3.4 Electronegativity Flashcards
What is electronegativity
. The power of an atom to attract the electron density in a covalent bond towards itself
What element has the highest electronegativity
Flourine is the best at attracting electrons, it is better than hydrogen at it so is more electronegative
What does electron density mean
. When chemists consider the electrons as charge clouds
Describes the way the negative charge is distributed in a molecule
What is the Pauling scale
Why don’t noble gases have an electronegative number
It is a measure of electronegativity.
It runs from zero to four
The greater the number, the more electronegative the atom is.
Noble gases don’t have a number because they do not form covalent bonds
What 2 things does electronegativity depend on
. Nuclear charge
. Size of atom:
- Distance between the nucleus and outer shell electrons
- Shielding of nuclear charge by electrons in inner shells
How does the size of an atom affect the electronegativity of it
. the smaller an atom, the closer the nucleus is to the shared outer main level electrons so they are more attracted to each other and it is easier to attract electron density
How does nuclear charge affect electronegativity
. The larger the nuclear charge (for a given shielding effect), the greater the electronegativity, because it can attract the outer shell more and so attract electron density.
How does electronegativity change going down a group
It decreases because the atoms get bigger so there is more shielding from electrons, and distance between nucleus and outer shell increases.
As a result it is less able to attract electron density.
How does electronegativity change across a period
. It increases because nuclear charge increases as atomic radii decreases and the number of energy levels remain the same.
As a result it is more able to attract electron density in a covalent bond
So where are the most and least electronegative elements found in the periodic table
. Most electronegative: top right eg fluorine, oxygen etc.
But ignore the noble gases which form few compounds
. Least electronegative elements are bottom left
What is the polarity of covalent bonds
. It is the unequal sharing of electrons between atoms that are covalently bonded.
It is a property of the bond
What type of covalent bonds are between two of the same element
. They are the same, eg in fluorine F2, the electrons in the bond must be shared equally between the atoms.
This is because both atoms have the same electronegativity and the bond is non polar.
What type of covalent bond is between two different atoms and why
Eg hydrogen fluoride
.Polar
In a covalent bond between two atoms of different electronegativity, the electrons in the bond will be shared unequally between the atoms.
So there will be an unsymmetrical distribution of them.
Eg HF hydrogen fluoride
. Hydrogen has an electronegativity of 2.1 and fluorine has 4.0.
This means the electrons in the covalent bond will be more attracted to the fluorine than the hydrogen.
So as a result the electron cloud is distorted towards the fluorine.
The fluorine end of the atom is relatively negative, and that of hydrogen is relatively positive/electron deficient.
What is the relationship between polarity and electronegativity of atoms
The greater the difference in electronegativity between two atoms, the more polar the covalent bond is
What does f+ and f- mean
Delta positive and delta negative
They represent the partial charges in a formula, so a small charge of less than 1 electron.
Because it is only a small change as the electron pair has only shifted towards the more electronegative atom.
They represent the relative polarity of each atom.
Eg in H-F there will be a f+ above the H and an f- above the F
What are the different intervals in electronegativity for each bond formed on the Pauling scale
If the difference in electronegativity is less than 0.4 for two atoms, they have a non polar bond. So pure and the electrons are shared equally.
If the difference in electronegativity is between 0.4 and 2.0, it is a polar covalent bond so the electrons aren’t distibuted evenly
If the difference in electronegativity between two atoms is from 2 to 4, it is an ionic bond so the electrons are so attracted to the other atom that they go completely.
When can a molecule with a large difference in electronegativity become non polar
. Polar covalent bonds have an unequal distribution of electron density with the more electronegative atom having a greater electronegativity.
. However non polar molecules are formed when the bond dipoles cancel each other out, so when the molecule is symmetrical around the central atom.
What is a dipole
When the electron density goes to one atom with a greater electronegativity than the other.
This separation of charge is called a dipole
What does
——I———-> mean
Same as delta (f+ and f-)
~The arrow points towards the more electronegative element
Describe the bonding in the molecule carbon dioxide
O==C==O
It has two carbon=oxygen double bonds, and both of these bonds are polar.
However these bonds point in opposite directions in a straight line, so their dipoles cancel each other out.
As a result, there is no overall polarity in carbon
Describe bonding in molecule tetrachloromethane CCl4
This molecule has 4 chlorine and one carbon, so 4 carbon to chlorine bonds which are polar
However, the molecule is symmetrical in all directions so the dipoles cancel each other out.
Overall, the molecule is non polar
Describe bonding in trichloromethane
CHCl3
This molecule has 3 chlorine and 1 carbon so overall 3 carbon-chlorine bonds.
The carbon to hydrogen bond is basically non polar as carbon and hydrogen have very similar Electronegativities.
In this case, the dipoles on the carbon to chlorine can’t cancel out as it is not symmetrical
So it is a polar molecule.
The side with hydrogen is positive and the side with chlorines is negative
Describe the bonding in a water molecule
It consists of two hydrogen atoms bonded to one oxygen atom.
It is polar.
This is because a water molecule has a non linear shape, so although the bonds point in opposite directions, they are not acting in a straight line.
As a result the bond polarities can’t cancel.
So bond angles are 104.5 degrees
