Chapter 6 - Shapes of molecules and intermolecular forces Flashcards
What is electron pair repulsion theory?
Electrons have a negative charge, therefore repel each other, so are arranged as far apart as possible
What is the difference in repulsions between a lone pair and bonded pair?
A lone pair is slightly closer to the central atom and occupies more space than a bonded pair - so it repels more strongly than a bonding pair.
What is the molecular shape and bond angle for a molecule with 4 bonded pairs?
Tetrahedral, 109.5°
What is the molecular shape and bond angle for a molecule with 3 bonded pairs and one lone pair?
Pyramidal, 107°
What is the molecular shape and bond angle for a molecule with 2 bonded pairs and 2 lone pairs?
Non linear, 104.5°
What is the molecular shape and bond angle of a molecule with 2 electron pairs/regions?
Linear, 180°
What is the molecular shape and bond angle for a molecule with 3 electron pairs/regions?
Trigonal planar, 120°
What is the molecular shape and bond angle for a molecule with 6 electron pairs/regions?
Octahedral, 90°
What is electronegativity?
The ability of an atom to attract a bonding pair of electrons
How does electronegativity change across the periodic table?
Electronegativity increases along each period and decreases down each group (due to nuclear charge increasing and atomic radius decreasing)
What are the most electronegative atoms?
Nitrogen, oxygen, fluorine and chlorine
What is a non polar bond, and where does it occur?
A bond where the bonded electron pair is shared equally between the bonded atoms. It occurs when the bonded atoms are the same or the bonded atoms have similar/the same electronegativity
What is a polar bond and where does it occur?
A bond where the bonded electron pair is shared unequally between the bonded atoms. Bonds are polar when the bonded atoms are different and have different electronegativity values
What is a dipole?
The separation of charges in a molecule
What are the three types of intermolecular forces?
Induced dipole-dipole interactions, permanent dipole-dipole interactions, hydrogen bonding
How do induced dipole-dipole interactions work?
Movement of electrons in a molecule produces a changing dipole. At any time an instaneous dipole exists, but it is constantly shifting. Instantaneous dipole induces a dipole on neighbouring molecule, which induces further dipoles on other molecules, which then attract one another
Where do induced dipole-dipole interactions occur?
Exist between all molecules, whether polar or non polar
What increases the strength of induced dipole-dipole interactions?
Larger number of electrons mean larger induced dipoles (larger instantaneous and induced dipoles so stronger attractive forces)
How do permanent dipole-dipole interactions work?
Permanent dipoles in polar molecules have attractive forces between them
What structure are simple molecular substances, and how is this held together?
Simple molecular lattice, molecules held in place by weak intermolecular forces - but atoms within each molecule are bonded together strongly by covalent bonds
Why do simple molecular structures have low melting and boiling points?
Only have to break weak intermolecular forces to break lattice, so little energy is required
What is the solubility of non-polar simple molecular structures?
Soluble in non polar solvents as intermolecular forces form between molecules and solvent, breaking lattice
Not soluble in polar solvent as intermolecular bonding in polar solvent is too strong to be broken
What is the solubility of polar simple molecular substances?
May dissolve in polar solvents as polar solute molecules and polar solvent molecules can attract each other.
Solubility depends on strength of dipole
What is the electrical conductivity of simple molecular substances?
There are no mobile charged particles in simple molecular structures. With no charged particles to move, there is nothing to complete and electrical circuit
What is a hydrogen bond?
A type of permanent dipole-dipole interaction found between molecules containing:
- an electronegative atom with a lone pair of electrons
- a hydrogen atom attached to an electronegative atom
What are the strongest of the three intermolecular forces?
Hydrogen bonds
What are the anomalous properties of water?
The solid (ice) is less dense than the liquid (water), and it has a relatively high melting and boiling point
Why is ice less dense than water?
Hydrogen bonds hold water molecules apart in an open lattice structure, so the water molecules in ice are further apart than in water, it is therefore less dense
