Lecture on Atomic Number, Mass Number, and Isotopes

Key Concepts

• Atomic Number (Z):

  • Number of protons in the nucleus of an atom.
  • Defines the identity of an element.
  • Found above the elemental symbol on the periodic table.
  • Changing the number of protons changes the element itself.
  • Example: Hydrogen has 1 proton (Atomic Number = 1), Sulfur has 16 protons (Atomic Number = 16).

• Mass Number:

  • Sum of protons and neutrons in the nucleus.
  • Called the mass number because protons and neutrons account for most of an atom's mass.
  • Always a whole number, unlike the atomic mass which is an average value with decimals.

• Atomic Mass:

  • Average mass of an atom expressed in atomic mass units (AMU).
  • Different from mass number.
  • Example: Average atomic mass of hydrogen is 1.01 AMU.

Atomic Notation

• Notation Structure:
  • Atomic Symbol (e.g., C for Carbon)
  • Atomic Number (Z) is often omitted if known from the element (e.g., Carbon always has 6 protons).
  • Mass Number written as a superscript.

Isotopes

• Definition:

  • Atoms with the same number of protons but different numbers of neutrons.
  • Have the same atomic number but different mass numbers.
  • Example: Hydrogen isotopes - Hydrogen 1, Hydrogen 2 (Deuterium), Hydrogen 3 (Tritium).

• Isotope Naming:

  • Element name followed by mass number (e.g., Cobalt-60, Carbon-14).

Examples and Exercises

• Lead-206 Example:

  • Lead (Pb) has atomic number 82 (82 protons).
  • Mass number 206, so it has 124 neutrons (206 - 82).

• Boron Example:

  • Boron has an atomic number of 5 (5 protons).

• Silicon Example:

  • Average atomic mass is 28.09 AMU.

• Neon (Neutral Atom):

  • Has 10 protons and 10 electrons (atomic number 10).

• Electrically Neutral Atom:

  • Example with 27 protons and 27 electrons (Cobalt).

• Sodium with 10 Electrons:

  • Sodium has 11 protons (atomic number 11), so 10 electrons make it positively charged.

• Nitrogen-15 Example:

  • 15 mass number, 7 protons, 8 neutrons.

• Nickel-61 Example:

  • 28 protons, 33 neutrons.

• Naming Isotopes:

  • Uranium-238 is named as such based on its mass number.

Conclusion

• Understanding atomic numbers, mass numbers, and isotopes is crucial for grasping elemental identity and behavior.
• Further learning and practice can be done at the lecturer’s website getchemistryhelp.com.

Note: For nuclear chemistry involving changes in proton numbers, refer to lessons on nuclear chemistry.