Atomic Theory

Introduction to Chemistry and Matter

Chemistry is the scientific study of matter, which is composed of atoms. Matter can be classified into substances and mixtures.

• Substances are either elements (e.g., carbon, oxygen) or compounds (e.g., carbon dioxide).

• Mixtures consist of two or more substances physically combined (e.g., air is a mixture of oxygen and nitrogen).

Dalton’s Atomic Model and Mass Laws

Dalton’s atomic theory explained several important mass laws:

• Law of Conservation of Mass: Matter is neither created nor destroyed in chemical reactions.

• Law of Constant Composition: A given compound always contains the same proportion of elements by mass.

• Law of Multiple Proportions: When two elements form more than one compound, the ratios of the masses of the second element that combine with a fixed mass of the first element are simple whole numbers.

Divisibility of the Atom: Early Experiments

Early atomic models considered the atom indivisible, but later experiments showed otherwise.

• J.J. Thomson’s cathode ray experiments (1897) demonstrated the existence of electrons, indicating atoms are divisible.

• Millikan’s oil drop experiment (1909) measured the charge of the electron.

• Thomson’s "plum pudding model" (1904) proposed that electrons were embedded in a positively charged sphere.

The Nuclear Atom: Rutherford’s Gold Foil Experiment

The discovery of the nucleus revolutionized atomic theory.

• Alpha (α) particles are small, positively charged particles emitted by radioactive materials.

• Rutherford’s gold foil experiment (1909) showed that most α particles passed through gold foil, but some were deflected at large angles, suggesting a dense, positively charged nucleus at the center of the atom.

• This led to the rejection of the plum pudding model and the acceptance of the nuclear model of the atom.

Subatomic Particles

Atoms are composed of three main subatomic particles:

• Protons (p+): Positively charged, mass ≈ 1 amu

• Neutrons (n0): Neutral, mass ≈ 1 amu

• Electrons (e–): Negatively charged, mass ≈ 1/1836 amu

Key constants:

• e = C

• 1 amu = g

Isotopes

• Isotopes are atoms of the same element with different numbers of neutrons and thus different masses.

• Example:

  • 12C: mass = 12 amu, abundance = 98.892%

  • 13C: mass = 13.00335 amu, abundance = 1.108%

• The average atomic mass is calculated using the weighted average of all isotopes.

Isotope notation: where A = mass number, Z = atomic number, X = element symbol.

Ions

• Atoms can gain or lose electrons to form ions (charged species).

• Cations are positively charged (loss of electrons), anions are negatively charged (gain of electrons).

• Example:

  • 11 p+, 12 n0, 10 e–: Na+ ion

  • 17 p+, 18 n0, 18 e–: Cl– ion

Elements and the Periodic Table

Classification of Elements

The periodic table organizes elements by increasing atomic number and recurring chemical properties.

• Groups (columns): Elements with similar chemical properties.

• Periods (rows): Horizontal rows of elements.

• Main-group elements: Groups 1, 2, and 13–18.

• Transition elements: Groups 3–12.

• Metals, nonmetals, metalloids: Classified by physical and chemical properties.

• Alkali metals: Group 1

• Alkaline-earth metals: Group 2

• Halogens: Group 17

• Noble gases: Group 18

Naming Compounds and Writing Formulas

Ionic Compounds: Monatomic Ions

• Ionic compounds are formed from the combination of metal cations and nonmetal anions.

• Name the cation first, then the anion. The anion name ends with “-ide.”

• Examples:

  • CaBr2: Calcium bromide

  • Magnesium nitride: Mg3N2

Common Monatomic Ions:

Metals with Multiple Ions

Some metals can form more than one type of ion.

• Specify the charge using Roman numerals in parentheses.

• Examples:

  • FeCl3: Iron(III) chloride

  • Copper(II) selenide: CuSe

Some Metals That Form More Than One Monatomic Ion:

Ionic Compounds: Polyatomic Ions

• Polyatomic ions are ions composed of two or more atoms covalently bonded, acting as a single charged unit.

• Examples:

  • NaClO: Sodium hypochlorite

  • LiClO2: Lithium chlorite

  • CsClO3: Cesium chlorate

  • KClO4: Potassium perchlorate

  • NH4H2PO4: Ammonium dihydrogen phosphate

  • Strontium iodate: Sr(IO3)2

  • Lithium peroxide: Li2O2

  • Mercury(I) acetate: Hg2(CH3COO)2

Common Polyatomic Ions:

Millikan’s Oil Drop Experiment

• Millikan’s experiment determined the charge of the electron by observing the behavior of oil droplets in an electric field.

• Measured charges were always multiples of a fundamental value, confirming the quantization of electric charge.

Summary Table: Key Experiments in Atomic Theory

Additional info:

• For a deeper understanding, students should practice naming and writing formulas for both monatomic and polyatomic ionic compounds, and be familiar with the periodic table layout and group properties.