A neutral nitrogen atom has 7 protons and 7 electrons. After gaining three electrons to form a nitride ion (N3-), how many protons and electrons does this nitride ion have?

Introduction / Context:
Understanding the difference between atoms and ions is a fundamental concept in introductory chemistry. This question focuses on how the numbers of protons and electrons change when a neutral atom gains or loses electrons to form an ion. Nitrogen is a common example in questions about anions because it tends to gain three electrons to complete its valence shell and form the nitride ion, written as N3-. Recognising that proton number does not change during ion formation is a key idea being tested here.

Given Data / Assumptions:

• A neutral nitrogen atom has 7 protons and 7 electrons.
• The nitride ion is represented as N3-.
• An anion forms when an atom gains electrons.
• We assume that forming N3- does not change the nucleus, so the number of protons stays constant.

Concept / Approach:
The atomic number of nitrogen is 7, which means that every nitrogen atom has 7 protons in its nucleus. The number of protons defines the identity of the element and does not change when ions are formed. A neutral atom has an equal number of protons and electrons, so neutral nitrogen has 7 electrons. When nitrogen forms N3-, it gains three extra electrons, giving it a total of 10 electrons. Thus, the correct description of the nitride ion must show 7 protons and 10 electrons. Any answer that changes the number of protons is automatically incorrect.

Step-by-Step Solution:
Step 1: Note that the atomic number of nitrogen is 7, so a nitrogen nucleus always contains 7 protons.Step 2: A neutral nitrogen atom has 7 electrons to balance the 7 positive charges of the protons.Step 3: The nitride ion N3- has a 3 negative charge, which indicates that the atom has gained three extra electrons.Step 4: Add the three extra electrons to the original 7 electrons: 7 + 3 = 10 electrons.Step 5: The number of protons remains 7 because the nucleus is not altered when forming a simple ion.Step 6: Therefore, the nitride ion has 7 protons and 10 electrons.

Verification / Alternative check:
You can verify the charge by subtracting electron count from proton count. For N3-, the net charge should be -3. With 7 protons and 10 electrons, the total positive charge is +7 and the total negative charge is -10. The overall charge is +7 + (-10) = -3, which matches the notation N3-. If you tried any other combination of protons and electrons from the options, you would not obtain the correct -3 charge, which confirms that 7 protons and 10 electrons is the only consistent pair.

Why Other Options Are Wrong:
Option a, 10 protons and 7 electrons, wrongly changes the number of protons and also leads to a net charge of +3 instead of -3. Option c, 4 protons and 7 electrons, changes the element entirely, because an atom with 4 protons would be beryllium, not nitrogen. Option d, 4 protons and 10 electrons, again represents a different element and a charge of -6. None of these alternatives keep the atomic number 7 and simultaneously achieve the correct -3 charge.

Common Pitfalls:
A common mistake is to assume that both protons and electrons change when ions form. In reality, only the electron count changes in simple ionic processes, while the nucleus (and thus the number of protons) remains the same. Another pitfall is misinterpreting the 3- charge as meaning the ion has only three electrons, which is incorrect. Instead, the charge indicates a difference of three between the numbers of protons and electrons. Always start with the atomic number to fix the proton count, then adjust the electron count according to the charge.

Final Answer:
The nitride ion N3- contains 7 protons and 10 electrons.