Ionic Compounds
Chapter 5
What are ions?
► An
ion is an atom or group of atom that has
an electric charge because it has gained or
lost electron.
► [Na]= 1s2 2s² 2p6 3s1
► [Na+] = 1s2 2s² 2p6
► When
a sodium atom loses its outermost
electron, it becomes positively charged ion
called as sodium ion.
Cations and anions
► Ions
with a positive charge are called as
cations. They have more protons than
electrons.
► Negatively
charged ions are called as anions. They
have more electrons than protons.
► [Cl-] = 1s²2s²2p63s²3p6 =[Ar]
► [Cl] = 1s²2s²2p63s²3p5
► Collection of cations are never found without the
formation of similar number of anions. The
concept of having equal number of positive
charges is called electroneutrality.
Class Practice
► What
will be the electronic configuration of
► [Ca] and [Ca2+]
Write down the number of protons and
electrons.
Transition metals
► Not
all simple ions are isoelectronic with
noble gas atoms. The transition metals do
not fit this pattern. Lanthanides and
actinides elements form cations, mostly with
+3 charges, hence they are not
isoelectronic with noble gases.
Octet rule
► The
tendency of atoms of elements to gain
or lose electrons so that their outer s and p
orbitals are full with eight electrons.
The tendency of atoms to match the
electron configuration of the s and p orbitals
of a noble gas is called the octet rule.
Binary ionic compound
► An
ionic compound composed of simple
cation and simple anion is called a binary
ionic compound.
► Common table salt NaCl is composed of
equal number of sodium ions, Na+ and
chloride ions Cl-.
Naming Binary Compounds
► The
name consists of two words: the name of the
cation followed by the name of the anion.
► NaCl sodium chloride
► Mg3N2 magnesium nitride
► The magnesium ion,Mg2+ has two positive
charges, and the nitride ion, N3- has three negative
charges. We must combine in such a way that
there are as many negative charges as there are
positive charges.
► Three
Mg2+ cations are needed for every
two N3- anions to produce electrically neutral
compounds. Subscripts denote 3
magnesium ions and 2 nitride ions.
Home work
► Page
164 Section review
► Q1) b and c.
► Q2) a, d, e and h.
► Q3) e, c and d.
Ionic Bonding
► Sodium
a very reactive Group I metal
combines with chlorine a reactive Group 17
non metal ( a poisonous yellowish green gas
that consists of molecules with two chlorine
atoms bonded together) react a violent
exothermic reaction occurs. The white
residue formed from two dangerous
elements is the salt we eat every day.
Crystal structure of NaCl
► The
coulombic force of attraction of
oppositely charged ions is greater than the
coulombic force of repulsion from the ions
of like charges. Coulombic force is the
attraction or repulsion between two objects
that have electric charges.
Energy aspects of salt formation.
► Ionization
energy is the energy that must be
supplied to remove the outermost electron
from an atom. Atoms always resist having
their electrons removed so their ionization
energies are always positive.
► Na(g) +495 kJ/mol -- Na+(g) +e- (g)
► Electron
affinity is the energy needed to put
an extra electron into a neutral atom. Most
electron affinities are negative indicating
that the recipient atom easily accepts
another electron.
► Cl(g) +e-(g) -349 kJ/mol -- Cl-(g)
► Adding the above two equations we find
that the electron transfer from sodium to
chlorine atom is an endothermic reaction.
► Na(g) +Cl(g) +146
kJ/mol -- Na+(g) + Cl-(g)
► This is very different from what happens in the
flask. The formation if ions is only a part of the
reation.
► 1. The energy must be added to convert 1 mol of
sodium from a solid to a gas.
Na(s) + energy -- Na(g)
2. More energy must be added to remove one
electron from each sodium atom.
Na(g) +energy -- Na+ (g) +e-
►
►
►
3. No energy is needed to convert chlorine into gaseous
state because it is already a gas. However energy must be
added to break up ½ mole of Cl2 molecules to produce 1
mol of chlorine atoms.
½Cl2(g) + energy --- Cl(g)
4. Some energy is given off when an electron is added
to each chlorine atom to form Cl- ion. This is an exothermic
reaction.
Cl(g) + e- -- Cl- (g) + energy
5. Much more energy is given off when Na+ and Cl- ions
come together to form ionic crystal of NaCl.
Na +(g) + Cl- (g) -- NaCl (s) +energy.
Properties Of Binary ionic
compounds
►A
simple cation combined with a simple
anion results in binary compounds.
► Since they form strong ionic bonds they
have higher melting and boiling points.
► Since they have strong ionic bond these
solids are not good conductors of electricity.
► However in solution they form ions which
help in conducting electricity.
Polyatomic ions
► An
electrically charged group of two or
more chemically bonded atoms that
functions as a single ion.
Charges of poly atomic ions.
Oxidation numbers
►A
number assigned to an atom in a polyatomic
ion or molecular compound based on complete
transfer of electrons.
► Some guide lines:
► 1. The oxidation number of any free or combined
element is zero.
► 2. Oxidation number for mono atomic ion is the
charge on the ion.
► 3.The oxidation number on hydrogen is +1 unless
combined with a metal when it has a charge -1.
► The
oxidation number of halogen is -1 since more
electro negative.
► Oxygen has an oxidation number -2 except in
peroxides it is -1 and when combined with fluorine
it is +2.
► In compounds the metals in Group I,II and Al
have oxidation numbers +1,+2 and +3.
► The sum of the oxidation number for all the atoms
in a compound is zero.
► The sum of the oxidation number for all
polyatomic ion is equal to the charge on that ion.
Homework
► Page
181
► # 1-5
Silicates and Hydrates
Silicates are any compounds containing
silicon, oxygen, one or more metals, and
possibly hydrogen
► Ionic
compounds like copper sulfate CuSO4
with water molecules incorporated into their
crystal lattices are called hydrates. Some
salts form hydrates. E.g CuSO4.5H2O
► When
a hydrated compound is heated to
get rid of the water molecule the remaining
salt is anhydrous.
Homework
► Page
184
► Q.3 a, c, e, f, i ,j
► Q.4 all
► Q.5. a, c, d, e, h.
Class work
► Test
prep page 191.