2.1 Atomic Theory and Subatomic Particles

Vocabulary flashcards covering the historical development of atomic theory, key discovery experiments, and fundamental subatomic particles.

Atom

The smallest particle of an element that retains the characteristics of that element.

Democritus (400 B.C.)

Ancient Greek philosopher who proposed that nature's basic particle is an invisible, indivisible unit called the atom.

Aristotle

Ancient Greek philosopher who contended that matter is continuous and composed of four elements: earth, air, fire, or water.

Element

A pure substance that cannot be broken down into any other simpler substance by chemical means.

Law of Conservation of Matter

A law formulated by Antoine Lavoisier stating that matter is neither created nor destroyed during a chemical reaction. It can only change forms.

Law of Definite Proportions

States that compounds form from elements combining in small, fixed whole-number mass ratios.

Law of Multiple Proportions

States that different compounds can be formed from the same kinds of elements as long as they combine in different whole-number proportions.

By the 1700s we knew:

• Definition of an element

• Law of Conservation of Matter

• Law of Definite Proportions

• Law of Multiple Proportions

• Many Chemical Reactions

John Dalton

Chemist who proposed the first comprehensive atomic theory in $1808$, stating that

• matter consists of invisible, indivisible particles called atoms

• atoms of an element are identical, atoms of different elements are different

• atoms combine in small whole number ratios to form compounds

• in chemical reactions, atoms are joined, separated, and rearranged.

Cathode Ray Tube (CRT)

An evacuated tube that shoots a beam of electrons inside a vacuum toward a screen to create a visible image.

Proved they carried a negative charge: He placed charged metal plates and magnets around a cathode ray tube. The ray bent toward a positive plate and away from a negative plate, proving the beam consisted of negative charges.

J.J. Thomson

Physicist who discovered the electron in $1897$ using cathode ray experiments, demonstrating that rays consist of negatively charged particles with a constant charge-to-mass ratio(both charge and mass are fixed values), earning the Nobel Prize in Physics in $1906$.

Robert Millikan

Physicist who determined through experiments that every electron has the same specific negative charge, winning the Nobel Prize in Physics in $1923$.

Plum-Pudding Model

An atomic model proposed by J.J. Thomson where small negatively charged electrons are distributed equally throughout a sphere of positive charge.

Rutherford's Gold Foil Experiment

A $1910$ experiment led by Ernest Rutherford, Geiger, and Marsden firing alpha particles at thin gold foil, revealing that atoms contain a dense, positively charged nucleus.

• took a piece of gold foil set it up with a radioactive source that produced alpha particles, watched fluorescent screen to see where particles went — some went through some bounced back or deflected.

Alpha Particle

A small, positively charged particle produced by radioactive decay that was bombarded against gold foil during Rutherford's experiments.

Atomic Nucleus

The very small, dense region located in the center of an atom that is positively charged and contains most of the atom's mass.

Conclusions from gold foil experiment

• the center of atom has a nucleus → contains most of atoms mass → is positively charged

• the nucleus is very small compared to the whole atom

• the nucleus is very dense

• the majority of the atom is empty space

Proton

A positively charged subatomic particle located in the nucleus, identified by Rutherford et al. ($1914\text{--}1917$), with a charge of $+1$, symbol $p$ or $p^+$, and mass of 1.008 AMU $1840$ times the mass of an electron).

James Chadwick

Physicist who discovered the neutral neutron in $1932$ (winning the Nobel Prize in Physics in $1935$) nearly $15$ years after the proton, as its lack of charge made it difficult to detect.

Neutron

A neutral subatomic particle located in the nucleus with a charge of $0$, symbol $n$ or $n^0$, and a mass of $1.008$\,amu.

Electron

A negatively charged subatomic particle located outside the nucleus with a charge of $-1$, symbol $e^-$, relative size of $1/1840$, and a mass of $0.00055$\,amu.