electronegativity + polarity Flashcards
What is a covalent bond?
A covalent bond is the strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms
What is electronegativity?
Electronegativity is a measure of the attraction of a bonded atom for the pair of electrons in the covalent bond.
How can electronegativity be measured?
It’s measured on the Pauling scale
Pun: appauling
Scale made by Linus pauling
What are the trends in electronegativity ?
Electronegativity increases across a period and up a group
In all directions towards fluorine
Why does electronegativity increase across a period?
The charge on the nucleus increases across a period
The number of protons in the nucleus increases
Thus there is an increased attraction for the outer electrons
And so the bonding pair of electrons are attracted more strongly
Why does electronegativity increase up a group?
Down the group the bonding pair of electrons is held increasingly further away from the nucleus,
The number of shells increases
So the distance of the outer electrons from the nucleus increases
And so the bonding pair of electrons are attracted less strongly
(Sum up, it’s shielding)
What are polar bonds?
When one atom is more electronegative than the other (and they are bonded)
What is an example of polar bonds?
Chlorine is more electronegative than the hydrogen
Cl has a greater attraction for the electrons than H in the bond
The electrons are closer to the Cl than the H
-paulings scale readings-
Chlorine: 3.0
Hydrogen:2.1
So Cl>H in electronegativity
If you want a visual representation look at the video at 3:42
As a result the Chlorine becomes slightly negative and the reverse for hydrogen
This is a permanent dipole and the charge difference is always present
What are dipoles?
The differing attraction for the pair of electrons allows there to be a small charge difference between the atoms.
What are the conditions for a polar bond to form
If the two bonding atoms are different, their attraction for the shared pair of electrons is unequal
The bonding atom with a greater attraction for the shared pair of electrons is more electronegative
The bond is polarised
What are the conditions for a non polar bond to form?
If the two bonding atoms are identical, their attraction for the shared pair of electrons are equal
The electrons are equally distributed between the bonding atoms
The bond is perfectly covalent
What is the spectrum of bonds?
Rather than bonds existing as discreetly ionic and covalent they exist on a spectrum
Where is ionic bonding on the spectrum Of bonds and why?
The far left as the difference in electronegativity is so great that one atom effectively takes the electron from the other
(Left is greatest right is leastest which I’m aware isn’t a word)
Where is polar-covalent bonding on the spectrum of bonds?
The middle as the difference in electronegativity is small so the atoms share the electrons unequally and the bond is polarised
Where is covalent bonding on the spectrum of bonds?
At the far right as there is no difference in electronegativity so the molecule is electronically symmetrical
What is the difference between polar and non polar bonds
Molecules containing polar bonds are not always polar
The symmetry of polar bonds can cancel the effect of any permanent dipole
What are features of non symmetrical molecules?
A difference in charge exists across the molecule
There is an overall dipole
The molecule is polar
What are the features of symmetrical molecules?
The symmetry of the molecule means that the effect of any permanent dipoles is cancelled out
Linear, trigonal planar or tetrahedral shape (occurs in)
All atoms attached to the central atom are identical
No difference in charge exists across the molecule
The molecule is non polar
What is a non polar bond
The two bonding atoms are identical and the electrons are equally distributed between the bonding atoms
What is a polar bond?
The two bonding atoms are different and their attraction for the shared pair of electrons is unequal
What is a polar molecule
A difference in charge exists across the molecule
What is a non polar molecule
The symmetry of the molecule means that the effect of any permanent dipoles is cancelled out and there is no difference in charge exists across the molecule
Which of the following molecules is polar? A: SiCl4 B: CO2 C: NH3 D: BF3
CccCcCccccccccCccccccccCcC
It’s c
In magnesium iodide, some polarisation of the bond occurs. Explain, in terms of magnesium iodide, what the term polarisation means.
There is a small difference in electronegativity between Mg and I , as a result, the pair of electrons are shared unequally and there is an uneven distribution of charge . The bond is polarised
Both NF3 and BF3 molecules contain polar bonds. However, only one of these molecules is polar. State which molecule is polar and explain the difference.
NF3 is polar. It had a pyramidal shape and is not symmetrical, meaning that the dipoles do not cancel. BF3 is not overall polar it has a trigonal planar shape and is symmetrical so the dipoles cancel
Define the term electronegativity
The ability of an atom to attract electrons in a covalent bond
Chlorine is more electronegative than carbon and hydrogen, which have roughly equal electronegativity.
- they can form the molecule dichloromethane, CH2Cl2. Explain why it is a polar molecule
The molecule is not symmetrical and dipoles do not cancel out overall
