Chapter 6 - Shapes of Molecules and Intermolecular Forces Flashcards
What does molecular shape depend on?
Electron pairs around the central atom.
In what way do electron pairs interact?
They repel each other
Which type of pair repels the most?
Lone pairs
What is the order of pair repulsion, from weakest to strongest?
Bonding pair-bonding pair
Lone pair-bonding pair
Lone pair-lone pair
What is the way to predict a molecules shape?
Electron pair repulsion theory
What shape is given to a molecule with no lone pairs and 4 bonding pairs?
Tetrahedral
What is the bond angle in a tetrahedral molecule?
109.5 degrees
What shape is given to a molecule with 2 bonding pairs and 1 lone pair?
Non-linear
What shape is given to a molecule with 2 bonding pairs and 2 lone pairs?
Non-linear
What is the bond angle in a non-linear molecule with 2 lone pairs?
104.5 degrees
What shape is given to a molecule with 3 bonding pairs and 1 lone pair?
Pyramidal
What is the bond angle in a pyramidal molecule?
107 degrees
What shape name is given to a molecule with 2 bonding pairs around the central atom, with no lone pairs?
Linear
What is the bond angle in a linear molecule?
180 degrees
What shape name is given to a molecule with 3 bonding pairs around the central atom, with no lone pairs?
Trigonal planar
What is the bond angle in a trigonal planar molecule?
120 degrees
What shape is given to a molecule with 6 bonding pairs around the central atom, with no lone pairs?
Octahedral
What is the bond angle in an octahedral molecule?
90 degrees
What is electronegativity?
The attraction of a bonded atom for the pair of electrons in a covalent bond.
How is electronegativity measured?
Pauling scale
What does a high Pauling value mean?
A higher electronegativity and therefore greater attraction for an electron pair in a covalent bond
What makes a bond polar?
2 Atoms have different electronegativity values.
When the bonding electrons are pulled towards the more electronegative atom.
One atom becomes slightly positive charged, and one becomes slightly negative charged.
Forms polar covalent bond.
What does a polar bond form?
A permanent dipole
How is a dipole formed?
A difference in charge between the 2 atoms, caused by a shift in electron density in the bond.
Are diatomic gases polar or non-polar? Why?
They are non-polar because the atoms have equal electronegativity so the electrons are equally attracted to both nuclei.
What happens if the polar bonds are arranged symmetrically in a molecule?
The dipoles cancel each other out so the molecule has no overall dipole and is therefore non-polar.
What is a dipole?
The separation of opposite charges within a molecule. Has the delta + and delta - signs.
What happens if the polar bonds are arranged unsymmetrically in a molecule?
Uneven distribution of charge causing the molecule to have an overall dipole.
What can the electronegativity predict about bonding?
Whether bonding is ionic or covalent.
What can electronegativity tell us about ionic bonding?
In ionic bonding the difference in electronegativity is 1.8 or higher.
What can electronegativity tell us about covalent bonding?
Atoms bonded covalently have zero difference in electronegativity. This is a pure covalent bond.
What can electronegativity tell us about polar covalent bonding?
In polar covalent bonding the difference in electronegativity is between 0 and 1.8 and the molecule tends to be polar.
What are intermolecular forces?
Weak interactions between dipoles of different molecules
What is the order of intermolecular forces from weakest to strongest?
London forces
Permanent dipole-dipole interactions
Hydrogen bonding
How are induced dipole-dipole interactions (London forces) formed?
- Movement of electrons produces a changing dipole in a molecule.
- At any instant, an instantaneous dipole exists, which induces a dipole on a neighbouring molecule.
- The induced dipole further induces dipoles on neighbouring molecules, which then attract one another.
What is the effect of having more electrons in a molecule on London forces?
Larger instantaneous and induced dipoles
Greater induced dipole-dipole interactions
Causes stronger attractive forces between molecules
What is the effect of stronger London forces?
Higher boiling point
Why does boiling point increase when London forces are increased?
More electrons cause larger induced dipoles, so more energy is needed to overcome the intermolecular forces.
What are permanent dipole-dipole interactions?
Intermolecular forces that occur between polar molecules that have an uneven distribution of charge.
What is the effect of having permanent dipole-dipole interactions?
Higher boiling point
Which molecules have London forces?
ALL molecules
What is a simple molecular substance?
Substance with few atoms held together by covalent bonds like H2O, CO2, or Ne
Why do simple molecular substances have low melting and boiling points?
Because they have weak intermolecular forces that are broken by energy present at low temperatures.
Why don’t simple molecular substances usually dissolve in water?
There are little interactions between the molecules in the lattice and the water molecules.
Why do simple molecular substances have poor electrical conductivity?
There are no mobile charged particles.
How is hydrogen bonding formed?
A weak bond is polarised between a hydrogen atom of one molecule and a lone pair on a neighbouring molecules’ electronegative atom (e.g. oxygen, nitrogen, or fluorine)
What effect does hydrogen bonding have on a molecule
Soluble in water
Higher boiling points
Why does ice float on water?
Because hydrogen bonds hold water molecules apart in an open lattice structure, but when water freezes, the water molecules become further apart, causing ice to be less dense than water.
What are 4 anomalous properties of water?
High boiling point
More dense in liquid form
Surface tension
High viscosity
