Atoms, Molecules, and Ions

Atoms, Molecules, and Ions

• An atom is the fundamental unit of a chemical element and the building block of matter.
• An atom is composed of subatomic particles: protons, neutrons, and electrons.
• An element is a substance containing only one type of atom.
• Hydrogen consists of one proton and one electron.
• A molecule is formed when two or more atoms combine.
• Single atoms are not molecules, "O" isn't a molecule.
• Oxygen can bond, forming O2 or O3, or with other elements such as CO2 or Na2O.
• Molecules can be simple, like water (H2O), or complex, like glucose (C6H12O6).
• Common molecules are nitrogen (N2), ozone (O3), and ammonia (NH3).
• A compound contains at least two different elements.
• Water (H2O), carbon dioxide (CO2) and sulphuric acid (H2SO4) are examples of compounds.
• All compounds are molecules, but not all molecules are compounds.
• Oxygen (O2), hydrogen (H2), and chlorine (Cl2) are molecules but not compounds.
• Compounds include carbon monoxide, ethane, and ammonia.
• An ion is a substance with a positive or negative charge and does not have the same number of electrons and protons.
• A neutral particle shares the same number of protons and electrons.
• A helium atom (He+) missing an electron is an ion with a positive charge.
• Carbonate (CO32-) with two extra electrons forms an ion with a negative charge.

Early Atomic Theories

• Democritus, a Greek philosopher (460-370 B.C.), posited that matter consisted of basic elements.
• Democritus suggested that matter, when continuously divided, would eventually reach an "uncuttable" particle, which he named "atomos" (meaning indivisible).
• Democritus proposed atoms are small, hard particles of a single material that form different shapes and sizes and material by joining together.
• Aristotle (384-322 B.C.) disagreed, suggesting that matter could be divided indefinitely.
• Aristotle's view was favored for thousands of years.
• John Dalton (1776-1844), a British chemist introduced his atomic theory in 1803.
• Dalton's atomic theory states all substances are made of atoms, which are small particles that cannot be created, divided, or destroyed.
• Dalton proposed that atoms of the same element are identical, while atoms of different elements are different.
• Dalton's theory explains that atoms of different elements combine in whole-number ratios to form chemical compounds. (law of definite proportion).
• Dalton also proposed that in chemical reactions, atoms are combined, separated, or rearranged but never changed into atoms of another element (law of conservation of mass).
• Dalton's atomic theory provided a way to use chemical equations to describe chemical reactions.
• Dalton's atomic theory was accepted because it explained the laws of conservation of mass, definite proportions, and multiple proportions.
• Dalton's original theory that all elements of the same type are identical was proven to be wrong.
• Atoms of the same element can have different numbers of neutrons resulting in different mass numbers; these atoms are called isotopes.
• Frederick Soddy (1877-1956) proposed the concept of isotopes in 1912.
• Soddy won the Nobel Prize in Chemistry in 1921 for his work with isotopes and radioactive materials.

Development of Modern Atomic Theory

• Michael Faraday discovered atoms have an electrical component.
• During the 1830's two opposite electrodes were placed in a solution of water containing a dissolved compound.
• One of the elements of the dissolved compound accumulated on one electrode, and the other element was deposited on the opposite electrode.
• Sir William Crookes sent an electric current through a gas in a sealed tube in 1879.
• The tube had electrodes at either end and a flow of electrically charged particles moved from one of electrodes, named cathode rays.
• J.J. Thomson (1856-1940) discovered that atoms are made of smaller negatively-charged particles called electrons.
• In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of electrons.
• Thomson created the Plum Pudding Model where electrons are in a soup of positive charges.
• Thomson proposed that the atom was a sphere of positively charged material with electrons scattered throughout similar to plums in a pudding.
• Ernest Rutherford (1871 - 1937) disagreed with the "Plum Pudding Model" and devised an experiment to investigate the structure of positive and negative charges in the atom.
• Rutherford did the gold foil experiment, leading him to discover and propose the nucleus.
• An atom's mass is mostly in the nucleus.
• In 1911, Rutherford's gold foil experiment showed the diameter of the nucleus is 100,000 times smaller than the diameter of the gold atom.
• Rutherford's Revised Atomic Theory proposed most of the matter and positive charge of the atom is in a small, dense central region called the nucleus.
• Rutherford proposed that electrons surround the nucleus at some distance
• Rutherford's experiment determined that atoms are mostly empty space with a tiny, massive nucleus at the center.
• Neils Bohr (1885 – 1962), proposed a planetary model in 1913.
• The Bohr Model of the Atom suggests a nucleus surrounded by orbiting electrons at different energy levels (different distances from nucleus).
• Electrons have definite orbits and no paths in between.
• Electrons can "jump" from level to level which helps explain why different atoms interact and why they are reactive.
• Bohr proposed that electrons move in paths at certain distances around the nucleus, and they can jump from a path on one level to a path on another level.
• Electrons can only revolve in certain orbits, or at certain energy levels (ie, the energy levels are quantized).