Atomic Theory


Atomic theory states that matter is composed of discrete units called atoms, as opposed to the obsolete notion that matter could be divided into any arbitrarily small quantity. It began as a philosophical concept in ancient Greece (Democritus) and India and entered the scientific mainstream in the early 19th century. It was during this time when discoveries in the field of chemistry showed that matter did indeed behave as if it were made up of particles.

• Atomic Mass
Mass is a basic physical property of matter. The mass of an atom or a molecule is referred to as the atomic mass. The atomic mass is used to find the average mass of elements and molecules and to solve stoichiometry problems.
• Atomic Structure
An atom consists of a positively charged nucleus, surrounded by one or more negatively charged particles called electrons. The positive charges equal the negative charges, so the atom has no overall charge; it is electrically neutral. Most of an atom’s mass is in its nucleus; the mass of an electron is only 1/1836 the mass of the lightest nucleus, that of hydrogen. Although the nucleus is heavy, it is quite small compared with the overall size of an atom.
• Atomic Theory
• Dalton's Atomic Theory
John Dalton, a British school teacher, published his theory about atoms in 1808. His findings were based on experiments and the laws of chemical combination.
• Ionization Energies of Diatomic Molecule
The energies of electrons in molecular orbitals can be observed directly by measuring the ionization energy. This is the energy required to remove an electron, in this case, from a molecule.
• Isotopes
Atoms that have the same atomic number (number of protons), but different mass numbers (number of protons and neutrons) are called isotopes. There are naturally occurring isotopes and isotopes that are artificially produced. Isotopes are separated through mass spectrometry; MS traces show the relative abundance of isotopes vs. mass number (mass : charge ratio).
• Nuclide, Atomic Number, mass number
An atom is the smallest unit of an element that can exist. Every atom is made up of protons, neutrons, and electrons. These particles define a nuclide and its chemical properties and were discovered in the early 20th century and are described by modern atomic theory.
• Simple View of Atomic Structure
• Sizes of Ions
• The Atom
The atom is the smallest unit of matter that is composed of three sub-atomic particles: the proton, the neutron, and the electron. Protons and neutrons make up the nucleus of the atom, a dense and positively charged core, whereas the negatively charged electrons can be found around the nucleus in an electron cloud.
• The Mole and Avogadro's Constant
The mole, abbreviated mol, is an SI unit which measures the number of particles in a specific substance. One mole is equal to $$6.02214179 \times 10^{23}$$ atoms, or other elementary units such as molecules.

Atomic Theory is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.