Ionic Compounds

1
Ionic Compounds

• Chapter 5

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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.

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Cations and anions

• Ions with a positive charge are called as
  cations. They have more protons than electrons.

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• 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.

5
Class Practice

• What will be the electronic configuration of
• Ca and Ca2
• Write down the number of protons and electrons.

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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.

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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.

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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-.

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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.

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• Three Mg2 cations are needed for every two N3-
  anions to produce electrically neutral compounds.
  Subscripts denote 3 magnesium ions and 2 nitride
  ions.

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Home work

• Page 164 Section review
• Q1) b and c.
• Q2) a, d, e and h.
• Q3) e, c and d.

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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.

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Crystal structure of NaCl
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• 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.

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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)

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• 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.

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• 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-

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• 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.

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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.

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Polyatomic ions

• An electrically charged group of two or more
  chemically bonded atoms that functions as a
  single ion.

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Charges of poly atomic ions.
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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.

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• 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.

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Homework

• Page 181
• 1-5

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Silicates and Hydrates

• Silicates are any compounds containing silicon,
  oxygen, one or more metals, and possibly hydrogen

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• 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

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• When a hydrated compound is heated to get rid of
  the water molecule the remaining salt is
  anhydrous.

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Homework

• Page 184
• Q.3 a, c, e, f, i ,j
• Q.4 all
• Q.5. a, c, d, e, h.

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Class work

• Test prep page 191.