2.2.10 Electronegativity and Bond Polarity Flashcards
What does electronegativity measure? Which scale measures it, and what is the history behind it?
- Electronegativity measures the attraction of a bonded atom for the pair of electrons in a covalent bond.
- Since 1932, electronegativity has been measured by the Pauling scale - named after US chemist Linus Pauling
What are the most electronegative atoms?
Nitrogen, Oxygen, Fluorine and Chlorine
How does electronegativity vary across Mendeleev’s periodic table?
Electronegativity increases towards FLourine, so:
1. Across a period it increases
2. Across a group it decreases
Think of a period as the x axis and a group as the y axis, to get to ‘F,’ from the origin at the lanthanides, you must go up along a period and up a group. In the same say, it increases along the period and up the group.
What is a non polar molecule? Produce an example;
Two covalently bonded atoms are identical, and so each atom has an equal attraction to the electron pair, therefore resulting in a ‘perfect 100% covalent bond.’ An example: Hydrogen.
What happens to electronegativity in a polar atom?
Bonding atoms are different, so one of them attracts bonding electrons more. Therefore, it has more electronegativity than the other atom.
What is a permanent dipole? Produce an example;
In general:
1. A permanent dipole is a small charge difference across a bond
2. That results from a difference in the electronegativities of the bonded atoms
3. A polar covalent bond has a permanent dipole
HCl:
1. Cl is more electronegative than H (see Periodic Table),
2.So Cl has greater attraction for the bonding pair of electrons than the H atom
3. So bonding pair of electrons are held closer to Cl atom
4. The permanent charge difference between the two is called a permanent dipole
How do you show a permanent dipole? Produce an example;
𝛿- shows a small negative charge on an atom: Cl𝛿-
𝛿+shows a small positive charge on an atom: H𝛿+
How does the symmetry of a compound dictate its polarity? Give examples of polar and nonpolar molecules.
Polar molecules: HCl is not symmetrical because there is a large difference in the electronegativity of Cl and H.
Non polar molecules: CCl4 has four Cls with small negative charges situated symmetrically around the central C atom - and so their small negative charges cancel each other out, allowing the molecule to be non polar.
Why are not all of the molecules containing Oxygen polar?
When it is bonded to other atoms, it will be polar: but this does not mean all molecules containing Oxygen will be polar, as their symmetry may cancel out the overall dipole (eg. CO2, as it is in a linear structure), but this is not the case for water, which has a non-linear, bent structure.
How do you form a polar bond?
Take atoms with different electronegativity values and covalently bond them together.
When do you form a polar covalent bond?
When the charge difference between the atoms is only small and they share an electrons unevenly.
How do you increase the permanent dipole?
Increase the difference in electronegativity values. If the difference is very great, the most electronegative element may be able to take the electron, forming an ionic bond.
