diatomic molecules: Flashcards
bond length: definition
the mean distance between the centres of two atoms in a molecule.
What is the relationship between the length and strength of a bond?
The shorter the bond, the stronger the bond.
bond dissociation enthalpy: definition? symbol?
a measure of bond strength – defined as the standard enthalpy change of reaction in which the bond is broken.
given the symbol D
always endothermic
What does the minimum of a potential energy curve correspond to?
minimum of a potential energy curve corresponds to the equilibrium bond length and bond strength – the deeper the minimum curve = the stronger the bonds between atoms.
Bond order formula?
number of filled bonding orbitals - number of filled antibonding orbitals.
octet rule: definition
octet rule: bonding in molecules can be explained by the tendency for atoms to want to achieve the electronic configuration of the closest noble gas (i.e. a full valence shell).
Who introduced the first model of covalent bonding?
Gilbert Lewis in 1916
What did Gilbert Lewis find in 1916?
he recognised that it was possible for the atoms in a molecule to obey the octet rule by sharing electrons.
Successes of the Lewis Model:
provides a simple way of describing bonding.
can be extended to predict molecular shapes.
introduced concept of bond order – a measure of the number of bonds between a pair of atoms, e.g. a single bond has a bond order of 1, double bond has a bond order of 2
Failures of the Lewis Model:
there’re many stable atoms without full octets.
- e.g. NO –if drawn with double bond then N has only 7 valence electrons – so is electron deficient.
- if drawn with a triple bond then O has 9 valence electrons – gives an expanded octet.
- cannot draw a Lewis structure of NO where both atoms have a full octet.
doesn’t explain hypervalent atoms – atoms with expanded octets.
-e.g. PF5 - P atom contributes 5 electrons and each F atom contributes 1 electron each – overall there are 10 electrons around the central P atom.
can’t explain transition metal bonding.
electronegativity: definition
the ability of an atom in a molecule to attract electrons to itself in a covalent bond.
Who introduced the most commonly used scale of electronegativity?
introduced by Linus Pauling, known as Pauling electronegativity.
What did Pauling state?
Pauling noticed that the bond dissociation enthalpies (BDE) for heteronuclear diatomics (XY) were always greater than the average of the BDE’s of the homonuclear diatomics (X2 and Y2).
What is 1 eV in kJ/mol?
96.485 kJ/mol
What is the trend in electronegativity: across a period? down a group?
electronegativity generally increases across a period and decreases down a group
