature=relmfuhttp://www.youtube.com/watch?

v=rtNaEFXOdAc&feature=relmfu

Halogens• They want to be –1!• Easily reduced, high EA, high IE.• Density increases moving down the group. (mass rises

faster than atomic radius)• Reactions

– With metals to form metal halides, e.g. 2 Al(s) + 3 Cl2(g) 2 AlCl3(s)

– With hydrogen to form hydrogen halides (binary acids!), e.g.H2(g) + I2(s) 2 HI(g)

– With other halogens to form interhalogen compounds, e.g.Br2(l) + F2(g) 2 BrF(g)

• http://www.youtube.com/watch?v=F4IC_B9i4Sg

Noble Gases• Closed-shell electron configuration; very unreactive!• Used for lights, airtanks for divers, cryogens• Few reactions! Fluorides, oxides can be made under

severe conditions.

• Helium--helios (sun)• Krypton--kryptos (hidden)• Neon--neos (new)• Xeno--xenos (stranger)