Lecture Notes on Atomic Number, Mass Number, and Electric Charge

Key Concepts

Atomic Number (Z)

• Represents the number of protons in an atom.
• Identifies the element; an atom with 7 protons is nitrogen.
• Found in the periodic table:
  • Example: Nitrogen has an atomic number of 7.

Mass Number (A)

• Sum of protons and neutrons in an atom.
• Different for isotopes of the same element.
• Example:
  • Nitrogen-14 has a mass number of 14.
  • Nitrogen-15 has a mass number of 15.
• Noted as a whole number; average atomic mass is a decimal value.

Average Atomic Mass

• Reflects the weighted average of all isotopes of an element.
• Example: Nitrogen's average atomic mass is about 14.01, indicating that nitrogen-14 is more abundant than nitrogen-15.

Example: Boron

• Atomic number: 5 (5 protons).
• Average atomic mass: 10.81.
• Main isotopes: Boron-10 and Boron-11.
  • Boron-11 is more abundant since the average atomic mass is closer to 11.

Electric Charge

• Atoms are electrically neutral when protons = electrons.
• Ions have differing numbers of protons and electrons:
  • Positive charge: More protons than electrons.
  • Negative charge: More electrons than protons.

Practice Problems

Problem A

• Given: 6 protons, 7 neutrons, 6 electrons.
  • Atomic Number (Z): 6 (Carbon).
  • Mass Number (A): 6 + 7 = 13.
  • Charge: 6 - 6 = 0 (Neutral Atom).
  • Element: Carbon-13.

Problem B

• Given: 9 protons, 10 neutrons, 10 electrons.
  • Atomic Number (Z): 9 (Fluorine).
  • Mass Number (A): 9 + 10 = 19.
  • Charge: 9 - 10 = -1 (Ion).
  • Element: Fluorine-19 (ionic form, negative charge).

Problem C

• Given: 13 protons, 14 neutrons, 10 electrons.
  • Atomic Number (Z): 13 (Aluminum).
  • Mass Number (A): 13 + 14 = 27.
  • Charge: 13 - 10 = +3 (Ion).
  • Element: Aluminum (3+ charge, ionic form).

Summary

• Atomic Number identifies the element and corresponds to the number of protons.
• Mass Number is the sum of protons and neutrons and varies among isotopes.
• Electric Charge indicates whether an atom is neutral or an ion, based on the balance of protons and electrons.