Problem 38 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The number of neutrons in dipositive zinc ion with mass number 70 is (a) 34 (b) 36 (c) 38 (d) 40

Step-by-Step Solution

Answer

The number of neutrons is 40.

Step 1: Understanding the Question

We need to determine the number of neutrons in a dipositive zinc ion ( Zn^{2+} ) with a mass number of 70. The mass number is the total number of protons and neutrons in the nucleus.

Step 2: Find the Atomic Number of Zinc

Zinc ( Zn ) has an atomic number of 30. This means zinc has 30 protons in its nucleus. The atomic number is equivalent to the number of protons and also tells us about electrons in a neutral atom.

Step 3: Understand the Dipositive Ion

Since we are dealing with a dipositive ion ( Zn^{2+} ), the element loses two electrons. However, this doesn't affect the number of neutrons, as neutrons are located in the nucleus and neutral overall charge depends on electron balance.

Step 4: Calculate the Number of Neutrons

The number of neutrons is calculated by subtracting the number of protons from the mass number. Therefore, the number of neutrons in this zinc ion is given by:\[\text{Number of Neutrons} = \text{Mass Number} - \text{Atomic Number} = 70 - 30 = 40\]

Step 5: Conclusion

The number of neutrons in the dipositive zinc ion with a mass number of 70 is 40, corresponding to option (d).

Key Concepts

Atomic Number

The atomic number is one of the fundamental properties of an element. It's denoted by the letter \( Z \) and represents the number of protons in the nucleus of an atom. This number is crucial because it determines the chemical identity of the element.
For example, zinc has an atomic number of 30, meaning it has 30 protons. The atomic number also generally indicates the number of electrons in a neutral atom. Therefore, zinc has 30 electrons when it isn't ionized.

• Defines the element: Each element has a unique atomic number.
• Equals the number of protons.
• In a neutral atom, it's also the number of electrons.

In our exercise, knowing zinc's atomic number helps us understand how many protons are present and leads us to the charge balance in ions.

Mass Number

The mass number of an atom is the total count of protons and neutrons in its nucleus. It doesn't account for electrons because electrons contribute very little mass.
For zinc in this exercise, the mass number given is 70. This means the sum of protons (30, from the atomic number) and neutrons equals 70.

• Expressed as the sum of protons and neutrons.
• Indicates the overall mass of the nucleus.
• Essential for calculating the number of neutrons.

By subtracting the atomic number from the mass number (70 - 30), we find the number of neutrons, which is a key step in solving the problem.

Zinc Ion

A zinc ion typically forms when zinc atoms lose electrons. The process of losing electrons affects the ion's charge but not its mass number or the number of neutrons.
Zinc starts as an electrically neutral atom with 30 electrons, balanced by 30 protons. When zinc forms a positive ion, it does so by shedding electrons but still retaining its 30 protons.

• Originates from a neutral zinc atom.
• Electrically charged due to electron loss.
• Mass and composition of the nucleus remain unchanged.

In this case, we are dealing with a dipositive zinc ion \( (Zn^{2+}) \), which indicates two electrons have been lost.

Dipositive Ion

Dipositive ions are ions with a double positive charge, indicated by \( +2 \). This happens when an atom loses two electrons.
For zinc, this means the \( Zn^{2+} \) ion has lost two of the 30 electrons present in a neutral atom, resulting in a positive charge while the protons and neutrons in the nucleus are unchanged. The neutrality of neutrons assures that the mass and nuclear properties remain constant.

• Loses two electrons, hence \( +2 \) charge.
• Protons remain constant, maintaining atomic identity.
• Only affects the electron count, not neutron count.

Understanding dipositive ions helps solidify how charges can change during ionization without affecting the mass number or neutron count.