有關Molecular symmetry 的題目

AL/2006/I/3(c)(i)

Which of the following best represents the reaction of (CH₃)₃N with BF₃ to form (CH₃)₃NBF₃?

要答這條題目就得留意三隻反應物和生成物的分子形狀。N(CH₃)­₃的N有5粒最外層電子，有3粒和CH₃組共價鍵，另外2粒閒置著，所以一共有3組Bond pairs及1組lone pair。由這組合得知這分子的形狀是Trigonal pyramidal (所以C錯)。接著是BF₃︰B有3粒最外層電子，3粒全用作組單鍵，所以有3組Bond pair，所以BF₃的形狀是Trigonal planar (A及D錯)。

雖然同學已經知道答案，但亦應留意生成物的形狀。Nitrogen atom透過將Lone pair electrons分享給Boron atom而得出dative covalent bond，所以B現在有4組bond pairs、由planar變成Tetrahedral shape。而Choice A的生成物亦不對(N有4組bond pair所以亦應是tetrahedral shape)。

AL/2009/I/1(a)(i)

Draw the respective electronic structures of BF₃ and NF₃. Hence, deduce the shape of each species. (3 marks)

分子形狀來來去去都是由Central atoms的bond pair及lone pair的數量決定︰

BF₃

There are 3 bond- pairs (no lone-pair) in the outermost shell of B. To minimize electronic repulsion, the 3 bond-pairs in BF₃ will be arranged in a trigonal planar shape. (1.5)

NF₃

There are 3 bond-pairs and 1 lone-pair in the outermost shell of N. The electron pairs in NF­₃ will also be arranged tetrahedrally. The molecule is trigonal pyramidal in shape. The bond angle < 109^o as repulsion between lone-pair and bond-pair is stronger than that between bond-pair and bond-pair. (1.5)

(DSE不需知道Bond angle，上文提及的Bond angle純粹希望大家更清楚不同分子的形狀)

DSE/2012/1A/12

Which of the following molecules is planar?

A.          BF­₃

B.          NH₃

C.          CH₄

D.         PCl₅

逐個分析好快就搵到︰

A: Boron atom有3粒最外層電子，3組Bond pairs及0組Lone pair à Trigonal planar shape (Planar)

B: Nitrogen atom有5粒最外層電子，3組Bond pairs及1組Lone pair à Trigonal pyramidal shape (Not a planar)

C: Carbon atom有4粒最外層電子，4組Bond pairs及0組Lone pair à Tetrahedral shape (Not a planar)

D: Phosphorus atom有5粒最外層電子，5組Bond pairs及0組 Lone pair àTrigonal bipyramidal shape (Not a planar)

事實上得兩款組合可以得出planar shapes︰

1.          1組/2組Bond pair à Linear shape (e.g. HCl / CO₂)

2.          3組Bond pair à Trigonal planar shape (e.g. BCl₃)

DSE/PP/1B/3(b)

Consider the nitrogen compound NCl₃.

(i) Draw the electron diagram of NCl₃, showing electrons in the outermost shells only.

Nitrogen atom需要多3粒電子，3粒Chlorine atom各需要1粒電子，所以N和3個Cl分別組成共價單鍵就可滿足雙方要求︰

記得Nitrogen atom及Chlorine atom要分別用點或交差來顯示電子的不同來源。

(ii) The shape of NCl₃ is similar to that of NH₃. Explain why this is so. (total 3 marks)

因為大家都有一樣數量的Bond pair 及 Lone pair，所以電子對之間的排斥相近︰

The nitrogen in NCl₃ and that in NH­₃ both have the same number of electron bond-pairs and lone electron pairs / have three electron bond-pairs and one lone electron pair in their outermost shells. (1)

The repulsion between these electron pairs causes both NCl₃ and NH₃ to adopt a trigonal pyramidal shape. (1)