Questions Related to Science

In one molecule of H₂SO₄ (sulphuric acid):

Hydrogen (H) = 2 atoms
Sulphur (S) = 1 atom
Oxygen (O) = 4 atoms

Total atoms = 2 + 1 + 4 = 7 atoms
There are 7 atoms in one molecule of H₂SO₄.

Oxygen (O₂) is a diatomic gas.

To calculate the formula unit mass of calcium chloride (CaCl₂):

Atomic mass of Ca = 40
Atomic mass of Cl = 35.5

Formula unit mass = 40 + (2 × 35.5) = 40 + 71 = 111 u
So Formula unit mass of CaCl₂  is  111 u

There are 6.022 × 10²³ atoms in exactly  12 g of carbon.
This number is known as the Avogadro number, and 12 g of carbon represents 1 mole of carbon atoms.

The Latin name of  sodium is Natrium.
The Latin name of  mercury is Hydrargyrum.

Molecular mass is the sum of the atomic masses of all the atoms present in a molecule. It represents the total mass of one molecule of a substance and is expressed in atomic mass units (amu).

Example:
The molecular mass of water (H₂O) = (2 × 1) + 16 = 18 amu.

The number 6.023 × 10²³ is called the Avogadro number or Avogadro constant. It represents the number of particles (atoms, molecules, or ions) present in one mole of a substance.

The term used to represent the mass of 1 mole molecules of a substance is molar mass. It is expressed in grams per mole (g/mol).

The law of conservation of mass was given by Antoine Lavoisier in 1774.

He stated that mass can neither be created nor destroyed in a chemical reaction. This means that the total mass of the reactants is always equal to the total mass of the products.

Lavoisier proved this law through careful experiments, showing that even when substances change form during a reaction, their total mass remains the same.

The complete symbol of an atom is written as:

A (mass number)
Z (atomic number)
X (symbol of element)

Example: For sodium, atomic number = 11 and mass number = 23, the symbol is ²³₁₁Na.

The three isotopes of hydrogen are represented as:

1. Protium (¹₁H) – 1 proton and no neutron
2. Deuterium (²₁H) – 1 proton and 1 neutron
3. Tritium (³₁H) – 1 proton and 2 neutrons

Atomic number: Number of protons = 3
Mass number: Number of protons + Number of neutrons = 3 + 4 = 7
Valency: Atomic number 3 means the element is Lithium (Li). Its electronic configuration is 2, 1, so it has 1 valence electron -> Valency = 1.

Helium has zero valency because its outermost shell is completely filled with 2 electrons. It does not need to gain, lose, or share any electrons to become stable.
That’s why helium is chemically inactive and has valency = 0.

From the symbol ³²₁₆S:

i)  Atomic number of sulphur = 16
ii) Mass number of sulphur = 32
iii) Electronic configuration of sulphur = 2, 8, 6

The three subatomic particles are:

1. Proton (p⁺): Charge = +1, Relative mass = 1
2. Neutron (n⁰): Charge = 0, Relative mass = 1
3. Electron (e⁻): Charge = –1, Relative mass = 1/1836 (almost zero)

The maximum number of electrons that each shell can hold is given by the formula 2n², where n is the shell number.

• K shell (n = 1) → 2 electrons
• L shell (n = 2) → 8 electrons
• M shell (n = 3) → 18 electrons
• N shell (n = 4) → 32 electrons

Nitrogen (N):   Atomic number = 7  --->  Electronic configuration = 2, 5
                              Number of electrons in the valence shell = 5

Argon (Ar):   Atomic number = 18   --->  Electronic configuration = 2, 8, 8
                          Number of electrons in the valence shell = 8

Elements that have a tendency to lose electrons are called metals.
They usually have 1, 2, or 3 electrons in their outermost shell and lose them easily to form positive ions (cations).

Example:

1. Sodium (Na) loses one electron to form Na⁺.
2. Magnesium (Mg) loses two electrons to form Mg²⁺.

Valence electrons are the electrons present in the outermost shell (energy level) of an atom. These electrons take part in chemical bonding and determine the valency of an element.

Example:

• In sodium (Na), the electronic configuration is 2, 8, 1 --->  it has 1 valence electron.
• In oxygen (O), the configuration is 2, 6  ---->  it has 6 valence electrons.

Achievement:
J.J. Thomson’s model explained that an atom is electrically neutral, as the positive and negative charges are equal in magnitude and balance each other.

Limitation:
His model could not explain the arrangement of charges inside the atom and failed to explain the results of Rutherford’s alpha particle scattering experiment.