Module 2 Test: Atomic Mass Number and Nuclear Structure

Course and Assessment Context

• Course Identifier: Application to Health (NSG312.1.FALL2020ONLINE)
• Curriculum Module: Module 2: Chemistry, Matter and Life
• Assessment Title: Session 2 Test
• Test Parameters and Specifications:
  • Current Item: Question 3 (Page 3 of 20)
  • Point Value: 1 Point
  • Assessment Progress: 18 of 20 questions remaining
  • Timer Status: 23 minutes remaining
  • Total Time Limit: 25 minutes
  • Attempt Allowance: 1 attempt

Examination Question and Answer Analysis

• Question Stem:
  • "The atomic mass number of an atom is equivalent to the number of ______ contained in its nucleus."
• Available Choices:
  • A. neutrons
  • B. protons and electrons
  • C. protons
  • D. protons and neutrons
  • E. electrons
• Correct Answer: D. protons and neutrons
• Rationale and Evaluation of Choices:
  • A. neutrons (Incorrect): Neutrons represent only a portion of the nuclear mass. The count of neutrons alone is given by the formula $N = A - Z$, which is not equivalent to the total mass number $A$.
  • B. protons and electrons (Incorrect): Electrons reside outside the nucleus in electron shells or orbitals, not within the nucleus. Furthermore, because electron mass is negligible compared to nucleons, electrons are excluded from the mass number.
  • C. protons (Incorrect): The total count of protons defines the atomic number ($Z$), which dictates the chemical identity of the element, rather than the mass number ($A$).
  • D. protons and neutrons (Correct): The atomic mass number ($A$) represents the sum of all nucleons (protons and neutrons) residing within the nucleus of an atom.
  • E. electrons (Incorrect): Electrons do not reside within the nucleus and have an extremely small relative mass ($\approx \frac{1}{1836}\,\text{amu}$), making them irrelevant to the atomic mass number.

Fundamental Principles of Atomic Structure

• The Atomic Nucleus:
  • The dense, positively charged region at the center of an atom.
  • Accounts for virtually all ($> 99.9\%$) of the total mass of the atom.
  • Composed of two types of subatomic particles collectively referred to as nucleons:
  • Protons
  • Neutrons
• The Extranuclear Region (Electron Cloud):
  • The vast space surrounding the nucleus organized into principal energy levels, sublevels, and atomic orbitals.
  • Occupied exclusively by electrons moving in probabilistic regions of space.

Subatomic Particles Summary

• Proton:
  • Location: Inside the nucleus
  • Electrical Charge: Positive ($+1$ fundamental charge, or $+1.602 \times 10^{-19}\,\text{C}$
  • Mass: Approximately $1\,\text{amu}$ ($1.6726 \times 10^{-27}\,\text{kg}$
  • Function: Defines the elemental identity and atomic number ($Z$
• Neutron:
  • Location: Inside the nucleus
  • Electrical Charge: Neutral ($0$
  • Mass: Approximately $1\,\text{amu}$ ($1.6749 \times 10^{-27}\,\text{kg}$
  • Function: Contributes to nuclear stability by offsetting electrostatic repulsion between protons via the strong nuclear force; defines the isotopic identity
• Electron:
  • Location: Outside the nucleus (orbitals/shells)
  • Electrical Charge: Negative ($-1$ fundamental charge, or $-1.602 \times 10^{-19}\,\text{C}$
  • Mass: Approximately $0.00055\,\text{amu}$ ($9.109 \times 10^{-31}\,\text{kg}$
  • Function: Dictates chemical bonding, reactivity, and ionic charge

Mathematical Relationships in Atomic Mass Calculations

• Atomic Number ($Z$):
  • Equivalent to the number of protons in the nucleus:
  • $\text{Atomic Number } (Z) = \text{Number of Protons}$
  • In a neutral (uncharged) atom:
  • $\text{Number of Protons} = \text{Number of Electrons}$
• Atomic Mass Number ($A$):
  • The total count of nucleons residing in the atomic nucleus:
  • $A = Z + N$
  • Where:
  • $A$ = Mass number
  • $Z$ = Atomic number (number of protons)
  • $N$ = Neutron number (number of neutrons)
• Deriving the Number of Neutrons ($N$):
  • By rearranging the fundamental equation:
  • $N = A - Z$
  • $\text{Number of Neutrons} = \text{Mass Number} - \text{Atomic Number}$
• Standard Isotopic / Nuclear Notation:
  • Elements and isotopes are represented as:
  • \sideset{^A_Z}{}\text{X}
  • Where:
  • $\text{X}$ = Chemical symbol of the element
  • $A$ = Atomic mass number (protons + neutrons)
  • $Z$ = Atomic number (protons)
  • Illustrative Examples:
  • Carbon-12: \sideset{^{12}_6}{}\text{C}
    • Protons ($Z$) = $6$
    • Neutrons ($N$) = $12 - 6 = 6$
    • Mass Number ($A$) = $6 + 6 = 12$
  • Carbon-14: \sideset{^{14}_6}{}\text{C}
    • Protons ($Z$) = $6$
    • Neutrons ($N$) = $14 - 6 = 8$
    • Mass Number ($A$) = $6 + 8 = 14$