450 BC Model: 4 Element, All matter was made up of a combination of four basic components: Earth, Water, Air, and Fire

400 BC Model by Democritus, The ultimate underlying structure to matter is the basic smallest unit of matter: "atomos"- not cuttable concept was developed by reason

1803 Model by Dalton, An atom of any givien element has its own fixed mass

1898 Model by JJ Thompson "Plum Pudding Model", An atom is spherical in shape and consists of a thin cloud of positively charged particles sprinkled throughout technology used were Cathode ray tubes

1911 Model by Rutherford "Solar System Model", An atom is spherical in shape with a tiny central nucleus containing all of the positive charge and most of the mass, with electrons orbiting in mostly empty space. Technology used was Gold Foil experiment using recently discovered radioactive material

1913 Model by Bohr Similar to Rutherford's except: When the atom receives energy from an outside source, an e- jumps from a lower energy level to a higher one. As the e- returns to the lower level, energy in the form of light is released technology used was Hydrogen Spectrum

20th Century Model of Quantum Mechanics, The atom consists of a central nucleus containing protons and neutrons together with electrons existing outside of the nucleus having definite energy but no definite definite orbits around the nucleus

Each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, and hence differ in relative atomic mass but not in chemical properties; in particular, a radioactive form of an element.

Gold is an example of a pure substance.

The average atomic mass of an element is the sum of the masses of its isotopes, each multiplied by its natural abundance (the decimal associated with percent of atoms of that element that are of a given isotope).

Lewis diagrams are based on the amount of outer electrons and they are represented by dots surrounding the elements symbol with a maximum of 8

The elements in the groups or families share the same characteristics like chemical reactivity and composition like the metals or non metals

The only trend of periods are as you go across a period, electrons are added to the same energy level. At the same time, protons are being added to the nucleus. The concentration of more protons in the nucleus creates a "higher effective nuclear charge." In other words, there is a stronger force of attraction pulling the electrons closer to the nucleus resulting in a smaller atomic radius.