Chapter 1: Atoms

States and Classification of Matter

Matter is anything that has mass and occupies space. It exists in different states and can be classified based on its composition and properties.

• States of Matter: Matter changes from solid to liquid to gas as temperature increases.

  • Solid: Definite shape and volume; particles are closely packed in an ordered arrangement.

  • Liquid: Definite volume but no definite shape; particles are close together but move freely in a random arrangement.

  • Gas: No definite shape or volume; particles are far apart and move randomly.

• Classification of Matter: Matter can be divided into two principal classes: pure substances and mixtures.

  • Pure Substances: Cannot be separated into simpler matter by physical processes.

    • Element: A pure substance that cannot be separated into simpler substances by chemical processes. Each element is defined by its atomic number.

    • Compound: A pure substance composed of two or more elements chemically combined in fixed proportions. Compounds can be broken down into elements by chemical means.

  • Mixtures: Combinations of two or more substances in which each retains its individual properties.

    • Homogeneous Mixture (Solution): Components are distributed uniformly; composition is the same throughout.

    • Heterogeneous Mixture: Components are not distributed uniformly; composition varies from one part to another.

Example: Air is a homogeneous mixture of gases, while sand and iron filings form a heterogeneous mixture.

Structure of the Atom

Atoms are the smallest particle of an element that retains its chemical properties. They consist of a nucleus (protons and neutrons) and electrons.

• Proton: Positively charged particle in the nucleus; mass ≈ 1 amu.

• Neutron: Neutral particle in the nucleus; mass ≈ 1 amu.

• Electron: Negatively charged particle outside the nucleus; mass ≈ 0.0005 amu.

Example: A carbon atom has 6 protons, 6 neutrons, and 6 electrons.

Atomic Number, Mass Number, and Isotopes

Atoms are identified by their atomic number and mass number. Isotopes are atoms of the same element with different numbers of neutrons.

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

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

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

Example: Carbon-12 and Carbon-14 are isotopes of carbon.

Formula:

• where is mass number, is atomic number, and is number of neutrons.

Atomic Mass and Average Atomic Mass

The atomic mass is the weighted average of the masses of all isotopes of an element, based on their natural abundance.

• Atomic Mass Unit (amu): Standard unit for atomic mass; 1 amu = 1/12 the mass of a carbon-12 atom.

• Average Atomic Mass: Calculated using the masses and abundances of isotopes.

Formula:

Avogadro's Number and the Mole Concept

The mole is a counting unit for atoms, ions, or molecules. Avogadro's number defines the number of particles in one mole.

• Avogadro's Number (): particles per mole.

• Mole: The amount of substance containing as many entities as there are atoms in 12 grams of carbon-12.

Example Calculation:

• atoms = 22.99 g Na

• atoms = atoms He

Conversions Between Mass, Moles, and Number of Particles

It is essential to convert between mass, moles, and number of atoms, ions, or molecules using molar mass and Avogadro's number.

• Mass to Moles:

• Moles to Particles:

• Example: atoms S

Key Terms and Definitions

Students should be able to define and use the following terms:

• Atom

• Molecule

• Element

• Compound

• Atomic number, Z

• Mass number, A

• Atomic mass

• Isotope

• Cation

• Anion

Additional info:

• When comparing two elements, the one with the smaller atomic mass will have a greater number of atoms per gram.

• Be able to convert between the mass of a substance and the amount (in moles) and number of particles.