Atoms: Structure, Properties, and the Foundations of Chemistry

Matter from the Particulate Point of View

Chemistry is the science that seeks to understand matter and its properties. Matter is defined as anything that has mass and occupies space (i.e., has volume). At the most fundamental level, matter is composed of particles—subatomic particles such as neutrons, protons, and electrons make up atoms, and atoms make up molecules. The way these particles combine determines the physical properties of matter.

• Atoms: The basic submicroscopic particles that constitute the fundamental building blocks of ordinary matter.

• Molecules: Particles formed when two or more atoms bond together in specific geometric arrangements.

• Key Point: The arrangement and type of atoms and molecules determine how matter behaves.

The Classification of Matter

Matter can be classified by its physical state (solid, liquid, gas) and by its composition (elements, compounds, mixtures).

• States of Matter:

  • Solid: Atoms or molecules are closely packed in fixed locations; solids have a fixed volume and rigid shape (e.g., ice, aluminum, diamond).

  • Liquid: Atoms or molecules are close but can move relative to each other; liquids have a fixed volume but not a fixed shape (e.g., water, alcohol).

  • Gas: Atoms or molecules have significant space between them and are free to move; gases are compressible and take the shape and volume of their container.

• Classification by Composition:

  • Pure Substance: Made up of only one component with invariant composition (e.g., elements, compounds).

  • Mixture: Composed of two or more components in variable proportions.

Classification of Pure Substances

• Element: A substance that cannot be chemically broken down into simpler substances; composed of a single type of atom (e.g., helium).

• Compound: A substance composed of two or more elements in fixed, definite proportions (e.g., water, sugar).

Classification of Mixtures

• Heterogeneous Mixture: Composition varies from one region to another; different components are visible (e.g., salt and sand mixture).

• Homogeneous Mixture: Uniform composition throughout; appears as a single substance (e.g., sweetened tea).

The Scientific Approach to Knowledge

The scientific method is a systematic approach to understanding nature through observation and experimentation.

• Observations: Data collected about the characteristics or behavior of nature.

• Hypothesis: A tentative explanation of observations; must be falsifiable.

• Scientific Law: A brief statement summarizing past observations and predicting future ones (e.g., law of conservation of mass).

• Theory: A well-established explanation for observations, validated by experiments (e.g., Dalton’s atomic theory).

Early Ideas About the Building Blocks of Matter

Ancient philosophers like Leucippus and Democritus proposed that matter is composed of small, indestructible particles called atoms. Later, John Dalton provided experimental evidence supporting the atomic theory, leading to the formulation of several fundamental laws:

• Law of Conservation of Mass: In a chemical reaction, matter is neither created nor destroyed.

• Law of Definite Proportions: All samples of a given compound have the same proportions of their constituent elements.

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

Atomic Theory and Subatomic Particles

Dalton’s atomic theory explained the laws of chemical combination:

• Each element is composed of tiny, indestructible particles called atoms.

• All atoms of a given element have the same mass and properties.

• Atoms combine in simple, whole-number ratios to form compounds.

• Atoms of one element cannot change into atoms of another element in chemical reactions.

The Discovery of the Electron

• J.J. Thomson’s Cathode Ray Experiment: Discovered the electron, a negatively charged, low-mass particle present in all atoms. Measured the charge-to-mass ratio of the electron.

• Millikan’s Oil Drop Experiment: Determined the charge of a single electron and, using Thomson’s data, calculated the electron’s mass.

The Structure of the Atom

• Plum-Pudding Model: Proposed by Thomson; electrons embedded in a positively charged sphere.

• Rutherford’s Gold Foil Experiment: Showed that atoms have a small, dense, positively charged nucleus surrounded by electrons; most of the atom is empty space.

• Neutrons: Discovered by Chadwick; neutral particles in the nucleus with mass similar to protons.

Subatomic Particles

Elements, Isotopes, and Ions

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

• Isotopes: Atoms of the same element with different numbers of neutrons; have different mass numbers (A = protons + neutrons).

• Ions: Atoms that have gained or lost electrons, resulting in a net charge. Cations are positively charged (loss of electrons), anions are negatively charged (gain of electrons).

Isotope Notation

• Symbol: AZX, where X is the element symbol, A is the mass number, and Z is the atomic number.

• Alternative: Element name or symbol followed by a dash and the mass number (e.g., C-13).

Atomic Mass and the Mole Concept

• Atomic Mass: The weighted average mass of an element’s isotopes, based on natural abundance.

• Calculation:

• Mole (mol): The amount of substance containing Avogadro’s number () of particles (atoms, molecules, etc.).

• Molar Mass: The mass of one mole of atoms of an element (g/mol), numerically equal to the atomic mass in amu.

Conversions Involving Moles

• Number of atoms to moles: Divide by Avogadro’s number.

• Moles to number of atoms: Multiply by Avogadro’s number.

• Mass to moles: Divide by molar mass.

• Moles to mass: Multiply by molar mass.

Example Calculation

• To find the number of atoms in 2.45 moles of copper:

Mass Spectrometry

Mass spectrometry is a technique used to measure the masses of atoms and the percent abundances of isotopes by separating particles according to their mass.

Summary Table: Classification of Matter

Key Equations

• Atomic Mass:

• Mole Conversions:

  • Number of particles

  • Mass

  • Moles