Periodic properties of the Periodic properties of the elementselements

A journey across the table is a A journey across the table is a journey through chemistryjourney through chemistry

The shape of the table is explained The shape of the table is explained by the orbital modelby the orbital model

2 groups 10

groups

14 groups

6 groups

Number of elements per row Number of elements per row matches the capacity of the matches the capacity of the

orbitalsorbitals

Relating chemical and physical Relating chemical and physical properties to the tableproperties to the table

• Mendeleev’s table was constructed 50 Mendeleev’s table was constructed 50 years before the discovery of the nucleusyears before the discovery of the nucleus

• The arrangement of elements was by The arrangement of elements was by atomic weightatomic weight

• Atomic number (Moseley 1915) provides a Atomic number (Moseley 1915) provides a more systematic orderingmore systematic ordering

• Note: X-ray photon energy varies smoothly Note: X-ray photon energy varies smoothly with atomic number, providing a method of with atomic number, providing a method of identifying elementsidentifying elements

Explaining periodicity: effective Explaining periodicity: effective nuclear charge and shieldingnuclear charge and shielding

• Inner shell (core) electrons screen the Inner shell (core) electrons screen the outer (valence) electrons from the nuclear outer (valence) electrons from the nuclear chargecharge– Shielding is more effective for electrons with higher Shielding is more effective for electrons with higher

angular momentum (higher l value: 3d > 3p > 3s)angular momentum (higher l value: 3d > 3p > 3s)– Decreases as the valence shell fillsDecreases as the valence shell fills

• Increased shielding means lower effective Increased shielding means lower effective nuclear chargenuclear charge

Effective nuclear chargeEffective nuclear charge

• Effective nuclear charge Effective nuclear charge

ZZeff eff = Z – S= Z – S

– Z = atomic numberZ = atomic number– S = screening constant (roughly number of S = screening constant (roughly number of

core electrons)core electrons)

• Model is approximate because the valence Model is approximate because the valence electrons penetrate the inner shellselectrons penetrate the inner shells– Actual ZActual Zeffeff > Z - S > Z - S

ZZeffeff and angular momentum and angular momentum

• Electron in 2s orbital spends more time Electron in 2s orbital spends more time close to nucleus than electron in 2pclose to nucleus than electron in 2p

• 2s orbital lower in energy than 2p2s orbital lower in energy than 2p• Shielding of 2s lower than 2pShielding of 2s lower than 2p• Shielding follows order: 3d>3p>3sShielding follows order: 3d>3p>3s

Shielding and effective nuclear chargeShielding and effective nuclear charge

• The “shell” picture helps to explain these The “shell” picture helps to explain these observationsobservations

• Electrons in same shell experience stronger Electrons in same shell experience stronger attraction to nucleus as shell fillsattraction to nucleus as shell fills

Correlate atomic radius and Correlate atomic radius and effective nuclear charge (Zeffective nuclear charge (Zeffeff))

Atomic radii and periodic variationAtomic radii and periodic variation

Isoelectronic ionic series: same Isoelectronic ionic series: same electrons, different chargeelectrons, different charge

• In a period the ions will tend to be In a period the ions will tend to be isoelectronic (all have filled shells)isoelectronic (all have filled shells)– NaNa++, Mg, Mg2+2+, Al, Al3+3+

– PP3-3-, S, S2-2-, Cl, Cl--

• For the cations, ZFor the cations, Zeffeff ↑, radius ↓ as Z ↑↑, radius ↓ as Z ↑

• For the anions, ZFor the anions, Zeffeff ↑, radius ↓ as Z ↑ ↑, radius ↓ as Z ↑

Ionization energyIonization energy

• Ionization energy (I):Ionization energy (I):

The energy required to remove an electron The energy required to remove an electron from an isolated gaseous atomfrom an isolated gaseous atom

M(g) = M(g)M(g) = M(g)++ + e + e

• As I increases, atoms are harder to ionizeAs I increases, atoms are harder to ionize

• Successive ionizations for electrons are Successive ionizations for electrons are represented by Irepresented by Iii (i = 1,2,3,…) (i = 1,2,3,…)

II11 < I < I22 < I < I33 <… <…

Ionization energy and shellsIonization energy and shells

• There is always a huge increase in IThere is always a huge increase in I ii when i when i

represents an electron in an inner shellrepresents an electron in an inner shell

Kink and ionization energy Kink and ionization energy

• Unlike atomic radius Unlike atomic radius the trend is not as the trend is not as smoothsmooth

• Note the dip at Be – B Note the dip at Be – B and N – Oand N – O

• Random or rational?Random or rational?– B adds electron to 2p B adds electron to 2p – O pairs electron (2pO pairs electron (2p44) )

Electron affinityElectron affinity

• Electron affinity:Electron affinity:

The electron affinity is the energy released The electron affinity is the energy released when an electron is added to a gaseous when an electron is added to a gaseous

neutral atomneutral atom

X(g) + e = X(g)X(g) + e = X(g)--

• Unlike ionization, addition of an electron is Unlike ionization, addition of an electron is usually favourable – even for usually favourable – even for electropositive elementselectropositive elements

Periodicity and electron affinityPeriodicity and electron affinity

• Smooth trends are Smooth trends are not in evidencenot in evidence

• Affinity tends to be Affinity tends to be zero or positive when zero or positive when electron is added to electron is added to new (Be, Mg, He, Ne new (Be, Mg, He, Ne etc) or half-filled (N) etc) or half-filled (N) shellshell

Electronegativity and periodicityElectronegativity and periodicity

Density and periodicityDensity and periodicity

Trend in melting point not as easily Trend in melting point not as easily explainedexplained

Periodicity and chemical propertiesPeriodicity and chemical properties

• Most obvious trend is from metallic to non-Most obvious trend is from metallic to non-metallicmetallic

• Acid-base properties of oxides:Acid-base properties of oxides:– Metal oxides tend to be basic Metal oxides tend to be basic

• (Na(Na22O = NaOH in HO = NaOH in H22O)O)

– Nonmetal oxides tend to be acidicNonmetal oxides tend to be acidic• (SO(SO22 = H = H22SOSO33 in H in H22O)O)