Calcium chloride
4 marks · NormalA student attempts to draw the dot-and-cross diagram for lithium fluoride (LiF) but makes some mistakes. Examine the student’s diagram and answer the following questions.
Identify two mistakes in the student's dot-and-cross diagram for lithium fluoride.
Draw a correct dot-and-cross diagram for lithium fluoride (LiF). Use dots (•) for lithium electrons and crosses (×) for fluorine electrons, or vice versa. Show only the outer shell electrons.
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(a) Mistake 1: Lithium should have only 1 outer electron, not 2. Mistake 2: Fluorine's 8th outer electron is missing or all are the same symbol. · must show Identifies lithium has 1, not 2, electrons in outer shell, Identifies fluorine should have 8, not 7, outer electrons, Notes that electron symbols should be different for Li and F
(b) Correct diagram: lithium ion (no outer electrons), fluoride ion (8 electrons, with one cross showing electron received from Li); symbols must be distinguished for each atom. · must show One electron transferred from Li to F, Li+ ion has no electrons in outer shell, F- ion has full outer shell (8 electrons), Different symbols (dot and cross) for electrons from lithium and fluorine
Common mistakes: Not showing correct number of outer electrons; Not showing electron transfer; Wrong charges on ions
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Marking points
B1 (a) · 1 mark · Correctly identifies that lithium should have only 1 electron in its outer shell.
B2 (a) · 1 mark · Correctly identifies that fluorine should have 8 electrons in its outer shell, or must use a different symbol for the transferred electron.
B3 (b) · 1 mark · Correctly shows one electron transferred from lithium to fluorine (distinct symbols for each atom).
B4 (b) · 1 mark · Correct dot-and-cross diagram showing Li+ (no outer electrons), F- (8 electrons, one from Li); correct use of symbols.
Worked solution
a) Two mistakes in the student's diagram:
1. Lithium should have only 1 electron in its outer shell (it has 1 valence electron), but two are shown.
2. Fluorine should have 8 outer electrons after gaining one from lithium, but only 7 are shown, and all are the same symbol; a transferred electron should be shown with a different symbol.
b) A correct dot-and-cross diagram: