CH 11 TOPIC 24 PREDICTING MOLECULAR SHAPES 1 You have mastered this topic when you can: 1) determine the shape of a molecule based on its Lewis Structure.. PREDICTING THE SHAPE OF A MOLECULE FROM ITS STRUCTURAL FROMULAE I) THE SHAPE OF A MOLECULE PLAYS A KEY ROLE IN DETERMINING WHETHER IT IS POLAR OR NON-POLAR. Molecules may be linear, tetrahedral, angular, or trigonal pyramidal. Obtain a molecule kit and build models of the examples described below to help you understand why a given molecule has the shape it does. A) DIATOMIC MOLECULES ARE MOLECULES COMPOSED OF ONLY 2 ATOMS. 1) Consider HF. HF is composed of only 2 atoms and therefore does not have a central atom to which other atoms are bonded; as a result it is a linear molecule. i.e. H F : a) Compounds composed of only 2 atoms will always be linear. B) MOLECULES COMPOSED OF 3 OR MORE ATOMS HAVE A CENTRAL ATOM AND THUS MAY BE TETRAHEDRAL, TRIGONAL PYRAMIDAL, ANGULAR OR STRAIGHT. 1) According to Lewis Theory, when a molecule is composed of 3 or more atoms it has a central atom to which its other atoms are bonded. Because the central atom has a stable octet it is surrounded by 4 pairs of Each pair of valence electrons occupies an area called the electron domain. Because all electrons are negatively charged the 4 pairs of valence electrons repel each other and move as far away from each other as possible. Thus, electron domains areas containing un-bonded pairs of valence electrons and bonded pairs of valence electrons are as far apart form each other as possible. Since molecules exist in a 3-dimensional world the electron domains surrounding the central atom exist at angles greater than 90. 2) TETRAHEDRAL MOLECULES ARE COMPOSED OF 5 ATOMS: 4 ATOMS BONDED TO A CENTRAL ATOM. a) Consider methane, CH 4. CH 4 is a tetrahedral molecule because it is composed of 5 atoms: 4 hydrogen atoms bonded to a single central carbon atom. The 4 electron domains surrounding the central carbon atom move as far away from each other as possible as indicated in the diagrams below. The left-hand diagram is not a true representation of the molecule because the drawing is on 2-dimensional paper. Thus this representation is limited to having the 4 electron domains around the central carbon atom at a maximum angle of 90 away from each other. The right hand 3-dimensional diagram more accurately represents the true shape of the molecule. It has the 4 electron domains surrounding the central carbon atom positioned at angles greater than 90. This angle has been accurately measured at 109.5º. The shape illustrated by right hand 3-dimensional diagram is called a TETRAHEDRON. NOTICE that the tetrahedron resembles a 4-sided pyramid with atoms at each corner. As most molecules are represented on the 2-dimensional paper, the 2-dimensional diagram below left is included to assist you in recognizing how this 2-dimensional Lewis structure cross translates to 3-dimensional molecular tetrahedral shape. NOTICE ALSO THAT EVERY ELECTRON DOMAIN AROUND THE CENTRAL ATOM LACKS UN-BONDED PAIRS OF VALENCE ELECTRONS. 2-D Structural Diagram 3-D tetrahedron shape H H C H H b) All molecules composed of 5 atoms where 4 of them are bonded to a single central atom form tetrahedrons. This occurs because the central atom of tetrahedral molecules has 4 electron domains that repel each other forcing them to be as far away from each other as possible. Build a model of methane to help you help you visualize its tetrahedron shape.
CH 11 TOPIC 24 PREDICTING MOLECULAR SHAPES 2 a) Consider ammonia, NH 3. According to Lewis Theory, the central nitrogen atom is surrounded by 4 electron domains consisting of 3 pairs of bonded valence electrons and 1 pair of un-bonded valence electrons. The 4 electron domains move as far away from each other as possible giving ammonia the trigonal pyramidal shape illustrated in the 3-dimensional diagram given below right. Since most molecules are represented on the 2-dimensional paper, the 2-dimensional diagram below left is included to assist you in recognizing this 2-dimensional Lewis structure T-shape translates to 3-dimensional molecular trigonal pyramidal shape. 2-D diagram 3-D trigonal pyramidal shape 107º i) Ammonia molecules are trigonal pyramidal because their electron domains consist of 3 pairs of bonded valence electrons and 1 pair of un-bonded Because all electrons are negatively charged they have a natural repulsion for each other causing them to move as far away from each other as possible. Build a model of ammonia to help you visualize its trigonal pyramidal shape. ii) The trigonal pyramidal shape may be considered a modified tetrahedral because it has 4 electron domains. The bond angle for trigonal pyramidal molecules is about 107º, less than the 109.5º bond angle of a tetrahedral molecule. This is because the electron domain consisting of the pair of unbonded valence electrons is closer to the central atom than are the valence electron pairs in the bonds. This pushes the bonded pairs of valence electrons closer to each other creating a smaller bond angle of about 107º. b) A trigonal pyramidal molecule held together by polar bonds is polar whereas a trigonal molecule held together by non-polar bonds is non-polar. 3) ANGULAR (BENT OR V-SHAPED) MOLECULES ARE COMPOSED OF 3 ATOMS: 2 ATOMS BONDED TO A CENTRAL ATOM HAVING TWO UN-BONDED PAIRS OF VALENCE ELECTRONS AROUND THE CENTRAL ATOM. a) Consider water, H 2 O. According to Lewis Theory, the central oxygen atom is surrounded by 4 electron domains consisting of 2 pairs of bonded valence electrons and 2 pairs of un-bonded The 4 electron domains move as far away from each other as possible giving water the angular shape illustrated in the two bottom 3-dimensional shapes given to the right. Since most molecules are represented on the 2- dimensional paper, the two 2-dimensional diagrams above right are included to assist you in recognizing how those 2- dimensional Lewis structure translates to 3-dimensional molecular angular/bent/vshape molecular shapes. H H 105º
CH 11 TOPIC 24 PREDICTING MOLECULAR SHAPES 3 i) Water molecules are angular/bent/v-shaped because they are composed 3 atoms, 2 of which are bonded to a central atom. This means the central atom s 4 electron domains consist of 2 pairs of un-bonded valence electrons and 2 pairs of bonded Because all electrons are negatively charged they have a natural repulsion for each other causing them to move as far away from each other as possible. Build a model of water to help you help you visualize its angular shape. ii) The angular/bent/v-shaped shape may be considered a modified tetrahedral because it has 4 electron domains. The bond angle for angular/bent/v-shaped molecules is about 105º, less than the 109.5º bond angle of a tetrahedral molecule. This is because the 2 electron domains consisting of the pairs of un-bonded valence electrons are closer to the central atom than are the 2 pairs of bonded valence electron. This pushes the bonded pairs of valence electrons closer to each other creating a smaller bond angle of about 105º. b) An angular/bent/v-shaped molecule held together by polar bonds is polar whereas an angular molecule held together by non-polar bonds is non-polar. 4) WHEN A MOLECULE IS COMPOSED OF 3 ATOMS: 2 ATOMS BONDED TO A CENTRAL ATOM SURROUNDED BY 4 BONDED PAIRS OF VALENCE ELECTRONS IT WILL BE LINEAR (STRAIGHT). a) Consider hydrogen cyanide, HCN. According to Lewis Theory, the central carbon atom is surrounded by 4 pairs of bonded valence electrons arranged as a single bond between the hydrogen and the carbon atom and a triple bond between the nitrogen and the central carbon atom. This gives the central atom 2 electron domains. The 2 electron domains move as far away from each other as possible giving hydrogen cyanide the linear shape illustrated in the 2 & 3-dimensional shape given below. i.e. 2-D Lewis Structure 3-D Molecular shape i) Hydrogen cyanide molecules are linear because the central atom has only 2 electron domains. Because all electrons are negatively charged they have a natural repulsion for each other, they move as far away from each other as possible. As this linear molecule has only 2 electron domains, the greatest distance they can be from each other is 180. Build a model of hydrogen cyanide to help you visualize that it is a straight molecule. NOTICE that the central atom lacks un-bonded pairs of b) A straight molecule held together by polar bonds is polar whereas a straight molecule held together by non-polar bonds is non-polar. 5) IF A CENTRAL ATOM HAS 1 OR 2 PAIRS OF UN-BONDED PAIRS OF ELECTRON DOMAINS THE MOLECULE WILL BE ANGULAR OR TRIGONAL PYRAMIDAL AND IF ITS BONDS ARE POLAR THE MOLECULE WILL BE POLAR, IF ITS BONDS ARE NON-POLAR THE MOLECULE WILL BE NON-POLAR. II) Required Practice 1: Draw the Lewis structure for each molecule and indicate which atoms are slightly positive and slightly negative, then state and justify the 3-D shape. {Answers are on page 4 of these notes.} 1. PCl 3(g) 2. SF 2(g) 3. CO 2(g) 4. CCl 4(g) 5. NH 4 + (aq) 6. H 3 O + (aq)
CH 11 TOPIC 24 PREDICTING MOLECULAR SHAPES 4 Required Practice 1 found on page 3 1) δ : Cl 2) δ : F ANSWERS TO THE REQUIRED PRACTICE P δ+ Cl : δ trigonal pyramidal because the central atom has 4 electron domains and 1 un-bonded pair of : Cl : δ S : δ+ angular because the central atom has 4 electron domains and 2 un-bonded pairs of : F :δ 3) O = C = O linear because the central atom has 2 electron domains and lacks un-bonded pairs of valence electrons. : Cl : δ 4) δ : Cl C δ+ Cl : δ tetrahedral because the central atom has 4 electron domains and lacks un-bonded pairs of : Cl : δ H δ + + 5) δ+ H N δ H + tetrahedral because the central atom 4 electron domains and lacks un-bonded pairs of valence electrons. H δ+ 6) H δ+ O δ H δ+ + trigonal pyramidal because the central atom has 4 electron domains and 1 un-bonded pair of H δ+
CH 11 TOPIC 24 PREDICTING MOLECULAR SHAPES 5 ASSIGNMENT At the top of your assignment, please print T24 Predicting Molecular Shapes, your Last then First name, block and date. For questions requiring calculations you must show all your work. Complete these questions in the order given here. [Marks indicated in italicized brackets.] 1. Draw the Lewis structure for each molecule and indicate which atoms are slightly positive and slightly negative, then state and justify the 3-D shape. [16] a. CF 4(g) b. H 2 S (g) c. NBr 3(s) d. O 3(g) 2. Write the name, the chemical formula [0.5 ea] and the classification as acid, base, salt, hydrate or covalent [0.5 ea]. [6] a. ammonium hydroxide c. manganese(vi) oxide e. lead(iv) phosphite b. sulphurous acid d. oxygen dichloride f. tin(ii) sulphide 3 Write the formula, the name [0.5 ea] and the classification as acid, base, salt, hydrate or covalent [0.5 ea]. [6] a. P 2 O 5(s) c. H 2 C 2 O 4(aq) e. Ca(OH) 2(aq) b. MgSO 4 7H 2 O (s) d. N 2 O (g) f. HBr 3(aq)) [28 marks in total]