Structure and Bonding Flashcards
(29 cards)
What is the Lewis bonding theory?
What are valance electrons?
Bonding occurs to achieve a completely filled or empty valence shell via sharing electrons or transfer of electrons between atoms
Valence electrons are the electrons of the outer shell
What is electronegativity? What is the important order of them?
A measure of the relative ability of an atom to attract valence electrons to itself
F>O>Cl>N
What are ionic bonds? What are covalent bonds? When do they occur? And polar covalent?
Ionic: between atoms of very different electronegativities, achieved by the full transfer of electrons e.g NaCl
Covalent: between atoms of similar electronegativity, via sharing of electron pairs
Where there is still a difference in electronegativity e.g HF, the shared pair is not equally distributed, bond polarisation via inductive effects, producing a dipole
What is the formula for the formal charge of an atom?
N. of valence electrons - (N of lone pair electrons + 1/2 bonding pair electrons)
i.e compare how many bonds it should form normally vs how many bonds it actually has
How can the hydronium ion be modelled? Which is more accurate and used more?
More accurate= square brackets, as charge spread over 3 hydrogens aswell
But use formal charge of oxygen as easier to draw mechanisms
Why is it favourable for a lone pair to be protonated?
Covalent bonding is in balance between electron-electron and nucleus-nucleus repulsion and nucleus-electron attraction, stabilised at the internuclear distance of the bond length
Lone pairs formally have interactions with one nucleus, so inter electron repulsion is greater than a bonding pair, thus the lp are at higher energy and more available for reaction
Protonation increases bonding pairs, thus increasing the stabilising nucleus-electron attraction
What is an overview of the shapes of molecules?
Using VSEPR:
- if 4 electron pairs around the central atom, tetrahedral base 109.5 , reducing the bond angle by 2.5 degrees for each lone, i.e pyramidal, non-linear
- 3 electron pairs= trigonal planar, 120 degrees
- 5 electron pairs= trigonal bipyramidal, 2 at 90, 3 at 120
- 6 Octahedral
As lp-lp>lp-bp>bp-bp in terms or repulsion
What is an atomic orbital? What are the orbitals for C and H involved in bonding?
Orbitals: Mathematical (wave) functions which relate the probability of finding an electron in a region of space
C: 2s and 3 x 2p
H: 1s
What do the S and P orbitals look like and their properties?
S- spherical, 2s and beyond have nodes
P- 3 x,y,z dumbell shaped, orthogonal, lobes have opposite phases, node at the centre where the phase changes sign
How are molecular orbitals formed?
Overlap of 2 atomic orbitals
Results in two molecular orbitals (MO) forming:
- bonding MO: lower in energy and stabilising, from the constructive overlap of atomic orbitals of the same phase
- anti-bonding MO: higher in energy and destabilising, from the destructive overlap of atomic orbitals of opposite phases
- antibonding MO destabilising as a node forms, resulting in greater nucleus-nucleus repulsion
- destabilisation of antibonding MO>stabilisation of bonding MO
What does the MO diagram for H2 look like?
Making sure the gap between the AO and antibonding is larger than AO and bonding
How can p orbitals overlap to MOs? What do these look like?
Either head on to form σ and σ*
Or overlap edge on, above and below atoms, leading to π bonding
See below for photos
What does the MO diagram look like for the C=C?
Don’t forget to separate p and sp2
And that pi<sigma but pi>sigma in energy
And antibonding is more destabilising than bonding stabilising
What is hybridisation?
Mixing of orbitals to form hybrid orbitals
e.g in C: s + 3xp = 4 x sp3
As orbitals are just wave functions which can be added/subtracted
What is the bonding in ethane composed of?
Each carbon posseses 4 x sp3 orbitals
3 are used in bonding with 1s of H for each carbon, 3σ C-H each
Arranged tetrahedrally
And 1 sp3 of each is used to form C-C σ
What is the bonding in ethene?
Both carbons sp2
1 s orbital and 2 p orbitals combine to form 3 sp2 orbitals, with each carbon also having 1 p orbital, arranged in a trigonal planar shape
The sp2 orbitals will be used to form 2 C-H σ and 1 C-C σ each
And the remaining p orbitals will overlap at the edges above and below to form a π bond
What is the bonding in ethyne?
Both sp
1 s orbital and 1 p orbital are combined to form 2 sp orbitals
These are arranged linearly, forming 1 C-H σ and 1 C-C σ each
Each p orbital remaining (2 of each) will overlap at the edges forming two π bonds which are orthogonal to each other
What happens to rotation around π bonds and σ bonds?
σ bonds enable rotation, leading to conformations
π bonds restrict rotation as electron density is above and below the plane, rotations around the C-C axis would reduce the p orbital overlap and break the π bond, leading to geometric isomers
How can you quickly determine the hybridisation of atom of a molecule?
Count the number of substituents around an atom, including lone pairs
4=sp3
3=sp2
2=sp
What is the structure and shape of allene?
2HC=C=CH2
which means the central carbon is sp hybridised, end carbons sp2
The molecule cannot be planar as the central carbon must have π bonds orthogonal to each other
Leads to chirality
What is the hybridization of the oxygen/nitrogen in ethers and amides?
Ether: 2lp and 2 substituents, Sp3
Amides: 1 substituent, and 2 lp, Sp2
Nitrogen= sp2 as well as delocalisation occurs within amides
What is bond order?
(N of bonding electrons - N of antibonding electrons) / 2
When bond order>0 bonding possible
But this is because there is greater stabilisation via the bonding MO than destabilisation via the antibonding MO
What does the MO diagram of C=O look like? How does this relative to electronegativity?
The orbitals of O are lower in energy than C as O has a greater Zeff, greater orbital contraction
As a result, the orbitals of O have a greater orbital coefficient and contribution to the bonding bonding MO than antibonding
And so the electron pair is closer to the oxygen than C, whilst the opposite is true for the antibonding
When do you have to be careful with formal charges?
Mechanisms, sometimes formal charges are negative but no lone pairs e.g BH4 -
Or positive and no orbital empty e.g H30+
