ionic model

chemical bonds

strong forces of attraction that hold atoms or ions together in a substance

all chemical bonds occur due to

electrostatic attractions between positively and negatively charged species

ionic bonding

due to the electrostatic attraction between the positively charged cations and negatively charged anions

ion properties

crystalline and brittle

poor electrical conductors when solid

good electrical conductors when molten or dissolved

cations

ions with more protons than electrons → positively charged

cation formation is an oxidation reaction because it involves the loss of electrons

anions

ions with more electrons than protons → negatively charged

anion formation is a reduction reaction because it involves the gain of electrons

isoelectronic

two species with the same electron configuration

octet rule

atoms react by gaining or losing electrons so as to acquire the stable electron structure of a noble gas, usually eight valence electrons

H+

H+ is a hydrogen nucleus → proton with no electrons surrounding it

- high density of charge and therefore readily combines with other species

H-

hydride anion

achieves noble gas configuration

transition element

an element with a partially filled d sub level

can form multiple ions with different charges

2+ transition metals

2+ occurs because 4s electrons are lost before 3d electrons

why do further successive ionisations occur

because the 3d sub level is similar in energy to the 4s sub level

electronegativity + trend

measure of the ability of an atom to attract a pair of covalently bonded electrons

trend: electronegativity increases across periods and up groups

what is the most electronegative element

fluorine →high tendency to attract pairs of covalently bonded electrons

greater chance of ionic character if

the difference in electronegativity between two elements is larger - when electronegativity distance is greater then 1.8

ammonium

NH4+

hydroxide

OH-

nitrate

NO3-

Hydrogen Carbonate

HCO3-

carbonate

CO3 2-

sulfate

SO4 2-

phosphate

PO4 3-

cyanide

CN-

chromate

CrO4 2-

dichromate

Cr2O7 2-

acetate

CH3COO-

thiocyanate

SCN-

ionic lattices

within ionic crystals, ions are arranged in a lattice structure

continuous, 3d networks of repeating units of positive and negative ions

- arrangement depends on size and charge ratio of ions

why are ionic lattices strong

ionic bonds are non-directional

- ions will attract all oppositely charged species surrounding it, with the attraction being equal in all directions

lattice enthalpy general equation

tells you how strong the ionic bonds are in a particular ionic lattice

lattice enthalpy

defined as the standard enthalpy change that occurs on the formation of gaseous ions from one more of the solid lattice

- endothermic (+)

lattice energy is affected by 2 factors

ionic radius: decreases with increasing ionic radius

ionic charge: increases with increasing ionic charge

lattice enthalpy: KF, NaF and CaF2

Na is greater than K because ionic radius is smaller

Ca is greater than both because it has a greater charge (more significant than ionic radii)

volatility

refers to the tendency of a substance to vaporise (turn into a gas)

volatility of ionic compounds

to turn an ionic compound into a gas, the strong electrostatic forces of attraction holding ions together must be overcome

low volatility

- implies high boiling points + melting points

what needs to happen for electrical conductivity

substances must contain mobile charged particles

ionic compounds electrical conductivity

solid lattice - cations and anions vibrate around a fixed point but they cannot move and therefore cannot conduct electricity

aqueous/molten - both cations and anions are free to move past each other, allowing them to conduct electricity when a potential difference is applied

solubility of ionic compounds

ionic compounds are typically soluble in polar solvents (water) and insoluble in non-polar solvents (hexane)

ionic compounds + water

in water, the water molecules position themselves so that their partial negative charges point towards the cations and their partial positive charges toward the anions

- thus, individual ions are pulled out of the lattice and become surrounded by water molecules

ionic compounds + non-polar solvents

there is no attraction between the ions in the ionic compound and the solvent so the cations and anions remain in the lattice

ionic compounds that do NOT dissolve in water

calcium carbonate and silver chloride

- when the attractions between cations and anions in the lattice are stronger than the association between the ions and water molecules