2.2 electronegativity, polarity & predicting molecular structure Flashcards
explain the valence shell electron pair repulsion theory (VSEPR)
the electron pair repulsion theory is based on the fact that the electron pairs in the outermost shell from the central atom will be arranged as far apart as possible from other electron pairs to minimise repulsion (minimum repulsion = maximum separation)
explain the two types of electron pairs.
lone (non-bonding) pair - outermost shell pair of electrons not involved in chemical bonding.
bonding pair - outermost shell pair of electrons shared between two atoms to form a covalent bond.
list the three types of bonding pairs, from the least to the most repulsion.
bonding + bonding = least repulsion.
bonding + lone = moderate repulsion.
lone + lone = most repulsion.
give the shape and bond angle of a molecule with two electron pairs.
- two bonding pairs around the central atom gives rise to a linear shape.
- bond angle is 180°
give the shape and bond angle of a molecule with three electron pairs, and explain why this shape is formed.
- three bonding pairs around the central atom gives rise to a trigonal planar shape.
- this is because electron pairs repel each other equally.
- bond angles are 120°
give the shape and bond angle of a molecule with four electron pairs, and explain why this shape and bond angle are formed.
- four electron pairs around the central atom repel one another as far apart as possible into a tetrahedral arrangement.
- lone pairs repel bonding pairs slightly closer together, which decreases the bond angle between the bonding pair of electrons.
- bond angle is reduced by about 2.5° for each lone pair present.
give the shape and bond angles of a molecule with five electron pairs, and explain why this shape is formed.
- five bonding pairs around the central carbon atom give rise to a trigonal bipyramidal shape.
- this is because electron pairs repel each other equally.
- bond angles are 90°, 120°, 180°
give the shape and bond angles of a molecule with six electron pairs.
- six bonding pairs around the central atom gives rise to an octahedral shape.
- bond angles are 90° and 180°
define the term ‘electronegativity’.
the ability of an atom to attract the bonding pair of electrons in a covalent bond, with the electron pair drawn towards the more electronegative atom.
what is a polar covalent bond?
a bond between atoms of different elements where the shared pair of electrons are pulled more strongly towards the more electronegative atom.
what is a polar molecule? give an example.
a molecule that contains polar bonds which do not cancel each other out, so that the whole molecule is described as ‘polar’, for example NH₃.
why is fluorine the most electronegative element?
- fluorine has a small atomic radius, and very high effective nuclear charge.
- this means that it can strongly attract a shared pair of electrons towards its valence shell, making it the most electronegative element.
electronegativity is usually measured using what?
the Pauling scale.
explain why, with the exception of the noble gases, electronegativity increases across periods and up the groups of the periodic table.
- more electronegative elements have higher nuclear charges, due to an increased number of protons in the nucleus, and smaller atomic radii.
- therefore with the exception of the noble gases, electronegativity increases across periods and up the groups of the periodic table.
describe how polar bonds form between atoms.
- polar bonds form between atoms of different electronegativities, where the bonding electrons are pulled towards the more electronegative atom.
- as a result, the electrons are spread unevenly, which produces a charge across the bond.
