Atomic Models: Thomson vs. Rutherford

Atomic Structure

• Understanding atomic structure helps us comprehend how atoms combine to form compounds, collide, and exhibit various properties.

Atomic Models

• Democritus' atomic model (460 BC): • Atoms are uniform, solid, hard, incompressible, and indestructible. • Atoms move in infinite numbers through empty space until stopped. • Differences in atomic shape and size determine the various properties of matter.

• John Dalton's atomic model (1804): • All matter is made of atoms, which are indivisible. • All atoms of a given element are identical in mass and properties. • Compounds are combinations of two or more different types of atoms.

Subatomic Particles

• J.J. Thomson's discovery (1856): • Cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles called electrons. • The plum pudding model of the atom, with negatively-charged electrons embedded in a positively-charged "soup."

• Ernest Rutherford's discovery (1871): • The atom consists of a tiny, dense, positively charged core called a nucleus, with nearly all the mass concentrated. • The nucleus is surrounded by light, negative constituents called electrons at some distance. • The atom is mostly empty space.

The Atom

• Electrons: • The smallest of the three particles that make up atoms. • Found in shells or orbitals that surround the nucleus. • Negative charges (-).

• Protons: • Found in the nucleus. • Positive charges (+).

• Neutrons: • Found in the nucleus. • No charge (neutral).

The Periodic Table of Elements

• The organized array of all chemical elements in order of increasing atomic number.
• The periodic law: elements in the same column (group) have similar properties.

Atomic Number and Mass

• Atomic Number: • The number of a chemical element in the periodic system. • The number of protons in the nucleus determines the identity of the element.

• Atomic Mass (Mass Number): • A weighted average of all the isotopes of an element. • The mass of each isotope is multiplied by its abundance.

Isotopes

• Atoms with the same number of protons but different numbers of neutrons.
• Share almost the same chemical properties but differ in mass and physical properties.
• Stable isotopes do not emit radiation, while unstable isotopes do emit radiation.

Nuclear Symbol Notation

• A shorthand notation that identifies the element (by symbol or atomic number) and the mass number of the element.
• The mass number is the sum of the number of nucleons (protons and neutrons) in the atomic nucleus.