Atoms and Elements Notes (CHAPTER 4)

Dalton’s Atomic Theory

  • Proposed in the early 1800s.
  • Elements are composed of small, indivisible particles called atoms.
  • Atoms of the same element are identical in mass and size.
  • Compounds are formed by combining two or more atoms of different elements.
  • Atoms combine to form compounds in simple whole number ratios.
  • Atoms of two elements may combine in different ratios, leading to the formation of different compounds.
  • Law of Constant Composition: all samples of a compound contain the same proportions (by mass) of the elements.

Atom Structure

  • An atom contains a tiny dense center called the nucleus.
  • The nucleus is essentially the entire mass of the atom.
  • The nucleus is positively charged; the amount of positive charge of the nucleus balances the negative charge of the electrons.
  • The electrons move around in the empty space of the atom surrounding the nucleus.
  • The radius of the atom is about 10510^5 times larger than the radius of the nucleus.

Nucleus Composition

  • The nucleus is composed of two kinds of particles: protons and neutrons.
  • Protons:
    • Charge = +1
    • Mass is about the same as a hydrogen atom (1 amu).
  • For an atom to be neutral, there must be equal numbers of protons and electrons.
  • Neutrons:
    • Has no charge.
    • Has a mass slightly more than a proton (1 amu or u).

Electron

  • Tiny, negatively charged particle.
  • Very light compared to mass of atom (11836)(\frac{1}{1836})th the mass of a H atom (amu) or 0.00055 amu.
  • Move very rapidly within the atom.

Atomic Number

  • The atomic number of an element is equal to the number of protons in the nucleus of that element.
  • The atomic number of an atom determines which element the atom is.
  • The number of electrons is equal to the number of protons in an atom.

Periodic Table

  • Includes metals, nonmetals, and semimetals (metalloids).
  • Representative (Main-group elements) and transition elements (metals).
  • Family (Group): Noble gases (inert gases), alkali metals, alkaline earth metals, halogens.
  • Ionic Compounds: compounds that contain ions (cations, anions).
  • Diatomic molecules.

Isotopes

  • Atoms of an element with different numbers of neutrons are called isotopes.
  • All isotopes of an element are chemically identical and undergo the exact same chemical reactions.
  • Isotopes of an element have different masses.
  • Isotopes are identified by their mass numbers: mass number = protons + neutrons.

Mass Number

  • Mass number=protons+neutronsMass \space number = protons + neutrons

Atomic Mass Calculation

  • To calculate the atomic mass, multiply the atomic mass of each isotope by its percent abundance and add the results.

Laws of Chemical Combination

  • Law of Definite Composition: A compound always contains two or more elements chemically combined in a definite proportion by mass.
  • Law of Multiple Proportions: Atoms of two or more elements may combine in different ratios to produce more than one compound.

Ions

  • Cations: Positive ions.
  • Anions: Negative ions.

Rutherford Experiment

  • Most of the alpha particles passed through the gold foil with little or no deflection.
  • A few were deflected at large angles, and some even bounced back.