Cambridge IGCSE Chemistry Teachers Resource (fourth edition) by ... - Free Printable

To solve this worksheet, we need to draw the outer shell electrons for each molecule. Here is the step-by-step logic for how covalent bonding works in these diagrams:

1. Identify Valence Electrons: First, determine how many electrons are in the outer shell of each atom based on its Group in the Periodic Table.
* Hydrogen (H): Group 1 $\rightarrow$ 1 valence electron. Needs 2 total to be full.
* Carbon (C): Group 4 $\rightarrow$ 4 valence electrons. Needs 8 total.
* Nitrogen (N): Group 5 $\rightarrow$ 5 valence electrons. Needs 8 total.
* Oxygen (O): Group 6 $\rightarrow$ 6 valence electrons. Needs 8 total.
* Chlorine (Cl): Group 7 $\rightarrow$ 7 valence electrons. Needs 8 total.

2. Drawing Rule: When atoms bond, they share pairs of electrons.
* In the overlapping area of the circles, draw one electron from each atom to make a shared pair (a single bond).
* Draw the remaining non-bonding electrons (lone pairs) in the rest of the atom's circle so that every atom (except Hydrogen) has 8 electrons in its outer shell.
* Symbol Tip: Use different symbols (like $\times$ and $\bullet$, or crosses and dots) to show which electrons belong to which atom, as requested by the instructions.

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Part 1a: Simple Molecules

1. Hydrogen molecule ($H_2$)
* Each H has 1 electron.
* They share their electrons in the middle.
* Draw: Put one cross in the left H (outside overlap), one dot in the right H (outside overlap). In the overlapping middle section, put one cross and one dot.
* *Check:* Each H now "sees" 2 electrons.

2. Chlorine molecule ($Cl_2$)
* Each Cl has 7 valence electrons.
* They share 1 pair to complete their octets.
* Draw:
* Overlap: One cross and one dot (the shared pair).
* Left Cl (non-overlap): Draw 3 pairs (6 electrons) using crosses.
* Right Cl (non-overlap): Draw 3 pairs (6 electrons) using dots.
* *Check:* Each Cl has $6 \text{ (own)} + 2 \text{ (shared)} = 8$ electrons.

3. Hydrogen chloride ($HCl$)
* H has 1 electron; Cl has 7 valence electrons.
* Draw:
* Overlap: One cross (from H) and one dot (from Cl).
* H (non-overlap): Empty (it only needs the shared pair to reach 2).
* Cl (non-overlap): Draw 3 pairs (6 electrons) using dots.
* *Check:* H has 2 electrons. Cl has $6 \text{ (own)} + 2 \text{ (shared)} = 8$ electrons.

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Part 1b: Complex Molecules

4. Methane molecule ($CH_4$)
* Central C has 4 valence electrons. Four H atoms each have 1.
* Draw:
* Overlaps: There are 4 overlapping areas (top, bottom, left, right). In each overlap, draw one cross (from C) and one dot (from H).
* Central C: No extra electrons outside the overlaps (all 4 are used in bonding).
* H atoms: No extra electrons.
* *Check:* C has 8 shared electrons. Each H has 2 shared electrons.

5. Ammonia molecule ($NH_3$)
* Central N has 5 valence electrons. Three H atoms each have 1.
* Draw:
* Overlaps: In the three overlapping areas, draw one cross (from N) and one dot (from H).
* Central N: You have used 3 electrons for bonding. You have 2 left. Draw one pair of crosses in the empty space of the N circle (usually at the top).
* H atoms: No extra electrons.
* *Check:* N has $2 \text{ (lone pair)} + 6 \text{ (shared)} = 8$ electrons. Each H has 2.

6. Water molecule ($H_2O$)
* Central O has 6 valence electrons. Two H atoms each have 1.
* Draw:
* Overlaps: In the two overlapping areas, draw one cross (from O) and one dot (from H).
* Central O: You have used 2 electrons for bonding. You have 4 left. Draw two pairs of crosses in the empty space of the O circle.
* H atoms: No extra electrons.
* *Check:* O has $4 \text{ (lone pairs)} + 4 \text{ (shared)} = 8$ electrons. Each H has 2.

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Final Answer:
Here is the description of exactly what to draw in each diagram:

1a. Hydrogen molecule ($H_2$):
* Draw 1 electron in the overlapping region from the left H and 1 electron in the overlapping region from the right H. (Total 2 electrons in the middle).

1a. Chlorine molecule ($Cl_2$):
* Overlap: Draw 1 pair of electrons (one from each atom).
* Rest of Left Cl: Draw 3 pairs (6 electrons).
* Rest of Right Cl: Draw 3 pairs (6 electrons).

1a. Hydrogen chloride ($HCl$):
* Overlap: Draw 1 pair of electrons (one from H, one from Cl).
* Rest of Cl: Draw 3 pairs (6 electrons).
* Rest of H: Leave empty.

1b. Methane ($CH_4$):
* 4 Overlaps: In each of the four intersections between C and H, draw 1 pair of electrons (one from C, one from H).
* Rest of atoms: Leave all other spaces empty.

1b. Ammonia ($NH_3$):
* 3 Overlaps: In each intersection between N and H, draw 1 pair of electrons (one from N, one from H).
* Rest of N: Draw 1 lone pair (2 electrons) in the non-bonding part of the Nitrogen circle.
* Rest of H: Leave empty.

1b. Water ($H_2O$):
* 2 Overlaps: In each intersection between O and H, draw 1 pair of electrons (one from O, one from H).
* Rest of O: Draw 2 lone pairs (4 electrons total) in the non-bonding part of the Oxygen circle.
* Rest of H: Leave empty.