How we got there
Read this like a teacher at the board. We go slowly, name every number, and say why we take each step.
Explain in another language · 11
1. What is the question asking?
Read the question twice. Circle the unknown. Underline every number and unit. The paper is asking: Predict the shape of an ammonia molecule, NH₃.
In other words — as if you have never seen this before
If you have never seen this before: The shape is named from the atoms, but the lone pair still pushes. Four electron groups and one lone pair make a pyramid, not a flat triangle. We will name a method in words, write the formula with no numbers, then substitute one line at a time.
2. What do we already know?
Nitrogen bonded to three hydrogen atoms, with one lone pair on nitrogen.
3. Which method — and why this one?
The shape is named from the atoms, but the lone pair still pushes. Four electron groups and one lone pair make a pyramid, not a flat triangle.
4. Step-by-step working
01 / 05
Identify the given
Three bond pairs and one lone pair. That is four electron groups.
3 bonds, 1 lone pairWhy this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
02 / 05
Apply the law
The lone pair takes the place of a fourth bond and pushes the three bonds closer.
not trigonal planarWhy this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
03 / 05
Sense-check
A flat triangle would need three groups and no lone pair, as in BF₃.
trigonal pyramidalWhy this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
04 / 05
Step F — Box the answer with its unit
Trigonal pyramidal.
Why this step: A number without a unit is half a mark on a Physics paper, and a marker can miss a Maths answer that is buried in working.
05 / 05
Step H — What the marker is looking for
On an NSC memo you usually earn: (1) a method mark for naming the right formula or idea in words, (2) an accuracy mark for substituting the given numbers with units, (3) an accuracy mark for the boxed answer with its unit. Write those three lines even if you are unsure of the arithmetic — blank pages score zero.
Why this step: First-timers chase the final number and leave the method blank. The memo still pays out for the idea and the substitution even when the last line is wrong.
5. The answer
Trigonal pyramidal.
Copy onto your answer sheet — in this order
Formula: 3 bonds, 1 lone pair
Answer: Trigonal pyramidal.
6. Check it like a marker
Calling it trigonal planar forgets the lone pair. The atoms are not in one plane.
7. A trap first-timers fall into
First-timers often grab the first formula they remember, mix units (cm with m, minutes with seconds), or drop a minus sign. Write the formula in words first: “this equals that, because…”