Questions Related to Structure of Atom

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.

Isotopes are atoms of the same element having the same atomic number but different mass numbers.

From the given atoms:
³³A₁₇, ⁴⁰B₂₀, ³⁷C₁₇, ³⁹D₁₉

The atoms A (³³A₁₇) and C (³⁷C₁₇) have the same atomic number (17) but different mass numbers (33 and 37).

Therefore, A and C are isotopes.

Charges on subatomic particles are -

• Proton → Positive charge (+1)
• Electron → Negative charge (–1)
• Neutron → No charge (neutral)

a) Atomic number: -The atomic number of an element is the number of protons present in the nucleus of an atom. It is denoted by Z. For a neutral atom, the number of protons equals the number of electrons.

b) Mass number: -The mass number of an atom is the sum of the number of protons and neutrons present in its nucleus. It is denoted by A.

Formula:
Mass number (A) = Number of protons + Number of neutrons

The protons and neutrons present in the nucleus determine the mass of an atom.

This is because electrons have a very small mass compared to protons and neutrons, so their contribution to the total mass is negligible.

Isobars are atoms of different elements that have thesame mass number but different atomic numbers.

This means they have the same total number of protons and neutrons combined, but the number of protons in each atom is different.
Example:
⁴⁰Ar (Argon) and ⁴⁰Ca (Calcium) are isobars because both have a mass number of 40, but their atomic numbers are 18 and 20 respectively.

The number of protons in an atom is equal to its atomic number.

For ³⁵Cl₁₇, the atomic number is 17.
So, the atom of chlorine (³⁵Cl₁₇) has 17 protons.

The nucleus of an atom has a positive charge.

This is because the nucleus contains protons, which are positively charged particles, and neutrons, which have no charge. The positive charge of the protons gives the nucleus its overall positive charge.

The first shell of an atom can hold a maximum of 2 electrons.

According to Bohr’s model of the atom, the maximum number of electrons that can be present in any shell is given by the formula 2n², where n is the shell number.

For the first shell (K shell),
n = 1
So, maximum electrons = 2 × (1)² = 2 electrons.

Therefore, the K shell or first energy level can have only 2 electrons.

Thomson said that an atom looks like a sphere of positive charge with electrons stuck inside it, just like seeds in a watermelon or plums in a pudding. The total positive and negative charges balance each other, making the atom neutral.