What is a covalent bond?
5 marks · HardBoron trifluoride (BF) is a covalent compound. Answer the following questions about its bonding.
How many electrons are in the outer shell of a boron atom and of a fluorine atom?
Draw a dot-and-cross diagram to show the covalent bonding in a molecule of BF. Use dots (•) for boron’s electrons and crosses (×) for fluorine’s electrons.
Explain, using your diagram, why the central boron atom in BF does not have a full octet in its outer shell.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Boron: 3; Fluorine: 7
(b) Dot-and-cross diagram with boron (central, 3 outer electrons = dots), surrounded by 3 fluorines (each 7 outer electrons = crosses), one shared pair B–F per bond, 3 lone pairs (×) on each fluorine. · must show B atom in centre with 3 dots, F atoms each with 7 crosses, 3 B–F single bonds each with a shared pair (1 dot + 1 cross per bond), Each fluorine shows 3 lone pairs, No more than 6 electrons around boron
(c) Boron only shares 3 pairs (6 electrons) so does not achieve a full octet (8 electrons) in its outer shell, even after bonding. · must show States boron has only 6 outer electrons, Octet not reached
Must show: For the diagram: B atom (dots) bonded to 3 F atoms (crosses), 3 single bonds; B with 3 shared pairs, F with 3 lone pairs each.
Common mistakes: Assigning 8 electrons to boron; Missing lone pairs on fluorine atoms
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States correct outer shell numbers: boron = 3, fluorine = 7.
B2 (b) · 1 mark · Draws correct dot-and-cross arrangement with B at centre, 3 F atoms, bonds as single shared pairs.
B3 (b) · 1 mark · Each F atom shows 3 lone pairs; boron shown with only 3 bonds (not octet).
B4 (c) · 1 mark · Explains boron is left with 6 electrons (not 8) after bonding.
B5 (c) · 1 mark · Mentions octet is not achieved or relates to incomplete outer shell.
Worked solution
a) Boron has 3 electrons in its outer shell. Fluorine has 7 electrons in its outer shell.
b) Each bond is a single shared pair (one dot from B, one cross from F). Each fluorine has three lone pairs (crosses unused in the bond).
c) Boron only forms 3 covalent bonds, sharing just 6 electrons in its outer shell. Therefore, it does not achieve a full octet.