Shapes of Molecules and Intermolecular Forces

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What charge does an electron have?
Negative.
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Do electrons attract or repel?
Repel.
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What do the electron pairs surrounding a central atom determine?
The shape of the molecule or ion.
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What do different numbers of electron pairs result in?
Different shapes.
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What does a solid line represent when drawing a 3D molecule?
A bond in the plane of the paper.
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What does a solid wedge represent when drawing a 3D molecule?
A bond coming out of the plane of the paper.
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What does a dotted wedge represent when drawing a 3D molecule?
A bond going into the plane of the paper.
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Which has more repulsion? Bonded pair/bonded pair or lone pair/lone pair.
Lone pair/lone pair.
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By how much is the bond angle reduced for each lone pair?
2.5 degrees.
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What are the different shapes of molecules?
Tetrahedral, pyramidal, non-linear, linear, trigonal planar and octahedral
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What is the bond angle for tetrahedral molecules?
109.5 degrees.
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What is the bond angle for pyramidal molecules?
107 degrees.
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What is the bond angle for non-linear molecules?
104.5 degrees.
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What is the bond angle for linear molecules?
180 degrees.
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What is the bond angle for trigonal planar molecules?
120 degrees.
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What is the bond angle for octahedral molecules?
90 degrees.
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What shape is methane, CH4?
Tetrahedral.
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What shape is ammonia, NH3?
Pyramidal.
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What shape is water, H20?
Non-linear.
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What shape is carbon dioxide, CO2?
Linear.
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What shape is boron trifluoride, BF3?
Trigonal Planar.
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What shape is sulfur hexafluoride, SF6?
Octahedral.
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What is electronegativity?
The attraction of a bonded atom for the pair of electrons in a covalent bond.
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What happens across the periodic table?
The nuclear charge increases and the atomic radius decreases.
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What does a large Pauling electronegativity value represent?
That atoms of the element are very electronegative.
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What elements are the most electronegative?
The non-metals nitrogen, oxygen, fluorine and chlorine are the most electronegative.
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Which elements are the least electronegative?
Group 1 metals.
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Why are the noble gases not included in electronegativity?
Because they tend not to form compounds.
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What is a non-polar bond?
A bond where the bonded electron pair is shared equally between the bonded atoms.
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When will a bond be non-polar?
When the bonded atoms are the same or have the same (or similar) electronegativity.
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What is a pure covalent bond?
A bond in which the bonded atoms come from the same element and the electron pair is shared equally. E.g. hydrogen, oxygen and chlorine.
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Why are hydrocarbon liquids (such as hexane) non-polar solvents?
Because carbon and hydrogen atoms have very similar electronegativities and form non-polar bonds.
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What is a polar bond?
A bond where the bonded electron pair is shared unequally between the bonded atoms.
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When will a bond be polar?
When the bonded atoms are different and have different electronegativity values.
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What is a dipole?
A separation in electrical charge so that one atom of a polar covalent bond (or one end of a polar molecule) has a small positive charge and the other has a small negative charge.
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What is a permanent dipole?
A dipole which does not change, for example in a polar covalent bond.
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Is hydrogen chloride polar?
Yes.
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Is water polar?
Yes.
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Which atom has the delta negative charge?
The one with the larger electronegativity value.
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Why is a water molecule polar?
The two O-H bonds each have a permanent dipole. The two dipoles act in different directions but do not exactly oppose one another. Overall, the oxygen end of the molecule is slightly negative and the hydrogen end has a slightly positive charge.
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Is carbon dioxide polar or non-polar? Why?
It is non-polar. This is because the two C=O bonds each have a permanent dipole, but the two dipoles act in opposite directions so exactly oppose each other. Therefore, the dipoles cancel and the overall dipole is zero.
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What is an intermolecular force?
A weak interaction between dipoles of different molecules.
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What are the 3 categories of intermolecular forces?
Induced dipole-dipole interactions, permanent dipole-dipole interactions and hydrogen bonding.
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What is the other name for induced dipole-dipole interactions?
London forces.
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What is an induced dipole-dipole interaction?
A weak intermolecular force which exists between all molecules (polar and non-polar).
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How are induced dipole-dipole interactions made?
1. Random movement of dipoles - an instantaneous dipole. 2. This causes an induced dipole in the neighbouring molecules.
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What are permanent dipole-dipole interactions?
Interactions between permanent dipoles (do not move randomly like instantaneous dipoles).
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What is hydrogen bonding?
A strong dipole-dipole attraction between an electron-deficient hydrogen atom of –NH, –OH or HF on one molecule and a lone pair of electrons on a highly electronegative atom containing N, O or F on a different molecule.
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What does a hydrogen bond act between?
The lone pair on an electronegative N, O or F atom and a hydrogen atom in another molecule.
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Why does ice float on water?
Because the hydrogen bonds extend outwards, holding water molecules slightly apart. This equals an open tetrahedral lattice full of holes. When ice melts = lattice collapses due to the holes and the molecules move closer together so it's less dense.
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Why does water have a relatively high melting and boiling point?
Because it requires more energy to break the London forces and the hydrogen bonds between molecules.
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List the intermolecular forces from strongest to weakest.
(Single covalent bonds), Hydrogen bonds, Permanent dipole-dipole interactions, Induced dipole-dipole interactions.
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How are simple molecular lattices held together?
The molecules are held in place by weak intermolecular forces. The atoms within each molecule are bonded together strongly by covalent bonds.
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Do simple molecular substances have high or low melting and boiling points?
They have low melting and boiling points. This is because the weak intermolecular forces can be broken by the energy at low temperatures.
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What happens when a simple molecular lattice is broken apart during melting?
Only the weak intermolecular forces break and the strong covalent bonds do not break.
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When are non-polar substances soluble? Why?
Soluble in non-polar solvents - because when a simple compound is added to a non-polar solvent, intermolecular forces form between the molecules and the solvent. The interactions weaken the forces in the lattice so the forces break + it dissolves.
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When are polar substances soluble? Why?
Soluble in polar solvents - this process is similar to the dissolving of an ionic compound.
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Why can solubility be hard to predict?
Because solubility depends on strength.
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Do simple molecular structures conduct electricity?
No because there are no mobile charged particles in simple molecular structures. With no charged particles that can move, there is nothing to complete an electrical circuit.
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How are hydrogen bonds represented in diagrams?
Dotted/Dashed line.
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Card 2

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Do electrons attract or repel?

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Repel.

Card 3

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What do the electron pairs surrounding a central atom determine?

Back

Preview of the front of card 3

Card 4

Front

What do different numbers of electron pairs result in?

Back

Preview of the front of card 4

Card 5

Front

What does a solid line represent when drawing a 3D molecule?

Back

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