Polarity

what is a polar bond

an unequal sharing of electrons (one atom is more electronegative than the other)

what is a non-polar bond

when the atoms involved in the bond are identical

what is electronegativity

the tendency of an atom to attract the electrons in a bond towards itself

how does electronegativity differ with periods and groups

• increase across a period

• decreases down a group

what has zero electronegativity

inert gases

what is the most electronegative element

fluorine (Fe)

why do polar bonds occur

• the most electronegative atom of a bond has the greater attraction for the shared electrons and has a slightly negative charge (holds the shared electrons closer)

• the less electronegative atom has a slight positive charge

• as a result a charge separation occurs across the bond (dipole)

what happens when there is a large difference in electronegativity between two bonded atoms

• it causes electrons to be completely transferred to the more electronegative atom (from the other atom)

• resulting in the formation of a negative ion from the most electronegative atom gaining an electron

• this is known as an ionic bond

what bond is it if the two atoms have the same electronegativity

non-polar covalent bond

what bond is it when there is an electronegativity different

polar covalent bond

what bond is it when the electronegativity difference is large

the bonding takes on more of an ionic character

how does electronegativity difference increase

description of non-polar covalent bonding

• atoms have the same tendency to attract bonding electrons

• electrons are shared equally

description of polar covalent bonding

• atoms have small differences in their tendency to attract bonding electrons

• electrons shared unequally between atoms

description of ionic bonding

• atoms have large differences in their tendency to attract bonding electrons

• one atom donates its valence electrons to the other

• ions are formed resulting in an ionic bond

when does a non-polar covalent bond occur

when identical non-metal atoms bond or when bonded atoms have nearly the same electronegativity values

when does a polar covalent bond occur

usually between two different non-metal atoms

when does an ionic bond occur

usually between the atoms of metals and non-metals

how does polarity interlink with the shape of the molecule

what will polar molecules have

one end of the molecule slightly positive and the other slightly negative - resulting in an uneven distribution of electron density

what does the polarity of a molecule depend on

• the molecule containing polar bonds

• the way the polar bonds are arranged

what happens in diatomic molecules if the bond joining the two atoms is polar

then the molecule as a whole will be polar

for molecules containing more than two atoms and has polar bonds what is the polarity of the molecule determined by

the shape (the arrangement of the polar bonds)

when can a polarity cancel and cause the molecule to be non-polar

when equivalent polar bonds are evenly arranged around a central atom

how can polar molecules be identified

• polar bonds in the molecule arranged unevenly around the central atom so that the bond dipoles do not cancel

• more than one type of atom bonded to the central atom

how to test for polarity in liquids

• when placed near an electrically charged object, the polar molecules align and are attracted to the charged object

• causes the liquid polar substance to deflect towards a charged object

• non-polar liquids flow without deflection past charged objects

(water and rod test)

why are water molecules polar

• they have polar bonds (O-H) arranged in a V shape around the oxygen atom

• the large electronegativity difference between hydrogen and oxygen atoms produces strong attractive forces between the molecules

substances that contain polar molecules

• hydrogen chloride HCl

• phosphorus trichloride PCl3

• ammonia NH3

• water H2O

• dichloromethane CH2Cl

substances that contain non-polar molecules

• carbon dioxide CO2

• oxygen O2

• hydrogen H2

• nitrogen N2

• methane CH4

polar shapes and why

• bent and v-shaped

• trigonal planar

asymmetrical shapes, dipoles don’t cancel → polar molecules

non-polar shapes and why

• tetrahedral

• linear

symmetrical shapes, dipoles can cancel if equal and opposite → non-polar molecules

what makes a polar molecule

• different electronegativity values between Z and X

• not arranged symmetrically around the central atom due to shape

• effect of dipoles is not cancelled

what makes a non-polar molecule

• bonds are arranged symmetrically around the central atom in a certain shape

• the effect of any dipoles formed by the different electronegativity of the X-Z bond is cancelled

what is the solubility rule of “like dissolves in like”

• polar molecules can dissolve in polar liquid

• non-polar molecules dissolve in other non-polar molecules but not in water