1.7.6Unit 1. Structure, Bonding and Introduction to Organic Chemistry
Predicting Shapes and Angles — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 6 parts Lesson 26 of 36 Unit quiz
Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
Section 1 of 6A Method That Always Works
The same four-step method applies to neutral molecules and to ions.
Step 1
- Step 1 Identify the central atom and count its outer electrons from the group number, adjusting for any ion charge.
Step 2
- Step 2 Add one electron for each bond made to the central atom, and take away or add electrons for positive or negative charges.
Step 3
- Step 3 Divide the total by two to find the number of electron pairs, then subtract the bonding pairs to find the lone pairs.
Step 4
- Step 4 Use the number of regions and lone pairs to read off the shape and its bond angle.
Section 2 of 6Worked Examples in Short
Worked Examples in Short
NH₃
- NH₃ Nitrogen has five outer electrons, three bonds add three,
- giving eight and four pairs, with three bonding and one lone,
- so pyramidal at about 107°.
NH₄⁺
- NH₄⁺ Five electrons plus four bonds minus one for the charge gives eight and four bonding pairs, so tetrahedral at 109.5°.
SF₆
- SF₆ Sulfur has six electrons plus six from the bonds giving twelve and six pairs, all bonding, so octahedral at 90°.
CO₂
- CO₂ Carbon forms two double bonds, which count as two regions, so the molecule is linear at 180°.
Section 3 of 6Common Shapes and Their Angles
1
Two regions with no lone pairs give linear molecules with an angle of 180°.
2
Three regions with no lone pairs give trigonal planar molecules with 120°.
3
Four regions give tetrahedral if there are no lone pairs, pyramidal with one and bent with two.
4
Five bonding regions give trigonal bipyramidal molecules such as PCl₅ with 90° and 120° angles.
5
Six bonding regions give octahedral molecules with 90°, as in SF₆.
Section 4 of 6Exam Skills
Exam Skills
Show the working
- Show the working State the number of bonding pairs and lone pairs before giving the shape.
Explain angles
- Explain angles Mention that lone pairs repel more strongly than bonding pairs when the angle is smaller than the basic value.
Drawing
- Drawing Use wedges and dashes to show three-dimensional shapes, and mark the bond angle clearly.
Expanded octets
- Expanded octets Phosphorus and sulfur can have more than eight electrons around them, as in PCl₅ and SF₆.
| Species | Pairs | Shape | Angle |
|---|---|---|---|
| BF₃ | 3 bonding | Trigonal planar | 120° |
| H₃O⁺ | 3 bonding, 1 lone | Pyramidal | About 107° |
| PCl₅ | 5 bonding | Trigonal bipyramidal | 90°, 120° |
| H₂O | 2 bonding, 2 lone | Bent | 104.5° |
Section 5 of 6Worked Example
Question Predict the shape and approximate bond angle of the H₃O⁺ ion.
- Oxygen has six outer electrons and three bonds add three, making nine;
- subtracting one for the positive charge gives eight electrons.
- Eight electrons form four pairs, three of them bonding and one a lone pair.
- Four regions with one lone pair give a pyramidal shape.
Answer H₃O⁺ is pyramidal with a bond angle of about 107°.
Section 6 of 6Exam Checklist & Tips
1.7.6 – Key Points for the Exam
- Shape The arrangement of atoms around the central atom, ignoring lone pairs.
- Pyramidal A three-sided pyramid shape with a lone pair on the central atom at the top.
- Trigonal bipyramidal A shape with five bonds, three in a plane at 120° and two perpendicular to it.
- Octahedral A shape with six bonding pairs at 90° to each other.
- Expanded octet A central atom with more than eight electrons in its outer shell.
Common Mistakes
- Forgetting to adjust the electron count when the species is an ion.
- Giving an exact value of 107° for ammonia; it should be described as about or approximately.
- Calling a trigonal pyramidal molecule trigonal planar.
- Counting each bond in a double bond as a separate region of repulsion.
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