Atoms: The Building Blocks of Matter

Matter from the Particulate Point of View

Matter is composed of particles, including subatomic particles such as neutrons, protons, and electrons, which make up atoms. Atoms, in turn, combine to form molecules. The arrangement and interaction of these particles determine the physical properties of matter. Chemistry is the science that seeks to understand matter and its properties.

• Matter: Anything that has mass and occupies space (has volume).

• 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.

The Classification of Matter

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

• Solid: Atoms or molecules are closely packed in fixed locations; solids have fixed volume and rigid shape.

• Liquid: Atoms or molecules are close but can move relative to each other; liquids have fixed volume but not fixed shape.

• Gas: Atoms or molecules have much 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. Can be an element or a compound.

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

Types of Pure Substances

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

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

Types of Mixtures

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

• Homogeneous Mixture: Uniform composition throughout (e.g., sweetened tea).

The Scientific Approach to Knowledge

The scientific method is an empirical approach based on observation and experimentation. It involves:

• Observations: Descriptions about the characteristics or behavior of nature (data).

• 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

• Leucippus and Democritus proposed that matter is composed of small, indestructible particles called atoms.

• Plato and Aristotle believed matter had no smallest parts and was composed of fire, air, earth, and water.

• John Dalton provided evidence for the atomic theory and formulated several laws:

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

  • Law of Definite Proportions: All samples of a 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.

Dalton’s Atomic Theory

• 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 experiments showed that cathode rays are composed of negatively charged particles (electrons).

• Thomson measured the charge-to-mass ratio of the electron.

• Millikan’s oil drop experiment determined the charge of a single electron.

• The mass of the electron was calculated using these experiments.

Models of the Atom

• Plum-Pudding Model: Electrons are embedded in a positively charged sphere (Thomson).

• Rutherford’s Gold Foil Experiment: Showed that atoms have a small, dense, positively charged nucleus surrounded by electrons.

• Nuclear Model: Most of the atom’s mass and all positive charge are in the nucleus; electrons occupy most of the volume; atoms are electrically neutral.

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

Subatomic Particles

• Protons: Positively charged, mass ≈ 1 amu.

• Neutrons: No charge, mass ≈ 1 amu.

• Electrons: Negatively charged, mass ≈ 1/1836 of a proton.

Elements and the Periodic Table

• The number of protons (atomic number, Z) defines the element.

• Elements are arranged in the periodic table by atomic number.

• Elements in the same column have similar properties.

Isotopes

• Atoms of the same element with different numbers of neutrons are called isotopes.

• Mass Number (A): Sum of protons and neutrons.

• Isotopes are represented as: or X-A (where X is the element symbol).

• Natural Abundance: The relative amount of each isotope in a naturally occurring sample.

Ions: Charged Atoms

• Atoms can lose or gain electrons to form ions.

• Cations: Positively charged ions (loss of electrons, typically metals).

• Anions: Negatively charged ions (gain of electrons, typically nonmetals).

Atomic Mass

• The atomic mass (atomic weight) is the average mass of an element’s isotopes, weighted by natural abundance.

• Calculated as:

Mass Spectrometry

• Technique used to measure the masses of atoms and the percent abundances of isotopes.

• Separates particles according to their mass.

The Mole and Avogadro’s Number

• A mole (mol) is a counting unit: objects (Avogadro’s number).

• The mole is defined as the number of atoms in exactly 12 grams of pure C-12.

Conversions Involving Moles

• To convert between moles and number of atoms:

• To convert between mass and moles:

• Molar mass (g/mol) is numerically equal to atomic mass (amu).

Example: Converting Between Mass and Number of Atoms

• Given mass of an element, convert to moles using molar mass, then to number of atoms using Avogadro’s number.

• Given number of atoms, convert to moles using Avogadro’s number, then to mass using molar mass.

Summary Table: Classification of Matter

Summary Table: Subatomic Particles

Key Equations

• Atomic Mass Calculation:

• Mole-Atom Conversion:

• Mass-Mole Conversion: