Dalton’s Atomic Theory and Early Atomic Structure Experiments

Dalton's Atomic Theory

John Dalton developed the first modern atomic theory, stating that all matter is made of atoms, atoms of the same element are identical, atoms of different elements are different, atoms combine in simple whole-number ratios to form compounds, and atoms can't be created or destroyed in chemical reactions — only rearranged.

J.J. Thomson - Cathode Ray Tube

Discovered the electron and found that atoms have tiny negatively charged particles, proposing the 'plum pudding model' where electrons are embedded in a positive sphere.

Ernest Rutherford - Gold Foil Experiment

Shot alpha particles at gold foil, finding that most passed through indicating that the atom is mostly empty space, while some bounced back, revealing that atoms have a dense positive nucleus.

Robert Millikan - Oil Drop Experiment

Measured the charge and mass of an electron, determining that the electron charge is -1.60 × 10⁻¹⁹ C.

Nuclear Model of the Atom

Describes an atom as having a small, dense, positive nucleus (composed of protons and neutrons) with electrons moving around it in space.

Proton

Subatomic particle with a symbol of p⁺, a charge of +1, a mass of 1 amu, and located in the nucleus.

Neutron

Subatomic particle with a symbol of n⁰, a charge of 0, a mass of 1 amu, and located in the nucleus.

Electron

Subatomic particle with a symbol of e⁻, a charge of -1, a mass of approximately 0 amu, and located outside the nucleus in the electron cloud.

Atomic Notation

A representation of an element where A is the mass number (protons + neutrons), Z is the atomic number (protons), and X is the element symbol.

Mass Number

The total number of protons and neutrons in an atom, represented by A in atomic notation.

Atomic Number

The number of protons in an atom, represented by Z in atomic notation.

Element Symbol

A one or two-letter abbreviation that represents an element, denoted as X in atomic notation.

Example of Atomic Notation

For sodium, the atomic notation is 11/23 Na, where protons = 11, neutrons = 12, and electrons = 11 if neutral.

Ion

An atom that has lost or gained electrons, resulting in a net charge; for example, Na⁺ has 10 electrons after losing one.

First 30 Elements

The first 30 elements are H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn.

Halogens

The group of elements consisting of F, Cl, Br, and I.

Noble Gases

The group of elements consisting of He, Ne, Ar, Kr, Xe, and Rn.