Electronegarivity And Forces Flashcards
What is eletronegativity
Electronegativity is the power of an atom to attract the electron density in a covalent bond towards itself.
Pauling scale
The Pauling scale is used as a measure of electronegativity. It runs from 0 to 4. The greater the number, the more electronegative the atom, see Table 1. The noble gases have no number because they do not, in general, form covalent bonds.
What does electronegativity depend upon
the nuclear charge
2 the distance between the nucleus and the outer shell electrons
3 the shielding of the nuclear charge by electrons in inner shells.
Polarity os some atom bonds and different atom bonds
When both atoms are the same, for example, in fluorine, F,, the electrons in the bond must be shared equally between the atoms (Figure 1) - both atoms have exactly the same electronegativity and the bond is completely non-polar.
Hydrogen has an electronegativity of 2.1 and fluorine of 4.0. This means that the electrons in the covalent bond will be attracted more by the fluorine than the hydrogen. The fluorine end of the molecule is therefore relatively negative and the hydrogen end relatively positive, that is, electron deficient. Covalent bonds like this are said to be polar. The greater the difference in electronegativity, the more polar is the covalent bond.
Types of intermolecular forces
van der Waals forces
act between all atoms and molecules.
• Dipole-dipole forces
act only between certain types of molecules.
• Hydrogen bonding
acts only between certain types of molecules.
Dipole to dipole forces
In molecules with more than one polar bond, the effects of each bond may cancel, leaving a molecule with no dipole moment. The effects may also add up and so reinforce each other. It depends on the shape of the molecule.
Van her waal forces
All atoms and molecules are made up of positive and negative charges even though they are neutral overall. These charges produce very weak electrostatic attractions between all atoms and molecules. These are called van der Waals forces.
The atom as a whole is neutral but at any moment in time the electrons could be anywhere,
This means the distribution of charge is changing at every instant. Any of the arrangements mean the atom has a dipole at that moment. An instant later, the dipole may be in a different direction. But, almost certainly the atom will have a dipole at any point in time, even though any particular dipole will be just for an instant - a temporary dipole. This dipole then affects the electron distribution in nearby atoms, so that they are attracted to the original helium atom for that instant.
The original atom has induced dipoles in the nearby atoms,
Induced dipoles
As the electron distribution of the original atom changes, it will induce new dipoles in the atoms around it, which will be attracted to the original one. These forces are sometimes called instantaneous dipole-induced dipole forces
• The dipole is caused by the changing position of the electron cloud, so the more electrons there are, the larger the instantaneous dipole will be.
Why is hydrogen bonding much stronger than dipole athaction
The oxygen atoms in water have lone pairs of electrons.
2 In water the hydrogen atoms are highly electron deficient. This is because the oxygen is very electronegative and attracts the shared electrons in the bond towards it. The hydrogen atoms in water are positively charged and very small. These exposed protons have a very strong electric field because of their small size.
When do hydrogen bonds form
• a hydrogen atom that is bonded to a very electronegative atom. This will produce a strong partial positive charge on the hydrogen atom.
• a very electronegative atom with a lone pair of electrons. These will be attracted to the partially charged hydrogen atom in another molecule and form the bond.
The boiling point for the hydrides
The noble gases show a gradual increase in boiling point because the only forces acting between the atoms are van der Waals forces and these increase with the number of electrons present.
The boiling points of water, H2O, hydrogen fluoride, HF, and ammonia,NH3 are all higher than those of the hydrides of the other elements in their group, This is because hydrogen bonding is present between the molecules in each of these compounds
The importance on hydrogen bonds
Although hydrogen bonds are only about 10% of the strength of covalent bonds, their effect can be significant - especially when there are a lot of them. The very fact that they are weaker than covalent bonds, and can break or make under conditions where covalent bonds are unaffected, is very significant.
