Chemistry: What is an Atom?

Study Notes

An atom consists of a positively charged nucleus surrounded by negatively charged electrons.
The nucleus contains protons and neutrons, accounting for most of the atom's mass.
Protons are positively charged, neutrons have no charge, and electrons are negatively charged.
Protons and neutrons have nearly equal masses, but differ in their electric charges.

Atoms are generally similar in size, measured in Angstroms (Å).
1 Å equals 1 × 10^-10 meters, with typical atomic diameters ranging from 2 to 3 Å.
Atomic radius is typically between 1 and 2.5 Å, while the nucleus radius is about 10^-5 Å.

Masses of atomic particles are incredibly small, often described in relative terms.
Atomic mass unit (amu) standardizes the measurement of particle mass.
A proton has a mass of 1.007 amu, a neutron 1.008 amu, and an electron 5.45 × 10^-4 amu.
Protons have a charge of +1, electrons -1, and neutrons have no charge.

Mass is a fundamental property of matter, which is anything occupying space and having mass.
Matter exists in three states: solid, liquid, or gas.

Atomic number, denoted as Z, identifies an element and equals the number of protons in the nucleus.
Atomic numbers are whole numbers.

Atomic mass refers to the mass of an atom or a molecule.
It helps in calculating average mass of elements and in solving stoichiometry problems.

Democritus: First proposed the concept of the atom, coining the term "Atomos" meaning uncuttable.
J.J. Thomson (1897): Discovered the electron; described negatively charged electrons in energy shells around the nucleus. Electron mass is 9.1 × 10^-28 kg.
Sir Ernest Rutherford (1919): Discovered the proton in the nucleus; the number of protons determines the element type. Proton mass is 1.67 × 10^-24 kg.
Sir James Chadwick (1932): Identified the neutron, which has no charge but similar mass to the proton. Useful for studying atomic structure. Neutron mass equals proton mass at 1.67 × 10^-24 kg.