Atomic number,mass, and isotopes

Introduction to Atomic Structure

• Overview of the nucleus and electron cloud in an atom.

Atomic Particles

• Proton:

  • Mass: 1.0073 u

  • Charge: +1

• Neutron:

  • Mass: 1.0087 u

  • Charge: 0 (neutral)

• Electron:

  • Mass: 0.0005 u

  • Charge: -1

Atomic Identity

• The identity of an atom is determined by the number of protons it contains.

• Examples of elements based on the number of protons:

  • 1 proton - Hydrogen (H)

  • 2 protons - Helium (He)

  • 3 protons - Lithium (Li)

  • 4 protons - Beryllium (Be)

Atomic Numbers and Mass Numbers

• Atomic Number (Z):

  • The number of protons in an atom.

  • Equal to the number of electrons in a neutral atom.

• Mass Number (A):

  • Total number of protons and neutrons in an atom (A = Z + N).

Isotopes

• Isotopes have the same atomic number but different mass numbers due to varying numbers of neutrons.

• Example: Three isotopes of hydrogen identified by their mass numbers:

  • Protium (1H), Deuterium (2H), and Tritium (3H).

Writing Atomic Symbols

• Format: Element Symbol with mass number and atomic number.

• Example for Helium:

  • Helium: 2 protons, 2 neutrons

  • Symbol:

    • [ _{2}^{4}He ]

Uranium Isotope Example

• Uranium isotope:

  • Symbol: [ _{92}^{235}U ] (92 protons and 143 neutrons)

  • Calculation of mass number: 92 protons + 143 neutrons = 235

Average Atomic Mass

• Definition: A weighted average of the different isotopes of an element, reflecting their relative abundances.

Summary of Key Concepts

• Key points to remember:

  • Protons define the identity of atoms.

  • Atomic number is the number of protons.

  • Mass number is the total of protons and neutrons.

  • Isotopes: Same number of protons but different neutrons.

  • The periodic table arranges elements by atomic number and average atomic mass.