The first ionisation energies of the p-block elements are given in the table below. Predict the missing value for the first ionisation energy of selenium. p-block element of period 4 Atomic number First ionisation energy/ kJmol - 1 gallium ( Ga ) 31 577 germanium ( Ge ) 32 762 arsenic ( As ) 33 966 selenium ( Se ) 34 bromine ( Br ) 35 1140 krypton ( Kr ) 36 1350

p -block element of period 4 Atomic number First ionisation energy/ kJmol - 1 gallium ( Ga ) 31 577 germanium ( Ge ) 32 762 arsenic ( As ) 33 966 selenium ( Se ) 34 941 bromine ( Br ) 35 1140 krypton ( Kr ) 36 1350 Ionization energy generally increases across a period as atomic size decreases and effective nuclear charge increases. Selenium has first ionisation energy lesser than that of arsenic because the electron removed from the arsenic is from an orbital that contains an unpaired electron. The electron removed from the selenium is from the orbital that contains a pair of electrons. The extra repulsion between the pair of electrons in this orbital results in less energy being needed to remove an electron. So, IE for selenium is lower, because of spin-pair repulsion.

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Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 10 - Periodicity>Question>Q 2

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By looking at the elements in Period 2 of the Periodic Table and using your knowledge of Period 3 elements, predict and explain the relative sizes of a nitride ion $N^{3 -}$ , and a fluoride ion, $F^{-}$ .

Important Questions on Periodicity

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Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 10 - Periodicity>Question>Q 3

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AS and A Level

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Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 10 - Periodicity>Question>Q 3

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AS and A Level

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Why is magnesium a better electrical conductor than sodium?

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Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 10 - Periodicity>Questions>Q 4

What is the general trend in first ionisation energies across Period 3?

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AS and A Level

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Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 10 - Periodicity>Questions>Q 4

Explain why aluminium has a lower first ionisation energy than magnesium.

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AS and A Level

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Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 10 - Periodicity>Questions>Q 4

Explain why sulphur has a lower first ionisation energy than phosphorus.

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AS and A Level

IMPORTANT

Chemistry for Cambridge International AS & A Level Coursebook with Digital Access (2 Years)>Chapter 10 - Periodicity>Questions>Q 4

The first ionisation energies of the p-block elements are given in the table below. Predict the missing value for the first ionisation energy of selenium.

p-block element of period 4	Atomic number	First ionisation energy/ ${kJmol}^{- 1}$
gallium $(Ga)$	31	577
germanium $(Ge)$	32	762
arsenic $(As)$	33	966
selenium $(Se)$	34
bromine $(Br)$	35	1140
krypton $(Kr)$	36	1350