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Chemistry Question

The first ionization energies, in kJ mol–1, of four elements with consecutive atomic
numbers are shown below.
A 1680
B 2080
C 496
D 738


(b) Which element could be X in a covalent compound with formula HX?
(1)
A
B
C
D
(c) Which element could be Y in an ionic compound with formula YH2?
(1)
A
B
C
D

The answers are A and D.
I dont understand why though? It is a little confusing. Please help. Thanks.
Reply 1
X must be in group 7 to form a covalent compound with formula HX. Since the pattern of first ionisation energies along the periods increases up to group 8, then drops for the first element in the next period, it means that B must be in group 8 so A is in group 7.

By similar reasoning Y must be in group 2 to form that compound, and by looking at the pattern, C is in group 1 and D is in group 2, so the answer is D.
Reply 2
Original post by Phil_Waite
X must be in group 7 to form a covalent compound with formula HX. Since the pattern of first ionisation energies along the periods increases up to group 8, then drops for the first element in the next period, it means that B must be in group 8 so A is in group 7.

By similar reasoning Y must be in group 2 to form that compound, and by looking at the pattern, C is in group 1 and D is in group 2, so the answer is D.


Thanks.
I get the first part

But the second part i don't get. Why must Y be in group 2
Because its h2, so 2 electrons. and if its in group 2, which has two outer electrons, another 2 electrons by the hydrogen won't built an octec.

:s-smilie: :|
Reply 3
Original post by EffKayy
Thanks.
I get the first part

But the second part i don't get. Why must Y be in group 2
Because its h2, so 2 electrons. and if its in group 2, which has two outer electrons, another 2 electrons by the hydrogen won't built an octec.

:s-smilie: :|


The information says that the hydrogen forms an ionic bond with compound Y. This means that the compound with formula YH2 will contain hydride (H-) ions and a Y 2+ ion to form the compound, so Y is in group 2.
Reply 4
Thanks, your explanation helped me to understand it.

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