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Learning Outcomes. Atomic radii (covalent radii only ). Explanations for general trends in values : ( i ) down a group (ii ) across a period ( covalent radii of main group elements only). First ionisation energies. - PowerPoint PPT PresentationTRANSCRIPT
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Learning Outcomes• Atomic radii (covalent radii only). Explanations for
general trends in values: (i) down a group (ii) across a period (covalent radii of main group elements only).
• First ionisation energies.• Explanations for general trends in values: (i) down a
group (ii) across a period (main group elements) and for exceptions to the general trends across a period.
• Second and successive ionisation energies.• Evidence for energy levels provided by successive
ionisation energy values.
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Atomic radii
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Atomic radii trends
• In general, the atomic radii values decrease across the period and increase down the group
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Atomic radius
• Half the distance • Between the nuclei of 2 atoms of the same
element• Joined by a single covalent bond
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trends
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Reasons for increase down a group
• The additional electrons are going into a new shell which is further from the nucleus
• Screening effect of inner electrons
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Screening effect
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Reasons for decrease across a period
• Increasing nuclear charge. • No increase in the screening effect
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IONISATION ENERGY• Some elements lose electrons very easily,
e.g. sodium and potassium • Silver and gold have very little tendency to
lose their electrons and hence are very unreactive
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definitionThe first ionisation energy is the energy required to remove the most loosely held electron from one mole of gaseous atoms to produce 1 mole of gaseous ions each with a charge of 1+.
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Na loses an electron
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equation
Na Na + + e-
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I. E. in groups I and II
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Chlorine increases in size
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Ionisation Energy decreases going down a group
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Reasons for decrease down
• Increasing atomic radius. • Screening effect of inner electrons
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Ionisation Energy increases across a period
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Reasons increase across
• Increasing nuclear charge. • Decreasing atomic radius
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IE and periodic table.
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Exceptions to the general trends• Beryllium and nitrogen have higher values than
expected • Reasons:• Be 1S2 2S2 (Full orbitals give greater stability)• N 1S2 2S2 2Sx1 2Py1 2Pz1 (3 half filled orbitals
give greater stability)
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EVIDENCE FOR EXISTENCE OF ENERCY LEVELS
• Suppose we measure the first, second, third, etc. up to the nineteenth ionisation energy of potassium
• K = 1S2 2S2 2P6 3S2 3P6 4S1
• K = 2,8,8,1• First ionisation energy has the lowest
ionisation energy value. Electron in the 4s sublevel is easiest to remove
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Potassium ionisation
• 1st ionisation energy: K(g) → K+(g) + e– n=1
• 2nd ionisation energy: K+(g) → K2+(g) + e– n=2
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Potassium IE’sn ionisation energy
(kJ mol–1)
1 419 2 3051 3 4412 4 5877 5 7975 6 9649 7 11343 8 14942 9 16964 10 48577 11 54433 12 60701 13 68896 14 75950 15 83152 16 93403 17 99771 18 444911 19 476075
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Sucessive Ionisation of Potassium
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Potassium IE’s• A, one electron has
been removed from potassium
• The second electron is much more difficult to remove since this electron is being removed from the K+ ion
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Sucessive Ionisation of Po-tassium
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Potassium IE’s• K+ This ion has
eight electrons in the outer shell ( B,C)
• The full outer sublevel (3p6) has extra stability and therefore will require more energy to remove electrons from it. (B,C)
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Potassium IE’s• B to C we are
removing eight more electrons
• (point C on the graph), there is another sudden jump D is being removed from a shell which is closer to the nucleus
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