3.6 Shapes of Molecules

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the central atom. CH H Electrons are negatively charged and will repel other electrons so each pair of electrons around an atom will repel all other electron pairs In a molecule, the bonding pair of electrons will repel other electrons around the central atom forcing the molecule to adopt a shape in which these repulsive forces are minimised. 3.6 The shapes of molecules and ions • The pairs of electrons will therefore take up positions as far apart as possible to minimise repulsion. 4 ▼ What is a pair of electrons referred to as? Coavlent bonds ▾ What does the shape of a simple molecule depend on: • The number of pairs of electrons that surround the central atom Drawing shapes of molecules Number of Bond Pairs and Lone Pairs BP = 2 LP = 0 BP = 3 LP = 0 Shapes of Molecules - no lone pairs Use the number of bond pairs and lone pairs of electrons to work out the shape of the molecule BP = 4 LP = 0 BP = 5 LP = 0 BP = 6 LP = 0 Number of Bond Pairs and Lone Pairs BP = 3 LP = 1 BP = 2 LP = 2 BP = 3 LP = 2 BP = 4 LP = 2 Example Chemical Name of Shape BeCl₂ BF₁ CH₂₁ PCI SF Example Chemical NH₂ H₂0 3.6 The shapes of molecules and ions CIF3 XeF4 Linear Trigonal Planar Tetrahedral Trigonal Bipyramidal Octahedral C Harris -Allery Chemistry Name of Shape Pyramidal Bent Trigonal Planar Square Planar 109.5° XX 180° 107° tx 104.5° XX XX 90° 90° 120° 90° XX XX 120° 120° 5 ▾ How do we find the number of electron pairs? Finding the number of electron pairs To work out the shape of a molecule or an ion you need to know how many lone pairs and how many bonding pairs of electrons are on the central atom. To find that out, you just follow these steps: 1. Find the central atom - it's the one all the other atoms are bonded to. 2. Work out how many electrons are in the outer shell of the central atom. This will be the same as its group number in the periodic table. 3. Add 1 electron for every atom that the central atom is bonded to. can work this out from the formula of the molecule or ion.) 4. If you're looking at an ion, you need to take its charge into account - add 1 electron for each negative charge or subtract 1 for each positive charge. 5. Add up all the electrons. Divide by 2 to find the number of electron pairs. 6. Compare the number of electron pairs to the number of bonds to find the number of lone pairs and the number of bonding pairs on the central atom. Example Carbon tetrafluoride, CF4 1. The central atom in this molecule is carbon. 2. Carbon's in group 4. It has 4 electrons in its outer shell. 3. The carbon atom is bonded to 4 fluorine atoms. 4. CF isn't an ion. 5. There are 4 + 4 = 8 electrons in the outer shell of the carbon atom, which is 8 ÷ 2 = 4 electron pairs. 6. 4 pairs of electrons are involved in bonding the fluorine atoms to the carbon so there must be 4 bonding pairs of electrons. That's all the electrons, so there are no lone pairs. 3.6 The shapes of molecules and ions F Figure 3: Dot-and-cross diagram for CF 6 Example Phosphorus trihydride, PH₂ 1. The central atom in this molecule is phosphorus. Phosphorus is in group 5, so it has five electrons in its outer shell. Phosphorus has formed 3 bonds with hydrogen atoms. 4. PH, isn't an ion. 5. There are 5 + 3 = 8 electrons in the outer shell of the phosphorus atom, so there are 8 ÷ 2 = 4 electron pairs. 2. 3. 6. 3 electron pairs are involved in bonding with the hydrogen atoms (bonding pairs), so there must also be 1 lone pair of electrons on the phosphorus atom. one lone pair- H PH 3.6 The shapes of molecules and ions H three bonding pairs Figure 4: Dot-and-cross diagram for PH₂. 2 pairs of electrons ▾ What degree apart are 2 pairs of electrons around an atom, and what shape: • 180° apart (furthest away from each other from other electron pairs) • The shape is linear • e.x Beryllium chloride, BeCl₂ (which is COVALENTLY bonded instead of ionic in the gas state) two groups of electrons Be 180° CI Be Cl 7 XX XX *C/ Be C/ XX XX XX С/— Ве —СI О XX х 180⁰ КОХО 3.6 The shapes of molecules and ions 180⁰ с 180⁰ O=C ХОХ ) = C = 0 180⁰ о XX XX 8 ▼ When looking at multiple bonds, such as double bonds or triple bonds, how does this affect the shape of a molecule? It does not affect the shape of the molecule, we treat multiple bonds as a single bonding area. ▾ Draw the linear shape structure CI- 2 pairs of electrons + 2 lone pairs ▾ What shape and angle does 2 boding pairs of electrons and 2 lone pairs form? 180° Be Cl If there are two bonding pairs of electrons and two lone pairs of electrons the lone-pair/lone-pair repulsion will squish the bond angle even further. The bond angle will be around 104.5° and the shape of the molecules is bent (or non-linear). Draw the non-linear shape Example-Maths Skills Water, H₂O In H₂O, the central oxygen atom has two bonding pairs shared with hydrogen atoms and two lone pairs so the shape of H₂O is bent (or non-linear). 3.6 The shapes of molecules and ions XX XX H 104.5° H *x H 104.5° H 9 3 pairs of electrons ▼ What degree apart are 3 pairs of electrons around an atom, and shape: • 120° apart (further away from each other from other electron pairs) Trigonal planar (planar because the pair of electrons are all on the same plane) • e.x Boron trifluoride, BF3 ▾ Draw the trigonal planar shape F 3.6 The shapes of molecules and ions B F 120⁰ 120° 3 pairs of Electrons + 1 lone pair ▼ What shape and angle does 3 bonding pairs and 1 lone pair form? 10 If there are three bonding pairs of electrons and one lone pair, the lone-pair/bonding-pair repulsion will be greater than the bonding-pair/ bonding-pair repulsion and so the angles between the atoms will change. There'll be smaller bond angles between the bonding pairs of electrons and larger bond angles between the lone pair and the bonding pairs. The bond angle is 107° and the shape of the molecules is trigonal pyramidal. Example Maths Skills Ammonia, NH, In NH₂, the central nitrogen atom has three bonding pairs of electrons and a lone pair so the shape of NH, is trigonal pyramidal. ▾ Draw the trigonal pyramidal shape •!!!!!! 4 ▾ Draw the T-shape: XX N. 3.6 The shapes of molecules and ions H H 107° XX !!!!! j 3 pairs of electrons + 2 lone pairs ▾ What shape do 3 bonding pairs and 2 lone pairs of electrons form? T-shape Example Maths Skills Chlorine trifluoride, CIF, In CIF, the central chlorine atom has three bonding pairs and two lone pairs so it has a T-shape. H 107° If there are three bonding pairs and two lone pairs of electrons, the molecule will be T-shaped. H 87.5° F 11 xx xx F CI 3.6 The shapes of molecules and ions F 87.5° -F 4 pairs of electrons ▼ What degree apart are 4 pairs of electrons around an atom, and shape: • 109.5° apart (furthest away from other electron pairs) • Tetrahedral, not planar since the shape is 3D, also why the angle doesn't sum to 360° each angle = 109.5⁰ A Figure 2 A tetrahedron has four points and four faces Note that the central atom is in the CENTRE of the shape • If a molecule is an ion, it does NOT affect the shape e.x: Methane, CHA 12 ● e.x: Ammonium, NH4 ▾ Draw the tetrahedral shape H 3.6 The shapes of molecules and ions 109.5⁰ 109.5⁰ I← JI!!!!!! N H CHip 109.5° 109.5° H H H H 109.5⁰ + 109.5⁰ H 109.5° 4 Pairs of electrons + 1 lone pair ▾ What shape does 4 bonding pairs and 1 lone pair form? 13 If there are four bonding pairs and one lone pair of electrons, the molecule forms a seesaw shape. The lone pair is always positioned where one of the trigonal planar atoms would be in a trigonal bipyramidal molecule. Example Maths Skills Sulfur tetrafluoride, SF 86.5°F F In SF, the central sulfur atom has four bonding 102° pairs and one lone pair so it has a seesaw shape. F ▾ Draw the seesaw shape: 86.5°F F 102° 4 Pairs of electrons + 2 lone pairs ▾ What shap does 4 bonding pairs and 2 lone pairs form? molecule will be square planar. S* If there are four bonding pairs and two lone pairs of electrons, the Draw the square planar shape: 3.6 The shapes of molecules and ions F Example Maths Skills Xenon tetrafluoride, XeF In XeF, the central xenon atom has four bonding pairs and two lone pairs of electrons its shape is square planar. Fllum 90° F toll 14 XX Xe 90° xx Topunil 5 pairs of electrons ▼ What degree apart are 5 pairs of electrons around an atom, and shape: • 120° and 90° apart (furthest away from other electron pairs) C/ 3.6 The shapes of molecules and ions 90⁰ C/ P 90⁰ C/ ▪▪▪|| C/ CI 15 ● 120⁰ C/ C/ P 3.6 The shapes of molecules and ions C/ ||| CI CI Trigonal bipyramidal, not planar since the shape is 3D, also why the angle doesn't sum to 360° 16 CI 3.6 The shapes of molecules and ions CI C/ e.x: Phosphorus pentachloride, PC15 P P C/ CI 120° C/ CI 17 C. Ophardt, c. 2003 120⁰° 3.6 The shapes of molecules and ions 90.0⁰ 90.0° 120⁰ 120° ▾ Draw the trigonal bipyramidal shape: POUS 90.0⁰ 90.0⁰ Phosphorus Pentachloride Trigonal Bipyramid Molecular Geometry 90° 120° 18 Cillum 120° CI 3.6 The shapes of molecules and ions CI ▾ Draw the square pyramidal shape: 90° 5 pairs of electrons + 1 lone pair ▾ What shape do 5 bonding pairs and 1 lone pair of electrons form? CI If there are five bonding pairs and one lone pair, the molecule forms a square pyramidal structure. (Molecules with this shape are very rare.) Example Maths Skills Chlorine pentafluoride, CIF, In CIF, the central chlorine has five bonding pairs and one lone pair of electrons, making its shape square pyramidal. F -CI F Filll!!! C CI 90° F Flum !!!)))/ >90° F 6 pairs of electrons ▼ What degree apart are 6 pairs of electrons around an atom, and shape: • 90° apart (furthest away from other electron pairs) 19 • Octahedral, not planar since the shape is 3D, also why the angle doesn't sum to 360° 3.6 The shapes of molecules and ions F", F FIII. F F 90⁰ S 90⁰ F FIS 90⁰ · ד F ד ד IMPORTANT Hint Take care. Octahedral sounds as if there should be eight electron groups, not six. Remember that an octahedron has eight faces but six points. F F F LL 20 • e.x: Sulfur hexafluoride, SF6 3.6 The shapes of molecules and ions COphard, 2005 ▾ Draw the octahedral shape: Sulfur Hexafluoride Electron Pair Geometry Octahedral 90° Bond Angles Octahedral Molecular Geometry ܒܝܢ 90 21 F 3.6 The shapes of molecules and ions Fillim F $90° F Effect of lone pairs on the shapes of molecules Molecules with lone pairs of electrons ▾ How do we find the shape of an unfamiliar molecule? 22 Finding the shape of an unfamiliar molecule In the exam you could be asked to draw the shape of a molecule you've never met before. Don't panic, just take it step by step. Work out how many electron pairs the molecule has, then work out how many of those are lone pairs. Decide what the bond angles are in the molecule, then draw and label it neatly. Example Predict the shape of the BF- ion. First, follow the steps on page 86 to find the number of electron pairs: 1. The central atom is boron. 2. Boron is in Group 3, so it has 3 electrons in its outer shell. 3. The boron atom is bonded to 4 fluorine atoms. 4. BF, is an ion with a 1- charge, so you need to add one extra electron to account for that. 5. That means there are 3 + 4 + 1 = 8 electrons in the outer shell of the central boron atom. That's 8 ÷ 2 = 4 pairs. 6. The boron atom has 4 electron pairs around it, and has made 4 bonds. So it has 4 bonding pairs and no lone pairs. A molecule with 4 bonding pairs and no lone pairs will have a tetrahedral shape, with bond angles of 109.5°. Now all you have to do is draw it. THE F 109.5° F Figure 7: The shape of the BF ion. ▼ Compare the strength between lone pairs than bonding pairs Lone pairs are more stronger then bonding pairs 3.6 The shapes of molecules and ions 23 lone pair of electrons H-N-H ▾ By how many degrees does the extra repulsion by the lone pairs decrease other bond angles by? The extra repulsion decreases other bond angles by 2.5 degrees ▾ Example: Ammonia 3.6 The shapes of molecules and ions 24 lone pair of electrons H-N-H H . This shows the structure of ammonia. • As you can see, the nitorgen atom has a lone pair. • So we've got three bonding pairs and one lone pair. • Four pairs of electrons form a tetrahedral structure. . SO the shape of the ammonia molecule is based on a tetrahedron. Normally the tetrahedral bond angle is 109.5 degrees celsius. • But a lone pair repels more strongly than a bonding pair. • This extra repulsion reduces the bond angle by 2.5 degrees celsisus. • That means the bond angle n the ammonia molecule is 107 degrees celsisus. • Scientists call this shape a pyramidal as it looks like a pyramid. 3.6 The shapes of molecules and ions 25 tetrahedron 109.5⁰ I ▾ What happens to the ammonia structure if it reacts with a hydrogen ion to form the ammonium ion NH4+ ammonia • In this case, the lone pair forms a dative covalent bond. • A dative covalent bond has the same level of repulsion as a regular covalent bond Ñ.….... H • So this means that the bond angle in the ammonium ion will return back to the tetrahedral angle of 109.5 degrees celsisus. 107⁰ 3.6 The shapes of molecules and ions 107⁰ "H H I "H H ammonium ion Z→I H 109.5⁰ _+ NH H 26 ▼ Example: Water • The Oxygen has two single covalent bonds to hydrogen atoms. • But the oxygen laso has two lone pair of electrons. If the oxygen atom did not have lone pairs then water would have a linear shape. ● H-O-H ● however the oxygen atom has two lone pairs. • This means we have four bonding areas in total and the shape will be based on the tetrahedron. tetrahedron 109.5⁰ 3.6 The shapes of molecules and ions H 104.5⁰ H 27 Remember that the tetrahedron bond angle is 109.5 degrees and because we have two lone pairs then it reduces the angle by 2.5x2=5 degrees celsius. This reduces it to 104.5 degrees. • Scientists call this a non-linear or a V-shaped molecule. ● *O** H H ▾ Why does water have an angular or V-shape: • ALWAYS DRAW LONE PAIR GHOSTS • Because there are 2 lone pairs, the H-O-H bond angle is reduced to 104.5° (109.5-2.5-2.5) • The shape is based on the tetrahedral shape, but due to the lone pairs it becomes angular/V-shaped 3.6 The shapes of molecules and ions 104.5⁰ 28 3.6 The shapes of molecules and ions 29