Question

Write answers to the following questions and explain your answers.

a. Explain how the element sodium is monovalent.

b. M is a bivalent metal. Write down the steps to find the chemical formulae of its compounds formed with the radicals, sulphate and phosphate

c. Explain the need for a reference atom for atomic mass. Give some information about two reference atoms.

d. What is meant by Unified Atomic Mass.'

e. Explain with examples what is meant by a 'mole' of a substance.

Answer 1

a. Sodium is a metal which has atomic number 11. It has 1 electron in its last shell which means it has 1 valence electron. In order to get stability by acquiring nearest noble gas configuration. Sodium prefers to loose 1 electron. Hence, we say that sodium has a valency of one and is thus monovalent.

b. M is a bivalent metal that is : M²⁺
Step 1 : M SO₄

Step 2 :
M SO₄

2+ 2-

Step 3 :


Step 4 : MSO₄


M is a bivalent metal that is : M²⁺

Step 1 :
M PO4

Step 2 : M PO4
2 3

Step 3 :


Step 4 : M₃(PO4)₂

c. Mass of an atom is concentrated in its nucleus. It is th sum of proton(p) and neutron(n). The number(p+n) is called atomic mass number.
As we know that atom is very small in size, then how do we determine its mass? Therefore, the concept of reference atom for atomic mass is taken into consideration.
Initially hydrogen atom being lightest was chosen as the reference atom.The relative mass of hydrogen atom was accepted as (1). this is how relative atomic masses of various elements were determined in reference of hydrogen. For example: the mass of one nitrogen atom is fourteen(14) times that of a hydrogen atom.

Finally, the carbon was selected as reference atom. The relative mass of carbon atom was accepted as 12. The relative atomic mass of hydrogen compared to the carbon atom becomes 12 x 1/12 =1.

d. The Unified Atomic Mass Unit or Dalton is a standard unit of mass that quantifies mass on an atomic or molecular scale.

• One unified atomic mass unit is approximately equivalent to 1g/mol.
• It is denoted by a symbol :1u or Da.

e. One mole is defined as the amount of substance containing as many elementary entities (atoms, molecules, ions, electrons, radicals, etc.) as there are atoms in 12 grams of carbon-12 (6.023 x 10²³⁾.
The mass of one mole of a substance equals to its relative molecular mass expressed in grams. Mole defines the quantity of a substance. One mole of any substance will always contain 6.022 × 10²³ particles, no matter what that substance is. Therefore, we can say:

• 1 mole of potassium atoms (K) contains 6.022 × 10²³ potassium atoms.
• 1 mole of potassium ions (K⁺) contains 6.022 × 10²³ potassium ions.
• 1 mole of hydrogen atoms (H) contains 6.022 × 10²³ hydrogen atoms.
• 1 mole of hydrogen molecules (H₂) contains 6.022 × 10²³ hydrogen molecules.