An atom has 7 electrons in its M shell and contains 18 neutrons in its nucleus. What is the mass number of this atom?

Introduction / Context:
This atomic structure question combines ideas about electronic configuration and nuclear composition. It asks you to interpret the number of electrons in a particular shell, identify the element and then calculate its mass number using the given number of neutrons.

Given Data / Assumptions:

• The atom has 7 electrons in its M shell (the third shell, n = 3).
• The nucleus contains 18 neutrons.
• The atom is assumed to be neutral unless stated otherwise, so number of protons equals number of electrons.
• We must find the mass number, which equals the sum of protons and neutrons.

Concept / Approach:
Electron shells are filled in the order K (n = 1), L (n = 2), M (n = 3) and so on according to well known rules and the capacity of each shell. The M shell can hold up to 18 electrons but may hold fewer depending on the element. If an atom has 7 electrons in the M shell, we can deduce its total number of electrons by considering typical configurations such as 2, 8, 7. Once we know the atomic number, which equals the number of protons, we can add the given neutrons to obtain the mass number A = Z + N.

Step-by-Step Solution:
1) For lighter elements with electrons reaching the M shell, a common pattern is K = 2 electrons, L = 8 electrons, M = remaining electrons.2) If the M shell has 7 electrons, a natural configuration is 2, 8, 7, giving a total of 2 + 8 + 7 = 17 electrons.3) In a neutral atom, number of electrons equals number of protons, so the atomic number Z = 17.4) The problem states that the nucleus contains 18 neutrons, so N = 18.5) Mass number A is defined as A = Z + N = 17 + 18 = 35.

Verification / Alternative check:
An element with atomic number 17 is chlorine. A common isotope of chlorine has mass number 35, written as Cl 35, with 17 protons and 18 neutrons. This matches the numbers we computed. The electron configuration of chlorine is indeed 2, 8, 7, which confirms that the assumption about distribution among shells is correct and that the calculated mass number of 35 is consistent with known isotopes.

Why Other Options Are Wrong:
25: This would imply 8 neutrons if Z = 17, which does not match the given 18 neutrons.
27: Again inconsistent, as 27 minus 17 equals 10 neutrons, not the 18 specified.
43: This would require 26 neutrons in addition to 17 protons, which does not match the question data.
37: This would imply 20 neutrons, whereas the nucleus is stated to contain 18 neutrons.

Common Pitfalls:
Students sometimes miscount the total electrons by assuming the M shell is full when they see it mentioned. It is important to use the actual number given, 7 in this case, and not the maximum possible. Another common error is to forget that mass number includes both protons and neutrons and mistakenly use only the number of neutrons. Always remember that A = Z + N and that Z is determined from the total electrons for a neutral atom.

Final Answer:
The mass number of the atom described is 35.