Atoms, Molecules and Ions

Introduction

This chapter introduces the basic structure of atoms and discusses the formation of molecules and ions. Understanding these concepts provides the foundation for all further study in chemistry.

Theory of Matter

Historical Development

• Democritus (Ancient Greece): Proposed that matter is made up of smallest indivisible particles called atomos.

• Advancements in the 18th and 19th centuries led to the development of atomic theory by John Dalton in the early 1800s.

Dalton's Atomic Theory

1. Elements are composed of extremely small particles called atoms.

2. All atoms of a given element are identical, but atoms of different elements are different.

3. Atoms cannot be changed into atoms of another element by chemical reactions; atoms are neither created nor destroyed in chemical reactions.

4. Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms.

Key Laws:

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

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

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

Discovery of Subatomic Particles

Cathode Rays and the Electron

• J. J. Thomson (1897): Discovered the electron using cathode-ray tubes.

• Negatively charged particles (electrons) were found to emanate from cathode tubes.

• Thomson measured the charge-to-mass ratio of the electron: (C/g).

Plum Pudding Model

• Thomson proposed that atoms are spheres of positive charge with electrons embedded within.

Radioactivity and the Nucleus

• Henri Becquerel: Discovered radioactivity.

• Marie and Pierre Curie: Further studied radioactive elements.

• Ernest Rutherford: Discovered the nucleus through the gold foil experiment.

Rutherford's Gold Foil Experiment

• Alpha particles were directed at a thin sheet of gold foil.

• Most particles passed through, but some were deflected at large angles, indicating a dense, positively charged nucleus.

Subatomic Particles

• Proton: Positively charged particle in the nucleus.

• Neutron: Neutral particle in the nucleus, discovered by James Chadwick.

• Electron: Negatively charged particle orbiting the nucleus.

Structure of the Atom

Atomic Number and Mass Number

• Atomic Number (Z): Number of protons in the nucleus; determines the element.

• Mass Number (A): Total number of protons and neutrons in the nucleus.

• Isotopes: Atoms of the same element with different numbers of neutrons.

Isotope Notation

• Written as , where X is the element symbol.

• Example: for carbon-12.

Atomic Mass Unit (amu) and Average Atomic Mass

Definition and Calculation

• Atomic Mass Unit (amu): Defined as one-twelfth the mass of a carbon-12 atom.

• 1 amu = g

• Average Atomic Mass: Weighted average of all naturally occurring isotopes of an element.

Calculation Formula

Periodic Table and Classification of Elements

Structure and Organization

• Elements are arranged in order of increasing atomic number.

• Periods: Horizontal rows.

• Groups: Vertical columns; elements in the same group have similar chemical properties.

Major Categories

• Metals: Good conductors, malleable, ductile.

• Nonmetals: Poor conductors, brittle.

• Metalloids: Properties intermediate between metals and nonmetals.

Special Groups

Molecular and Ionic Compounds

Molecular Compounds

• Composed of molecules formed by nonmetals sharing electrons (covalent bonds).

• Represented by molecular formulas showing the exact number of atoms of each element.

Ionic Compounds

• Formed by transfer of electrons from metals to nonmetals, resulting in cations and anions.

• Held together by electrostatic attraction (ionic bonds).

• Represented by empirical formulas showing the lowest whole-number ratio of ions.

Formation of Ions

• Cations: Formed by loss of electrons (usually metals).

• Anions: Formed by gain of electrons (usually nonmetals).

Nomenclature of Inorganic Compounds

Rules for Naming Ionic Compounds

• Name the cation first, then the anion.

• For metals with variable charge, indicate the charge with Roman numerals (e.g., iron(III) chloride).

• For monatomic anions, change the ending to -ide (e.g., chloride, oxide).

• For polyatomic ions, use the name of the ion (e.g., sulfate, nitrate).

Rules for Naming Molecular (Covalent) Compounds

• Name the element farther left in the periodic table first.

• Use prefixes to indicate the number of atoms (mono-, di-, tri-, tetra-, etc.).

• Change the ending of the second element to -ide.

Common Prefixes

Rules for Naming Acids

• If the anion ends in -ide, the acid name begins with 'hydro-' and ends with '-ic acid' (e.g., hydrochloric acid).

• If the anion ends in -ate, change the ending to '-ic acid' (e.g., sulfuric acid).

• If the anion ends in -ite, change the ending to '-ous acid' (e.g., nitrous acid).

Summary Table: Types of Compounds and Naming Rules

Example Applications

• NaCl: Sodium chloride (ionic compound)

• CO2: Carbon dioxide (molecular compound)

• H2SO4: Sulfuric acid (acid)

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