Our understanding of the atom has changed dramatically over time. Each model built on the last, getting closer to the truth about what matter is really made of.
Dalton's Model (1803) proposed that all matter is made of tiny, indivisible atoms. Different elements have different atoms, and chemical reactions rearrange atoms without creating or destroying them.
Thomson's Plum Pudding Model (1897) came after the discovery of electrons. Thomson imagined electrons embedded in a positive "pudding" -- like raisins in a cake. This was the first model to include subatomic particles.
Rutherford's Nuclear Model (1911) emerged from the famous gold foil experiment. Most alpha particles passed straight through gold foil, but a few bounced back. This proved that atoms have a tiny, dense, positive nucleus with electrons orbiting around it.
Bohr's Model (1913) refined Rutherford's idea by placing electrons in specific energy levels (orbits) around the nucleus. Electrons can jump between levels by absorbing or emitting energy.
The Quantum Mechanical Model (modern) replaces fixed orbits with electron clouds (orbitals) -- probability regions where electrons are most likely found. This is the model used in AP Chemistry.
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have identical chemical properties but different masses.
The mass number (A) equals the total number of protons and neutrons:
where \(Z\) is the atomic number (number of protons) and \(N\) is the number of neutrons.
We write isotopes using notation like \({}^{12}_{\ 6}\text{C}\), which means carbon with mass number 12 and atomic number 6. Since \(N = A - Z\), this atom has \(12 - 6 = 6\) neutrons.
Carbon-14 (\({}^{14}_{\ 6}\text{C}\)) is an isotope of carbon with 8 neutrons instead of 6. It is radioactive and used in carbon dating.