Simple Molecules and Their Shapes

CONTENT

• Covalent Molecules e.g methane, diamond, crystalline solid – their network structure e.g diamond.

Simple Molecules and their shapes

The factors that is  responsible for the shape of simple covalent molecules are

1. sharing of electron that leads to overlapping of two atomic orbital
2. the central atom and their valence shell electron.

Covalent Molecules

Let consider methane is an example (CH₄).

The central atom is carbon with electronic configuration of IS²2S²P² which can be spined as

During the bond formation between the carbon and hydrogen, one electron is promoted from 2S to 2P². That is IS²2S¹2P¹x2P¹y2P¹ or

In the molecule of methane, the carbon atom has four bond pairs of electrons in its valence shell (the octet rule is obeyed).

The C-H bond in methane are identical.  If the 2S and three 2P orbitals are hybridized to from four new orbital which are identical, this new hybrid orbitals are called SP³. That is one S and three P orbitals are combined.  The electron are negatively charged and they move to the corners of a regular  tetrahedron.  These carbon lies at the apices of the tetrahedron.

Shape of methane

EVALUATION

1. State two (2) factors that determines the shape of simple covalent molecule
2. Draw the shape of methane and explain its formation

Ammonia

In ammonia, NH₃, the central atom is Nitrogen with configuration 1S²2S²P3 or

The three unpaired electron in the 2P from the covalent bond with an election of hydrogen atom. It remain one lone pair in the outermost valence shell of nitrogen and the octect rule is satisfied.

The electron clouds of 4 pairs of electrons spaced out but not of the same shape as methane because ammonia contains one lone pair of electrons. This give ammonia a triagonal pyramidal shape.

Shape of Ammonia

Trigonal pyramidal shape of ammonia.

Crystalline Solids

1. the crystalline solids have definite geometric shape
2. the shape of the crystal depend on
3. the force of attraction between the particle
4. whether the particles are the same or different.
5. The relative sizes of the particles if they are different.

There are 3 types of unit cell crystal based on cubic structure.

1. Simple cubic: the particles are placed one at each corner of the cube
2. Faced – centred cubic: the particles are located at each corner and one in the centre of     each face of the cube.
3. Body-centred cubic: there is a particle at each corner and one at the centre of the cube.

Types of Crystalline Solids

Crystals can be grouped according to the  chemical nature of their particles

1. covalent crystalline solid
2. ionic crystalline solid
3. molecular crystalline solid
4. metallic crystalline solid.

Covalent Crystalline Solid

The best example is chemical crystal which is octahedral in shape.  The crystal lattice is build frfom a basic three- dimensional tetrahedral unit cell.  The carbon atom is lined to four other carbon atoms by covalent bonds which are directed towards the apices of a regular tetrahedron.  Thus, the unit cell is repeated several time to form a giant three dimensional molecules.

Tetrahedral unit of diamond crystal

Octahedral shape of the diamond crystal

Arrangement of the carbon atoms in the diamond

EVALUATION

1. Explain the crystalline solid
2. Draw the structure of

            (a)  ammonia (b) octahedral shape of diamond

Ionic Solids

Examples are NaCl and CuSO4 crystal.  The shape of these crystals are determined according to how positive and negative ions are arranged, and according to the sizes and changes of the ions.

Molecular Crystals

There the molecules are arranged in regular patterns to give lattices.  The molecules are held together by weak intermolecular forces e.gVander Waals force, hydrogen bond, depol-depole.

Examples of the molecules are Naphthalene, iodine and dry ice crystal.

Metallic Solid

The metallic particles are held together in a crystal lattice of closely – packed sphere.

The strength of the metallic bonds varies among different metals e.g iron is more stronger than sodium and potassium.

GENERAL EVALUATION

1. Write short notes on the following

    a. ionic solid

    b. molecular crystals

    c. metallic solid

2. What are the types of attractive forces present in each of the following substances at     room temperature and pressure?

    (a) Methane                         (b) Argon     

    (c) Diamond                         (d) Water  

    (e) Aluminium

READING ASSIGNMENT

New School Chemistry by O.Y. Ababio pages 286-294.

WEEKEND ASSIGNMENT

1. A lone pair of electron is found in (a) ammonia (b) methane (c) water (d) carbon(iv) oxide
2. Examples of covalent molecules with linear shapes are except (a) oxygen (b) hydrogen (c) water (d) hydrogen chloride
3. Example of compound with double bonds is (a) water (b) carbon(iv) oxide (c) methane (d) ammonia
4. The following are types of crystalline solid except (a) covalent (b) ionic (c) metallic (d) methane
5. The unit cells based on the cubic structure are the following except (a) simple cubic (b) complex cubic (c) body-centred cubic (d)face-centred cubic

THEORY

1. State three (3) examples of crystalline solids with their shape
2. Explain the following simple covalent molecules and draw their shape (a) methane (b) Water (c) carbon (iv) oxide.