Problem 49 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following notations shows the product incorrectly? (a) \({ }_{5} \mathrm{~B}^{10}(\alpha, \mathrm{n})_{7} \mathrm{~N}^{13}\) (b) \({ }_{96} \mathrm{Cm}^{242}(\alpha, 2 \mathrm{n}){ }_{97} \mathrm{BK}^{243}\) (c) \({ }_{7} \mathrm{~N}^{14}(\mathrm{n}, \mathrm{p}){ }_{6} \mathrm{C}^{14}\) (d) none of these

Step-by-Step Solution

Answer

Option (c) shows the product incorrectly.

Step 1: Understanding Notation

The nucleus notation \((Z, A)\), where \(Z\) is the atomic number and \(A\) is the mass number, is used to denote elements before and after a reaction. In \((x, y)\), \(x\) is the incident particle and \(y\) is the particle ejected in the nuclear reaction.

Step 2: Analyze Option (a)

Option \((a)\) \({}_{5} ext{B}^{10}( ext{α}, ext{n})_{7} ext{N}^{13}\) indicates a reaction where boron-10 interacts with an alpha particle, emits a neutron, and forms nitrogen-13. Checking mass and charge balance requires: \[ 5( ext{B}) + 2( ext{α}) = 7( ext{N}) + 0( ext{n}) \]\[ 10( ext{B}) + 4( ext{α}) = 13( ext{N}) + 1( ext{n}) \]Both reactions are balanced. Thus, this notation is correct.

Step 3: Analyze Option (b)

Option \((b)\)\({}_{96} ext{Cm}^{242}( ext{α}, 2 ext{n}){}_{97} ext{Bk}^{243}\) suggests curium-242 interacts with an alpha particle, ejects two neutrons, forming berkelium-243. Check balance: \[96 ( ext{Cm}) + 2 ( ext{α}) = 97 ( ext{Bk})\]\[242 ( ext{Cm}) + 4 ( ext{α}) - 2( ext{n}) = 243 ( ext{Bk})\]Both charge and mass are balanced, so this notation is correct.

Step 4: Analyze Option (c)

Option \((c)\)\({}_{7} ext{N}^{14}( ext{n}, ext{p}){}_{6} ext{C}^{14}\) suggests nitrogen-14 interacts with a neutron, emits a proton, converting to carbon-14. Checking balance: \[7 ( ext{N}) + 0 ( ext{n}) = 6 ( ext{C}) + 1 ( ext{p})\]\[14 ( ext{N}) + 1 ( ext{n}) = 14 ( ext{C}) + 1 ( ext{p})\]Charge balance correct, but mass doesn't match; this notation is incorrect.

Step 5: Conclusion

Among all options, option \((c)\) shows an incorrect notation as it violates the mass balance.

Key Concepts

Nuclear Notation

In nuclear physics, a specific kind of shorthand is used to describe elements and nuclear reactions. This shorthand is called nuclear notation. Each element is represented by its atomic number and mass number in the format

• Atomic number (Z) which shows the number of protons in the nucleus.
• Mass number (A) which is the sum of protons and neutrons.

When an element undergoes a nuclear reaction, we also indicate the particles involved. For example, in a nuclear equation like \[_{5}\text{B}^{10}(\alpha, \text{n})\,_{7}\text{N}^{13} \]This format describes a target nucleus (\(_{5}\text{B}^{10}\)), a bombarding particle (\(\alpha\)), the ejected particle (neutron), and the resulting nucleus (\(_{7}\text{N}^{13}\)). This keeps the complex interactions tidy and understandable.

Mass Balance

Mass balance in nuclear reactions is all about ensuring the total number of nucleons remains the same before and after the reaction. This means:

• The sum of the initial mass numbers on the left side of a nuclear equation should equal the sum on the right side.

For instance, using our example notation \[_{5}\text{B}^{10}(\alpha, \text{n})\,_{7}\text{N}^{13} \]we have:\[ 10(\text{B}) + 4(\alpha) = 13(\text{N}) + 1(\text{n}) \]By ensuring that both sides are balanced in mass, we respect the law of conservation of mass in nuclear reactions. Option (c) in the original exercise violated this principle, making it incorrect.

Charge Balance

Similar to mass balance, ensuring charge balance is crucial for writing correct nuclear equations. Charge balance involves preserving the total atomic number across a reaction. This is because the atomic number represents the charge (protons) within the nucleus. Therefore:

• The sum of atomic numbers before the reaction must equal the sum of atomic numbers after.

In the notation example:\[\alpha = _{2}^{4}\text{He} \]For reaction \[_{5}\text{B}^{10}(\alpha, \text{n})\,_{7}\text{N}^{13}\]Charge should remain:\[ 5(\text{B}) + 2(\alpha) = 7(\text{N}) + 0(\text{n}) \]Ensuring charge balance allows for the accurate prediction and identification of nuclear reactions. Again, in the original exercise, option (c) has a correct charge balance, but its mass balance was off.

Nuclear Equations

At the heart of understanding nuclear reactions lies the construction of nuclear equations. A nuclear equation is like a detailed roadmap of the changes happening during a reaction. It includes:

• Identities of reactants and products.
• All particles involved.

Writing a nuclear equation requires careful attention to:

• The balance of mass (as discussed).
• The balance of charge.

Taking option (b) from the original exercise, written as:\[_{96}\text{Cm}^{242}(\alpha, 2 \text{n})\,_{97}\text{Bk}^{243} \]shows the curium nucleus bombarded by an alpha particle, with the resulting ejection of two neutrons, correctly forming berkelium. Both mass and charge are balanced. Learning to write these nuclear equations accurately ensures that the principles of conservation are maintained, leading to a clearer understanding of nuclear processes.