Calculating Protons, Neutrons, and Electrons in Atoms and Ions

Key Concepts

• Atomic Number (Z): Represents the number of protons in an atom. Identifies the element.
• Mass Number (A): Sum of protons and neutrons in the nucleus. Specific to isotopes.
• Average Atomic Mass: Weighted average of all isotopes of an element found on Earth.

Sodium Example

• Atomic Number: 11
• Common Isotopes: Sodium-23 (most abundant), Sodium-24, Sodium-22
• Mass Number for Sodium-23: 23
• Protons: 11
• Neutrons: A - Z = 23 - 11 = 12
• Electrons in a Neutral Atom: Equals number of protons (11)

Formulas

• Protons: Equal to atomic number (Z)
• Neutrons: Mass number (A) - atomic number (Z)
• Electrons for Atoms: Equal to atomic number (Z) (neutral charge)
• Electrons for Ions: Z - charge

Calculation Examples

Neutral Atoms

• Example: Sodium
  • Protons = 11
  • Neutrons = 12
  • Electrons = 11

Cations

• Example: Aluminum 27 with a 3+ charge
  • Atomic Number = 13
  • Mass Number = 27
  • Protons = 13
  • Neutrons = 27 - 13 = 14
  • Electrons = 13 - 3 = 10
  • More protons than electrons due to positive charge

Anions

• Example: Sulfide ion (S²⁻)
  • Atomic Number = 16
  • Mass Number = 33
  • Protons = 16
  • Neutrons = 33 - 16 = 17
  • Electrons = 16 - (-2) = 18
  • More electrons than protons due to negative charge

Identifying Elements

• Example: Silicon 29
  • Mass Number = 29
  • Atomic Number (from periodic table) = 14
  • Protons = 14
  • Neutrons = 29 - 14 = 15
  • Electrons = 14 (assuming atom)

Challenge Problem

• Given: Cation with 4+ charge, 26 neutrons, 18 electrons
• Solution: Identify the Element
  • Electrons = Z - charge
  • 18 = Z - 4 → Z = 22
  • Protons = 22
  • Element 22 in periodic table is Titanium
  • Mass number (optional): 26 (neutrons) + 22 (protons) = 48

Summary

• Atoms: Protons = Electrons (neutral)
• Ions: Electrons differ by the charge value
• Identifying Elements: Use atomic number to determine element from periodic table.