Isotopes and Neutrons

Atomic Number

• Every element is defined by the number of protons in its atoms.
• The number of protons is called the atomic number.
• Example: Potassium has 19 protons.
• Example: Cobalt has 27 protons.
• The atomic number defines the chemical properties of an element.
• Elements are arranged in the periodic table based on their atomic number.

Isotopes

• Atoms of the same element can have different numbers of neutrons. These are called isotopes.
• Isotopes of an element have different masses due to the different numbers of neutrons.
• Most of the mass of an atom comes from the protons and neutrons in its nucleus.
• The sum of the number of protons and neutrons in an atom is called its mass number.
• Mass\ Number = Number\ of\ Protons + Number\ of\ Neutrons
• Isotopes of an element have similar chemical properties but different nuclear properties.
• Some isotopes are stable, while others are radioactive.

Isotope Notation

• Isotope notation represents isotopes in writing.
• Notation 1:
• ^{A}_X
  • X = chemical symbol (abbreviation of the element's name)
  • A = mass number (number of protons + neutrons)
  • Z = atomic number (number of protons)
• Example: Carbon (atomic number of 6)
  • Carbon isotope with 6 neutrons:
  • Mass number = 6 protons + 6 neutrons = 12
  • Notation: ^{12}_C
  • Carbon isotope with 7 neutrons:
  • Mass number = 6 protons + 7 neutrons = 13
  • Notation: ^{13}_C
  • The chemical symbol and atomic number are the same for both isotopes because they share the same identity of the element.
  • The mass number is different for each isotope depending on the number of neutrons.
• Notation 2:
  • X-A
  • X = chemical symbol or element's name
  • A = mass number
  • Example: Carbon isotopes
  • Carbon-12 (6 protons, 6 neutrons): C-12 or Carbon-12
  • Carbon-13 (6 protons, 7 neutrons): C-