Atomic Models: Contributions from Democritus to Bohr

Democritus

Initial view of the atom (atomos): Indivisible, indestructible particles. Solid, homogenous, and had different sides.

Dalton's Atomic Theory

All elements are made up of atoms. Atoms of the same element have the same mass and properties. Atoms of different elements have different masses and properties. Atoms combine in simple ratios to make compounds. Atoms cannot be created, destroyed, or divided in chemical reactions.

Dalton's Model of the Atom

Solid, spherical particle.

J.J. Thomson

Particle found by J.J. Thomson: Electron.

Thomson's Experiment

Used cathode ray tubes. He saw that cathode rays got deflected by electric/magnetic fields, which showed that there are negatively charged particles smaller than atoms.

Rutherford's Gold Foil Experiment

He focused a beam of positively charged alpha particles (two protons and two neutrons) at a thin sheet of gold foil.

Rutherford's Development

The Atom has a small, dense, positive nucleus at its center, which is orbited by electrons. The atom is mostly space.

Millikan's Oil Drop Experiment

He suspended tiny charged oil drops between 2 metal plates and then measured electric fields to find the charge of 1 electron.

Millikan's Development

Found the specific charge of an electron (1.602 × 10-19 coulombs).

Bohr's Model of the Atom

Electrons can orbit the nucleus in shells, can jump between shells, and release specific amounts of energy (quanta). Rutherford's model didn't think about this.

Protons

Charge: +1 (positive). Location: Nucleus. Mass: abt 1 atomic mass unit (amu). Importance: Determines atomic # and what the element is.

Electrons

Charge: -1 (negative). Location: orbiting the nucleus in shells. Mass: abt 1/1836 amu, very small compared to protons and neutrons. Importance: chemical properties and reactivity of atom, driving force of chemical bonds.

Neutrons

Charge: 0. Location: Nucleus. Mass: abt 1 amu. Importance: Contributes to atomic mass and stabilizes nucleus by preventing repelling of protons.

Atomic Number

Gives # of protons, which also defines an element.

Isotope

The same element with a different number of neutrons, which causes a different atomic mass.

Relative Atomic Mass

The mass of a single isotope compared to the mass of a carbon-12 atom (12 amu). Relative atomic mass is normally close to the atomic mass (protons + neutrons).

Average Atomic Mass

The average between all the isotopes (naturally occurring) of a certain element. Calculation: (Mass of Isotope 1 × % Abundance of Isotope 1) + (Mass of Isotope 2 × % Abundance of Isotope 2) + ... (where % abundance is expressed as a decimal).

Hydrogen

Symbol: H, Atomic Number: 1, Mass Number: 1, Atomic Mass: 1.008.

Helium

Symbol: He, Atomic Number: 2, Mass Number: 4, Atomic Mass: 4.003.

Lithium

Symbol: Li, Atomic Number: 3, Mass Number: 7, Atomic Mass: 6.941.

Beryllium

Symbol: Be, Atomic Number: 4, Mass Number: 9, Atomic Mass: 9.012.

Boron

Symbol: B, Atomic Number: 5, Mass Number: 11, Atomic Mass: 10.81.

Carbon

Symbol: C, Atomic Number: 6, Mass Number: 12, Atomic Mass: 12.011.

Nitrogen

Symbol: N, Atomic Number: 7, Mass Number: 14, Atomic Mass: 14.007.

Oxygen

Symbol: O, Atomic Number: 8, Mass Number: 16, Atomic Mass: 15.999.

Fluorine

Symbol: F, Atomic Number: 9, Mass Number: 19, Atomic Mass: 18.998.

Neon

Symbol: Ne, Atomic Number: 10, Mass Number: 20, Atomic Mass: 20.180.

Sodium

Symbol: Na, Atomic Number: 11, Mass Number: 23, Atomic Mass: 22.990.

Magnesium

Symbol: Mg, Atomic Number: 12, Mass Number: 24, Atomic Mass: 24.305.

Aluminum

Symbol: Al, Atomic Number: 13, Mass Number: 27, Atomic Mass: 26.982.

Silicon

Symbol: Si, Atomic Number: 14, Mass Number: 28, Atomic Mass: 28.085.

Phosphorus

Symbol: P, Atomic Number: 15, Mass Number: 31, Atomic Mass: 30.974.

Sulfur

Symbol: S, Atomic Number: 16, Mass Number: 32, Atomic Mass: 32.06.

Chlorine

Symbol: Cl, Atomic Number: 17, Mass Number: 35, Atomic Mass: 35.45.

Argon

Symbol: Ar, Atomic Number: 18, Mass Number: 40, Atomic Mass: 39.488.

Potassium

Symbol: K, Atomic Number: 19, Mass Number: 39, Atomic Mass: 39.098.

Calcium

Symbol: Ca, Atomic Number: 20, Mass Number: 40, Atomic Mass: 40.078.