Atomic Theory and Matter Classification: Core Chemistry Concepts

Classification of Matter: Pure Substances and Mixtures

Pure Substances

Substances that are formed by the same kind of particles (molecules, atoms, etc.).

• Elements: Formed by the same kind of atom. They cannot be divided by chemical means. Examples: Phosphorus (P), Oxygen (O), Carbon (C).
• Compounds: Formed by the same kind of molecules. They can be divided chemically. Examples: Water (H₂O), Carbon Dioxide (CO₂).

Mixtures

Substances that are formed by different kinds of particles.

• Homogeneous (Solutions): The different components cannot be visually distinguished. Example: Seawater.
• Heterogeneous: The different components can be visually distinguished. Example: Granite.

1. Defining the Atom

The atom is the smallest unit of a chemical element that retains its identity and properties. It cannot be divided by chemical processes.

Subatomic Particle Types

• Proton: Has a positive charge.
• Neutron: Has no charge (neutral).
• Electron: Has a negative charge.

2. Atomic and Mass Number

• Mass Number (A): The sum of the number of protons and the number of neutrons in an atom.
• Atomic Number (Z): The number of protons in an atom. This number determines the identity of the element.

3. Ions and Isotopes

Ions

An atom that has gained or lost one or more electrons, resulting in a net electrical charge.

Types of Ions

• Cation: A positively charged atom that has lost electrons.
• Anion: A negatively charged atom that has gained electrons.

Isotopes

Different forms of the same element, which have the same number of protons (same atomic number) but a different number of neutrons (different mass number).

4. Rutherford's Atomic Model (Gold Foil Experiment)

Rutherford conducted an experiment where he directed alpha particles at a very thin sheet of gold foil.

Observations and Conclusions

• Most alpha particles passed straight through the foil, suggesting that the atom is mostly empty space.
• Several alpha particles were deflected, changing their velocity and direction.
• A very small number of particles bounced back, indicating the presence of a dense, positively charged nucleus that repelled the alpha particles.

5. Bohr's Atomic Model

Bohr's model was developed to explain atomic emission spectra.

Key Postulates

• Electrons orbit the nucleus in fixed, stable orbits (energy levels) without releasing energy.
• If energy is supplied, electrons jump to a higher energy orbit (excited state).
• When electrons fall back to a lower energy orbit, they release the energy difference as light (photons).
• The atom is unstable when electrons occupy higher energy levels.

6. Electron Structure and Quantum Orbitals

Electrons move extremely fast around the nucleus. Quantum chemistry describes their location using probability.

Quantum Orbital Definition

An orbital is a region of space where there is the highest probability of finding an electron.

Types of Orbitals

• s Orbital: Spherical in shape and highly stable.
• p Orbital: Has three subtypes: p_x, p_y, and p_z.
• d Orbital: There are five types of d orbitals.
• f Orbital: There are seven types of f orbitals.

Periodic Properties of Elements

• Ionization Energy: The energy required for an atom to lose an electron and become a cation.
• Electronegativity: The ability of an atom to attract electrons toward itself in a chemical bond.
• Electron Affinity: The energy released when an atom gains an electron and becomes an anion.
• Atomic Volume/Radius: A measure related to the size of the atom.