Periodic Periodic PropertiesPropertiesPeriodic Periodic PropertiesProperties

Chapter 7Chapter 7

Overview

Periodic Table

Electron Shells & Sizes of Atoms

Ionization Energy

Electron Affinities

Metals, Nonmetals & Metalloids and

Trends

Periodic Table

1869 -- Dmitri Mendeleev & Lothar Meyer published classification schemes of elements based on order of increasing elemental atomic

weight which follows atomic number

some missing elements were “discovered” based upon prediction that they should be there

Henry Moseley related energy of nuclear x-rays to atomic number

Sizes of Atoms

Electron Shells quantum mechanical model predicts shells of

electron density as number of protons increases, inner shells are

held more tightly, closer to nucleus Sizes of Atoms

atom boundaries are difficult to assess define atom size as ½ the distance between

nuclei of two bound atoms

+ +

Cl Cl

Atomic radii = ½ distance between the nuclei

r

General Trends of Physical Properties most trends are related to Zeff, effective

nuclear charge due to increase in distance from nucleus and

shielding, Zeff decreases going down a group

due to ineffective shielding by electrons in the same shell, Zeff increases going across a

row

Zeffincreases

decreases

Compare valence e- ‘s of Na and Cs which will be held more tightly by nucleus?

• the e- that is closest to the nucleus

which will be closest to the nucleus?• Na’s 3s electron

Compare last electron of C and F which has more protons interacting?

• ZF = 9 while ZC = 4

• do e- in the same subshell shield their fellow e- ‘s very well?

–no• so which last e- will experience the greatest Zeff?

–F

General Trends in Atomic Size radius increases on going down a group

(Zeff decreases) radius tends to decrease on going left to

right across a row (Zeff increases)

size decreases

increases

We know that Zeff decreases going down a group, how does that affect size?

As Zeff (on a valence e- ) decreases, it is less tightly held, hence can expand somewhat

We know that Zeff increases going across a row, how does that affect size?

As Zeff (on a last e-) increases, it is more tightly held, hence contraction occurs and atom is smaller

Compare valence e- ‘s of Na and Cs which experiences the greatest Zeff ?

• Na’s 3s electron (bec. it is closest to nucleus)

which atom will be smaller?• Na < Cs

Compare last electron of C and F which has more protons interacting?

• ZF = 9 while ZC = 4

• do e- in the same subshell shield their fellow e- ‘s very well?

–no• so which last e- will experience the greatest Zeff?

–F• which atom will experience greatest contraction?

–F will be more contracted, hence smaller

Trends in Ionization Energy ionization energy decreases going down

(Zeff decreases) ionization energy increases going across

(Zeff increases)

I.E. increases

decreases

Ionization Energy -- endothermic process first ionization energy, I1 -- to remove first

electron• M M+ + 1e -

second ionization energy, I2 -- to remove second electron • M+ M2+ + 1e -

third ionization energy, I3 -- to remove third electron • M2+ M3+ + 1e -

I1 < I2 < I3

Note: removal of inner core electrons is very energy expensive

Element I1 I2 I3 I4

Na 496 4560

Mg 738 1450 7730

Al 578 1820 2750 11,600

The more tightly an e- is held, the more energy it takes to remove it in other words, greater Zeff = greater I.E.

Compare valence e- ‘s of Na and Cs which has greater Zeff?

• Na

which has greater I1?• Na (Cs’s 6s electron is further away, less tightly held and

easier to remove than Na’s 3s electron)

Compare last e- of C and F which has greater Zeff?

• F which has greater I1?

• F (more tightly held, more energy to remove)

transition metals and f-block metals show small variation in ionization energies across rows

representative elements show a larger range of values for I1

irregularities in ionization energies occur due to shielding I1 for Al < I1 for Mg -- removal of p versus s electron

I1 for O < I1 for N -- removal of p electron to produce stable configuration

Electron Affinities energy change associated with adding an

electron to gaseous atom measures attraction of atom for the added electron

• Cl(g) + 1e - Cl - E = -349 kJ/mol

exothermic process most electron affinities are exothermic

some exceptions & irregularities• nobel gases• Be & Mg• Group III elements

Metals general trends and properties

to the left on periodic table ability to lose electrons (low ionization energies) -- what

kind of ions? have a metallic or shiny luster, ductile form crystalline solids have good thermal and electrical conductivity transition metals vary in charge

• most have +2 in addition to other charges (+1 to +5)

form ionic compounds with non-metals• especially with oxides or halides

metal oxides are called basic oxides• react with water to form basic hydroxides

• Na2O(s) + H2O(l) 2NaOH(aq) • react with acid to form water and salt

Nonmetals general trends and properties

to the right on periodic table ability to gain electrons -- what kind of ions? non-conductors and thermal insulators have lower melting points – are molecular

solids rather than ionic solids seven exist as diatomic molecules

– H2, N2, O2, F2, Cl2, Br2, I2

form molecular compounds with other nonmetals such as oxides, halides & hydrides

nonmetal oxides are called acidic oxides• react with water to form acids

• CO2(g) + H2O(l) H2CO3(aq)

• react with bases to form salt and water

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

Metalloids have a mixture of properties some metallic,

some not

Group Trends for Active Metals Group 1A -- Alkali Metals

very reactive, only found in nature in oxidized form (compounds)

metals produced by electrolysis of molten compounds

reactivity increases down the group react rapidly with water

2K(s) + 2H2O(l) 2KOH(aq) + H2(g) react with acid:

2K(s) + 2HCl(aq) 2KCl(aq) + H2(g)

react with hydrogen to form hydrides• 2K(s) + H2(g) 2KH(s)

react with oxygen in different ways• 4Li(s) + O2(g) 2Li2O(s) containing O2-

• this is the more common reaction

other alkali metals form peroxides, O22-

• 2Na(s) + O2(g) Na2O2(s)

K, Rb, Cs form superoxides, O2-

• K(s) + O2(g) KO2(s)

• peroxides and superoxides are not as common

most alkali metal compounds are colorless

Group 2A -- Alkaline Earth Metals reactive – less than alkali metals (ionization energies

higher) reactivity increases down the group heavier metals react with water to form bases

• Ca(s) + 2H2O(l) Ca(OH)2(aq) + H2(g)

• lighter metals react similarly with hot water or steam

react with acids: • Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)

react with O to produce oxides• 2Mg(s) + O2(g) 2MgO(s)

Trends for Nonmetals

Hydrogen nonmetal, diatomic, colorless gas high ionization energy (no shielding) typically forms molecular compounds with

other nonmetals• these reactions are fairly exothermic

• 2H2(g) + O2(g) 2H2O(l) Ho = -571.7 kJ/mol

forms hydrides (H-) with metals• 2Na(s) + H2(g) 2NaH(s)

Group 6A: Oxygen Group density increases, metallic character

increases down group oxygen is diatomic, & the only gas in the

group allotrope (different form) is ozone, O3

• 3O2(g) 2O3(g) Ho = +284.6 kJ/mol

• is ozone more or less stable than O2?

ions are in different forms• O2-, oxide

• O22-, peroxide

• O2-, superoxide

less

Group 7A: Halogens increased nonmetallic character lighter elements are diatomic gases & more reactive than

heavier elements have very exothermic electron affinities and gain electron

easily F has highest electron affinity

• 2Na(s) + F2(g) 2NaF(s) Ho = -1147 kJ

• 2H2O(l) + 2F2(g) 4HF(aq) Ho = -758.7 kJ

Cl reacts somewhat more slowly• Cl2(g) + H2O(l) HCl(aq) + HOCl(aq)

Group 8A: Noble Gases all are monotomic, quite stable and unreactive Xe was the first noble gas compound identified

• XeF6

Kr is less reactive & has one known compound• KrF2

no compounds of He, Ne or Ar are known• however, there have been implications of a possible Ar

compound