CHEM Atomic Structure

Atomic Structure

What is an Atom Made Of?

  • Matter is made of atoms, the smallest particle retaining an element's chemical characteristics.
  • Atoms of different elements contain different numbers of sub-atomic particles.
  • The three main sub-atomic particles:
    • Protons: positive charge (+1), located in the nucleus.
    • Neutrons: neutral charge (0), located in the nucleus.
    • Electrons: negative charge (-1), located in electron shells around the nucleus.
  • Protons and neutrons are also called nucleons.
  • Atoms are electrically neutral; the number of protons equals the number of electrons.
  • Relative Mass of Sub-atomic Particles:
    • Protons: 1 unit
    • Neutrons: 1 unit
    • Electrons: 1/1840 of a proton or neutron

How Many Sub-atomic Particles Does an Atom Have?

  • Proton (Atomic) Number (Z): the number of protons in the nucleus, determining the element type.
  • Nucleon (Mass) Number (A): the total number of protons and neutrons in the nucleus.
    • A=Z+number of neutronsA = Z + \text{number of neutrons}
  • Number of neutrons = Nucleon number (A) - Proton number (Z)
Nuclide Notation
  • Represents proton and nucleon numbers of an atom.
  • ZAX^{A}_{Z}X, where X is the atomic symbol, A is the nucleon number, and Z is the proton number.
Sub-atomic Particles in Ions
  • Ions are formed when atoms gain or lose electrons; the number of protons and neutrons remains the same.
  • Charge numbers indicate the number of electrons gained or lost.
    • e.g., Ca2+\text{Ca}^{2+} has 20 protons, 20 neutrons, and 18 electrons.
    • O2−\text{O}^{2-} has 8 protons, 8 neutrons, and 10 electrons.
Isotopes
  • Isotopes are atoms of the same element with the same proton number but different nucleon numbers (different numbers of neutrons).
  • Isotopes have the same chemical properties but may have different physical properties.

How Are Sub-atomic Particles Distributed in an Atom?

  • Electrons are found in electron shells around the nucleus.
  • Electrons in the innermost shell have the lowest energy; those in the outermost shell have the highest energy.
  • Each electron shell can hold a certain number of electrons:
    • First shell: max 2 electrons
    • Second and third shells: max 8 electrons each
  • The outermost shell is called the valence shell; electrons in that shell are called valence electrons.
  • Atoms with full valence shells are stable and unreactive.
Electronic Configuration
  • Describes the arrangement of electrons in electron shells (e.g., Lithium: 2, 1).
Rules for Writing Electronic Configuration
  1. Locate the element in the periodic table to determine the number of electron shells.
  2. Distribute electrons based on the 2, 8, 8 rule, starting from the innermost shell.
  3. Elements in the same group have the same number of valence electrons and similar chemical properties.