Chapter2

Chapter Two: Atoms

Classes of Matter

  • Elements: Pure substances that cannot be broken down into simpler substances by a chemical reaction.

  • Compounds: Pure substances made up of two or more elements in a fixed proportion by weight.

  • Mixture: Two or more substances that are not chemically combined.

Dalton’s Atomic Theory

  • All matter is made up of atoms.

  • All atoms of an element have similar properties.

  • Atoms of different elements have different properties.

  • Atoms combine to form molecules.

  • Atoms cannot be split/divided (this aspect is incorrect).

  • Law of Conservation of Mass: Matter cannot be created or destroyed.

  • Law of Constant Composition: Any compound is always made up of elements in the same proportion by mass.

Subatomic Particles

  • Protons: Positive charge (+1), mass of one atomic mass unit (1 amu), located in the nucleus.

  • Neutrons: No charge, mass of 1 amu, located in the nucleus.

  • Electrons: Negative charge (-1), negligible mass (approximately 0), orbit the nucleus at fixed distances known as orbitals.

  • Nucleons: Protons and neutrons collectively are referred to as nucleons.

Properties and Location within Atoms (See Table 2.1)

  • Proton: Charge +1, Mass = 1 amu, Location = nucleus.

  • Electron: Charge -1, Mass = 0.0005 amu, Location = outside nucleus.

  • Neutron: Charge 0, Mass = 1 amu, Location = nucleus.

Page 2: Atomic Size and Isotopes

Isotopes

  • Atoms with the same number of protons but different numbers of neutrons.

  • Examples:

    • 12C (normal carbon, 6 neutrons)

    • 13C (6 protons, 7 neutrons; about 1% in nature, useful in chemistry)

    • 14C (6 protons, 8 neutrons; used in carbon dating)

Important Isotopes

  • Hydrogen Isotopes:

    • 1H (normal hydrogen or protium)

    • 2H (deuterium, D; heavy water D2O used in medical studies)

    • 3H (tritium, T; used in atomic bombs)

Ions

  • Charged atoms formed when electrons are added or removed:

    • Negative Ion: If an ion has a -1 charge, add one to the number of protons to determine the number of electrons. For -2, add 2, etc.

    • Positive Ion: If an ion has a +1 charge, subtract one from the number of protons to determine the number of electrons. For +2, subtract 2, etc.

Atomic Number and Mass Number

  • Atomic Number: Number of protons in an atom.

  • Mass Number: Total number of protons and neutrons in an atom. Calculation: # of neutrons = Mass Number - Atomic Number.

The Periodic Table Overview

  • Classification of elements, including categories such as Noble Gases and Halogens.