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Periodic properties of the elements A journey across the table is a journey through chemistry.

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Presentation on theme: "Periodic properties of the elements A journey across the table is a journey through chemistry."— Presentation transcript:

1 Periodic properties of the elements A journey across the table is a journey through chemistry

2 The shape of the table is explained by the orbital model 2 groups 10 groups 14 groups 6 groups

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

4 Relating chemical and physical properties to the table Mendeleev’s table was constructed 50 years before the discovery of the nucleusMendeleev’s table was constructed 50 years before the discovery of the nucleus The arrangement of elements was by atomic weightThe arrangement of elements was by atomic weight Atomic number (Moseley 1915) provides a more systematic orderingAtomic number (Moseley 1915) provides a more systematic ordering Note: X-ray photon energy varies smoothly with atomic number, providing a method of identifying elementsNote: X-ray photon energy varies smoothly with atomic number, providing a method of identifying elements

5 Explaining periodicity: effective nuclear charge and shielding Inner shell (core) electrons screen the outer (valence) electrons from the nuclear chargeInner shell (core) electrons screen the outer (valence) electrons from the nuclear charge –Shielding is more effective for electrons with higher angular momentum (higher l value: 3d > 3p > 3s) –Decreases as the valence shell fills Increased shielding means lower effective nuclear chargeIncreased shielding means lower effective nuclear charge

6 Effective nuclear charge Effective nuclear chargeEffective nuclear charge Z eff = Z – S –Z = atomic number –S = screening constant (roughly number of core electrons) Model is approximate because the valence electrons penetrate the inner shellsModel is approximate because the valence electrons penetrate the inner shells –Actual Z eff > Z - S

7 Z eff and angular momentum Electron in 2s orbital spends more time close to nucleus than electron in 2pElectron in 2s orbital spends more time close 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

8 Shielding and effective nuclear charge The “shell” picture helps to explain these observationsThe “shell” picture helps to explain these observations Electrons in same shell experience stronger attraction to nucleus as shell fillsElectrons in same shell experience stronger attraction to nucleus as shell fills

9 Correlate atomic radius and effective nuclear charge (Z eff )

10 Atomic radii and periodic variation

11 Isoelectronic ionic series: same electrons, different charge In a period the ions will tend to be isoelectronic (all have filled shells)In a period the ions will tend to be isoelectronic (all have filled shells) –Na +, Mg 2+, Al 3+ –P 3-, S 2-, Cl - For the cations, Z eff ↑, radius ↓ as Z ↑For the cations, Z eff ↑, radius ↓ as Z ↑ For the anions, Z eff ↑, radius ↓ as Z ↑For the anions, Z eff ↑, radius ↓ as Z ↑

12 Ionization energy Ionization energy (I):Ionization energy (I): The energy required to remove an electron from an isolated gaseous atom M(g) = M(g) + + e As I increases, atoms are harder to ionizeAs I increases, atoms are harder to ionize Successive ionizations for electrons are represented by I i (i = 1,2,3,…)Successive ionizations for electrons are represented by I i (i = 1,2,3,…) I 1 < I 2 < I 3 <…

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14 Ionization energy and shells There is always a huge increase in I i when i represents an electron in an inner shellThere is always a huge increase in I i when i represents an electron in an inner shell

15 Kink and ionization energy Unlike atomic radius the trend is not as smoothUnlike atomic radius the trend is not as smooth Note the dip at Be – B and N – ONote the dip at Be – B and N – O Random or rational?Random or rational? –B adds electron to 2p –O pairs electron (2p 4 )

16 Electron affinity Electron affinity:Electron affinity: The electron affinity is the energy released when an electron is added to a gaseous neutral atom X(g) + e = X(g) - Unlike ionization, addition of an electron is usually favourable – even for electropositive elementsUnlike ionization, addition of an electron is usually favourable – even for electropositive elements

17 Periodicity and electron affinity Smooth trends are not in evidenceSmooth trends are not in evidence Affinity tends to be zero or positive when electron is added to new (Be, Mg, He, Ne etc) or half-filled (N) shellAffinity tends to be zero or positive when electron is added to new (Be, Mg, He, Ne etc) or half-filled (N) shell

18 Electronegativity and periodicity

19 Density and periodicity

20 Trend in melting point not as easily explained

21 Periodicity and chemical properties Most obvious trend is from metallic to non- metallicMost obvious trend is from metallic to non- metallic Acid-base properties of oxides:Acid-base properties of oxides: –Metal oxides tend to be basic (Na 2 O = NaOH in H 2 O)(Na 2 O = NaOH in H 2 O) –Nonmetal oxides tend to be acidic (SO 2 = H 2 SO 3 in H 2 O)(SO 2 = H 2 SO 3 in H 2 O)


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