Covalent Bonding: Shared Pairs and Simple Molecules · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Covalent bonding: shared pairs and simple molecules

Science · Matter & Materials · ages 14-16
Name ______________________   Date ____________

  1. Why do simple molecular substances melt and boil at low temperatures?

    • their covalent bonds are unusually weak and snap easily
    • their molecules are too light to stay in one place at all
    • they have no bonds holding them together in the first place at all
    • whole molecules only have to be pulled apart from each other

  2. Why can two chlorine atoms not form an ionic bond with each other?

    • chlorine atoms are far too small to hold a charge
    • no atom here is prepared to hand an electron over
    • chlorine is a metal, and metals only bond with non-metals
    • chlorine atoms already have full outer shells

  3. Nitrogen is in group 5. How many covalent bonds does one nitrogen atom make?

    • five, one for each of its outer electrons
    • two, exactly the same as oxygen does
    • three, since it is three electrons short
    • eight, enough to fill the whole shell

  4. How many electrons are there in one shared pair?

    Answer: ______________

  5. Sulfur is 2,8,6 and hydrogen has one electron. What is the formula of hydrogen sulfide?

    • H₂S
    • HS
    • HS₂
    • H₆S

  6. Two atoms share one pair of electrons. How many electrons has each atom given away for good?

    • one each, which is why the bond is called a shared pair
    • two each, since a pair is two electrons
    • one atom gives two and the other gives none
    • none, because both atoms still count the pair

  7. Why do two oxygen atoms form a double bond rather than a single one?

    • oxygen atoms are heavier, so they need a stronger bond
    • a double bond takes less energy to make than a single one does
    • sharing once still leaves each atom incomplete
    • oxygen has two shells, so it needs two bonds

  8. In a dot-and-cross diagram of a chlorine molecule, why is the shared pair drawn where the two rings cross?

    • so the drawing takes up much less room on the page
    • because that is where the two nuclei sit
    • to show which atom won the tug of war
    • so that each atom counts both of those electrons

  9. A simple molecular substance conducts electricity once it has melted.

    Circle one:   True   False

  10. Carbon dioxide is O=C=O. How many electrons is the carbon counting in its outer shell?

    Answer: ______________

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Answer key

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Covalent bonding: shared pairs and simple molecules W1-mt_fn5Dq6qUjP-s1

  1. whole molecules only have to be pulled apart from each other · Melting and boiling separate whole molecules from each other. Those between-molecule pulls are weak, so not much heat is needed, and the strong bonds inside survive.
  2. no atom here is prepared to hand an electron over · An ionic bond needs a giver and a taker. Two chlorine atoms both want to gain, so there is nobody to hand an electron over.
  3. three, since it is three electrons short · Group 5 means five outer electrons, so nitrogen is 8 - 5 = 3 short. Three shared pairs close the gap.
  4. 2 · A pair is two electrons, one contributed by each atom. That is what makes it a shared pair rather than a transfer.
  5. H₂S · Sulfur is 8 - 6 = 2 electrons short, so it makes two bonds. Each hydrogen makes one, so two hydrogens are needed and the formula is H₂S.
  6. none, because both atoms still count the pair · Each atom puts one electron in, but neither loses it. The pair sits in the overlap and both atoms count both electrons, which is why neither ends up charged.
  7. sharing once still leaves each atom incomplete · Each oxygen is 8 - 6 = 2 electrons short. One shared pair leaves both still one short, so they share a second pair and both reach eight.
  8. so that each atom counts both of those electrons · Sitting inside both rings at once is what lets each atom count the pair. That double counting is the whole reason sharing fills two outer shells with one pair.
  9. False · Melting an ionic solid frees charged ions, but a molecular liquid still contains only neutral molecules. There is nothing charged to carry a current in any state.
  10. 8 · Carbon starts with four outer electrons and makes two double bonds, which brings four more electrons into its rings. 4 + 4 = 8, a full shell.
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