Chemcial Bonding part 3

Chemical Bonding

Learning outcomes:
(a) Describe ionic bonding, as in sodium chloride and magnesium oxide, including the use of ‘dot and cross’ diagrams.

Answer: NaCl
x
x
xx
x
x
x
xxNa + Cl x [ Na ] + [ x Cl ]-
x
x x


Sodium atom has one valence electron, it loses one to become a positive 1+ charged ion to achieve the stable octet arrangement.
Chlorine atom has seven valence electrons, it gains one electron from sodium atom to form a negative -1 charged ion to achieve the stable octet arrangement.
Being oppositely charged, they attract each other and are bonded in a single ionic bond to form the lattice structure, NaCl.

Answer: MgO
xx
xx

xx
xx Mg + x O à [ Mg ] 2+ [ O ] 2-




Magnesium atom has two valence electrons, it loses two electrons to become a positive 2+ charged ion to achieve the stable octet arrangement.
Oxygen atom has six valence electrons, it gains two electrons from Magnesium atom to form a negative 2- charged ion to achieve the stable octet arrangement.
Being oppositely charged, they attracted each other by electrostatic force and are bonded in a double ionic bond.

(i) Describe metallic bonding in terms of a lattice of positive ions surrounded by mobile electrons.

Answer: The valence electrons from metals are far away from the nucleus hence they are not strongly attracted to the positive charged of the protons in the nucleus. These valence electrons can easily leave the atom and becomes free mobile delocalized electrons. Metals are structures held up by metallic bonding which is the attraction force between the positive cations and the “sea of negative delocalized electrons”. The more the delocalized electrons, the stronger the metallic bonding, the higher the melting point of the metal.





(m) Describe, in simple terms , the lattice structure of a crystalline solid which is:
(i) ionic, as in sodium chloride, magnesium oxide.

Answer: NaCl
The lattice structure of sodium chloride consists of large number of Na+ and Cl- ions arranged in an orderly manner. One sodium ion is bonded three dimensionally to six chloride ions, each by a single ionic bond. The ions are arranged in straight rows and forms a large structures with flat sides, a crystal structure.The ions of Na+ and Cl- will be repelled when a force is applied to the lattice, bringing the like charged ions together, hence NaCl is hard but brittle.

Answer: MgO
The lattice structure of magnesium oxide consists of large number of Mg2+ and O2- ions arranged in an orderly manner. One magnesium ion is bonded three dimensionally with six oxygen ions, each by a double ionic bond. The ions of Mg2+ and O2- will be repelled when a force is applied to the lattice, bringing the like charged ions together, hence MgO is hard but brittle. Magnesium oxide also has a higher melting point than sodium chloride because the electrostatic force of attraction between the oppositely charged ions in magnesium oxide is twice that of sodium chloride’s.

(ii) Simple molecular, as in iodine.

Answer: Iodine is made up of small discrete iodine molecules. Each molecule is made up of two iodine atoms covalently bonded to one another. These iodine molecules attracted each other by weak Van Der Waal’s intermolecular forces to form a lattice structure which has very low melting point and boiling point.

(iii) giant molecular, as in graphite; diamond.

Answer: Graphite
Graphite is a carbon allotrope in which large number of carbon atoms are arranged in flat, hexagonal parallel layers to form a giant covalent structure. The carbon atoms in each layer are arranged in rings of six atoms with each carbon atom covalently bonded strongly to three other atoms in its layers. Weak Van Der Waal’s forces are present between the layers. The layers of atoms can slide over each other easily making graphite soft and slippery.

Answer: Diamond
Diamond is a carbon allotrope in which each carbon atom forms four covalent bonds with four other carbon atoms in a three dimensional tetrahedral arrangement. The structure is a giant network of carbon atoms held together by covalent bonds. Each carbon atom in diamond is at the centre of a tetrahedron.

(iv) Hydrogen-bonded, as in ice.

Answer: Ice has an open regular structure, with the O atom in H2O molecule forms two hydrogen bonds with two water molecules. The two H atoms form two more hydrogen bonds with two other water molecules. Each H2O molecule is therefore hydrogen bonded to four other H2O molecules in a tetrahedral arrangement.

(v) Metallic, as in copper.

Answer: Metals have giant structures. In metal like copper , the atoms are packed together closely in regular three-dimensional patterns to form giant lattice structures. In copper, it consisted of positive Cu2+ ions bonded to a sea of delocalized electrons.

(n) Outline the importance of hydrogen bonding to the physical properties of substances, including ice and water.

Answer: Each H2O molecule is hydrogen –bonded to four other H2O molecules in a tetrahedral arrangement. The open structure explains the fact that ice is less dense than liquid water at 0 oC . Hence, during winter, only the top layer of water is frozen. The bottom part of water is relatively warmer and keeps the aquatic life alive.