Problem 22 - A Complete Resource Book in Chemistry for JEE Main

Question

In which radiation, mass number and atomic number will not change? (a) \(\alpha\) (b) \(\beta\) (c) \(\alpha\) and \(2 \beta\) (d) \(\gamma\)

Step-by-Step Solution

Answer

The radiation is (d) b3 (gamma).

Step 1: Understanding the Question

The problem asks which type of radiation will result in no change in the atomic number or mass number of a nucleus.

Step 2: Analysis of Alpha Radiation

Alpha (b1) radiation involves the emission of an alpha particle, which consists of 2 protons and 2 neutrons. This changes both the atomic number and mass number.

Step 3: Analysis of Beta Radiation

Beta (b2) radiation involves the conversion of a neutron to a proton or vice-versa, changing the atomic number by 1, but the mass number remains unchanged.

Step 4: Analysis of Alpha and Beta Radiations Combined

If both b1 and 2b2 radiation occur, the combination will cause significant changes: the b1 decay decreases both atomic and mass numbers, while 2b2 decays alter the atomic number further.

Step 5: Analysis of Gamma Radiation

Gamma (b3) radiation involves the emission of energy without changing any particles in the nucleus. Therefore, neither the atomic number nor the mass number changes.

Step 6: Drawing the Conclusion

From the above analyses, the radiation that does not change the atomic or mass number is b3 radiation.

Key Concepts

Alpha Radiation

Alpha radiation is a type of radioactive decay where an unstable atomic nucleus discharges an alpha particle. An alpha particle consists of 2 protons and 2 neutrons, equivalent to a helium nucleus. When a nucleus emits an alpha particle:

• The atomic number decreases by 2 because of the loss of 2 protons.
• The mass number decreases by 4 due to the collective loss of 4 nucleons (protons and neutrons).

Alpha decay often happens in very heavy elements, like uranium and radium. This process makes the atom more stable by decreasing its size and rebalancing the nucleus.

Beta Radiation

Beta radiation occurs through beta decay, where a neutron in the nucleus is transformed into a proton, or vice versa. This transformation is accompanied by the emission of beta particles, which are high-energy, high-speed electrons or positrons. When a neutron becomes a proton, an electron (beta-minus particle) is emitted:

• The atomic number increases by 1, as there is an extra proton.
• The mass number remains unchanged because the overall number of nucleons is the same.

Conversely, when a proton turns into a neutron, a positron (beta-plus particle) is emitted and the atomic number decreases by 1. Beta decay is more common in lighter elements and plays a crucial role in balancing the neutron-to-proton ratio.

Gamma Radiation

Gamma radiation involves the release of gamma rays, which are photon emissions of high-energy electromagnetic radiation. This type of decay usually follows alpha or beta decay as the nucleus emits energy to reach a more stable state. Importantly, gamma radiation does not involve the release of matter, meaning:

• The atomic number remains unchanged, as no protons are lost or gained.
• The mass number remains the same, with no nucleons being added or removed.

Due to these properties, gamma decay only changes the energy state of the nucleus without altering its composition or mass.

Atomic Number

The atomic number is the number of protons in an atom's nucleus and defines the chemical element. For instance, hydrogen has an atomic number of 1, while carbon has an atomic number of 6. The atomic number:

• Determines the position of an element in the periodic table.
• Defines the chemical properties and how the atom interacts with other atoms.

During some types of radioactive decay, like beta decay, the atomic number might change, affecting the type of element the atom becomes. However, in gamma radiation, the atomic number remains constant, maintaining the element's identity.

Mass Number

The mass number of an atom is the sum of its protons and neutrons, often found at the top left of an element's symbol in notation. It is not to be confused with atomic mass, which is an average mass considering isotopes and electrons. The mass number:

• Represents the total number of nucleons in the nucleus.
• Does not include electrons, as they are too light to impact it significantly.

In alpha decay, the mass number decreases due to the release of 4 nucleons as an alpha particle, while beta decay keeps the mass number constant. Gamma decay likewise doesn't affect the mass number, only the energy of the nucleus.