Questions Related to Science

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.

A proton is about 1836 times heavier than an electron. The mass of a proton is much greater than that of an electron.

A neutron is slightly heavier than a proton.

The mass of a proton is approximately 1.672 × 10⁻²⁷ kg, while the mass of a neutron is about 1.675 × 10⁻²⁷ kg.
Although their masses are almost the same, the neutron has a very small additional mass.

Both are found in the nucleus of an atom, but since the neutron has no charge and a slightly greater mass, it is considered heavier than the proton.

The scientist was Ernest Rutherford. He performed the Alpha Particle Scattering Experiment, which showed that the nucleus of an atom is small, dense, and positively charged.

Formula: C₅H₁₀
Cyclopentane is a ring compound with five carbon atoms linked in a closed chain. Each carbon forms two bonds with neighboring carbon atoms and two bonds with hydrogen atoms.

     H   H
      \ /
  H - C   C - H
   |       |
  H C     C H
    \     /
      C
      |
      H

Each carbon (C) is surrounded by four dots or lines (representing shared pairs of electrons), showing covalent bonds with other carbons and hydrogens.

The two properties of carbon that lead to a huge number of carbon compounds are:

1. Catenation – Carbon atoms can join with each other to form long chains, branched chains, or rings.
2. Tetravalency – Each carbon atom can form four covalent bonds with other atoms like carbon, hydrogen, oxygen, or nitrogen.

A mixture of ethyne and air is not used for welding because it does not burn hot enough and can explode. When ethyne burns in pure oxygen, it gives a very hot flame that can melt metals, but with air, burning is incomplete and the flame is not hot enough for welding.