Grade 9 Chemistry: Isotopes and Ions

Isotopes

• Definition: Atom with the "wrong" number of neutrons (not right or wrong, but for memorization).
• Atomic Mass: Seen as a decimal on the periodic table because it's an average of many isotopes of the element.
• Example: Chlorine
  • Atomic mass: 35.45.
  • Can have 18, 19, or 20 neutrons (atomic weights 35, 36, 37).
  • Mixture of isotopes leads to decimal atomic masses.

Ions

• Definition: Atoms with the "wrong" number of electrons (more or fewer than usual).
• Charge: Due to imbalance between protons and electrons.
  • Removing an electron → positively charged.
  • Adding an electron → negatively charged.
• Examples:
  • Oxygen ion: 2-
  • Lithium ion: 1+
  • Noble Gases: Do not form ions because they have full valence shells.

Formation of Ions

• Reason: Elements form ions to achieve full valence shells like noble gases.
• Electron gain: Leads to negative charge (e.g., chlorine gains one electron, becomes 1-).
• Electron loss: Leads to positive charge (e.g., potassium loses one electron, becomes 1+).
• Proton Change: Impossible without changing the element itself.
• Determining Ion Charge:
  • Lithium: 3 electrons, 1 in valence shell → loses 1 electron, becomes 1+
  • Fluorine: 7 valence electrons → gains 1 electron, becomes 1-
  • Nitrogen: 5 valence electrons → gains 3 electrons, becomes 3-

Ion Charges in Periodic Table

• Groups: Elements in the same group have the same number of valence electrons.
  • Group 1: 1+, Group 2: 2+, Group 3: 3+.
  • Nonmetals: Form negative charges (Group 15: 3-, Group 16: 2-, Group 17: 1-).
  • Noble Gases: Do not form ions.
• Multivalent Elements: Some metals form multiple stable ions.
  • Example: Copper: 1+ or 2+.
  • Example: Iron: 2+ or 3+.
  • Note: First 20 elements have only one stable ion.

Polyatomic Ions

• Definition: Group of atoms that act like a single ion.
• Examples:
  • Carbonate: CO₃²⁻
  • Hydroxide: OH⁻
  • Sulfate: SO₄²⁻
  • Phosphate: PO₄³⁻
• Naming: Often end in “-ate” (carbonate, sulfate, phosphate).