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Past Scientists and their Discoveries


Greek philosopher; developed an atomic theory; came up with the name "atom" from the Greek word "atomia" = indivisible

Democritus's Atomic Theory

elaborated the idea that matter consisted of atoms having physical size and shape; atoms were in constant motion and moved in a void and interacted in different ways


had a pupil named Alexander; was a Greek philosopher, educator and scientist; undertook a large scale classification of plants and animals; introduced a method of scientific thinking that still plays a role today


first true chemist; first to write information down and hid it from the public in codes and left out items to keep their success to themselves; from them we got shorthand and symbols

Francis Bacon

English philosopher and scientist; introduced the idea that an understanding of the natural world could be gained from direct observation and experimentation

Antoine Laurent Lavoisier

French chemist who stated the first version of the law of conservation of mass; recognized oxygen (1778); disproved Phlogiston theory; helped to reform chemical nomenclature (naming)

Law of Conservation of Mass

mass is neither created nor destroyed; the total mass of materials present after a chemical reaction is the same as the total mass before the reaction

Joseph Proust

if atoms of A and B always have their own characteristic masses, then compound AB must always have the same composition by mass; must always have the same proportion in order to be that compound-- different proportion = different compound; Law of Definite Proportion

Law of Definite Proportion

a given compound always contains exactly the same proportion of elements by mass

John Dalton

middle school teacher; had an atomic theory that was published during the period of 1803-1807; designed his theory to explain several experimental observations; theory has remained basically intact up to the present

Dalton's Atomic Theory

each element is composed of extremely small particles called atoms; all atoms of a given element are identical (WRONG: isotopes); atoms of different elements have different properties including different masses; atoms of an element aren't changed into different types of atoms by chemical reactions; compounds are formed when atoms of more than one element combine; the relative # and kind of atoms are constant in a given compound

Dalton's Theory also includes...

law of multiple proportions, law of conservation of mass and law of definite proportion

Law of Multiple Proportions

if two or more different compounds are composed of the same two elements, the masses of the second element combined with a certain mass of the first element can be expressed as ratios of small whole #s

Law of Conservation of Mass

mass is neither created nor destroyed

Law of Definite Proportion

a given compound always contains exactly the same proportion of elements by mass

Joseph John Thomson

English physicist who researched atomic structure; determined charge:mass ratio; discovered that atoms contain particles which he called "electrons"; "plum pudding/raisin muffin" model of the atom which consisted of electrons embedded in a positive sphere of matter

Robert Millikan

devised a way to determine the charge of an electron; Millikan oil drop experiment

Millikan Oil Drop Experiment

drops are put between plates with the use of an atomizer and are seen through a microscope; when there is no electric field the drops will fall due to gravity; if an electric field is applied between the two plates and the electric force is greater than gravity, the drops will rise; the mass of a drop can be determined by applying the correct voltage across the two plates so the drop is stationary

Ernest Rutherford

revealed three types of radiation: alpha, beta and gamma; each type is different in its response to an electric field

Alpha Particles

more massive than beta; have positive charge; bends in the opposite direction of beta; experimented with alpha particles in gold foil experiment

Beta Particles

high speed electron; bends in the opposite direction of alpha particle

Gamma Particles

not affected by electric field

Gold Foil Experiment

alpha particles were positively charged with a mass of appx. 7500 x that of an electron; most particles went straight through; some were reflected backwards or deflected at large angles

Nuclear Atom

(developed by Rutherford) an atom with a dense center of positive charge (nucleus) around which tiny electrons move in a space that is otherwise empty; positive nucleus balances negative charge of electron and is small and dense; nucleus contains protons

Rutherford's Idea of Atomic Structure

if the nucleus was the size of an orange, then the radius of the atom would be 2.5 miles; the electron has a mass 1,830x less than proton or neutron

Refinements of Atomic Model

Rutherford's model was incomplete; light and electrons in atoms have some common properties; electrons have wave-like properties and light has certain particle-like properties; electrons and light have a dual wave-particle nature

Light as a Wave: Electromagnetic Radiation

a form of energy that exhibits wavelike behavior as it travels through space-- moves through a vacuum at a constant speed of 2.0 x 10^10 cm/s; has measurable wave properties of wavelength and frequency


the distance from the base of one wave to the base of the next wave


the number of waves that pass a given point in a specific amount of time, usually one second

Light as a Wave: Electromagnetic Spectrum

consists of all electromagnetic radiation arranged according to increasing wavelength

Kinds of Electromagnetic Radiation

long waves, radio waves--short waves, television, FM; microwave, radar, infrared, visible light, ultraviolet, x-rays, gamma rays

Light as a Wave: Light as Particles

scientists encountered two properties of light that could not be explained by wave theory: when an object made of a material that does not burn as it's heated, it gives off light and the photoelectric effect

Photoelectric Effect

the emission of electrons by certain metals when light shines on them

Max Planck

German physicist; proposed that when a hot object loses energy it did not do so continuously-- instead the object radiated energy in specific amts./packets called quanta


an individual quantum of light


finite quantity of energy that can be gained or lost by an atom (see notes slide 20)

Ground State

the state of lowest energy of an atom

Excited State

a state in which it has a higher potential energy than it has in its ground state

Niels Bohr

Bohr model: proposed a model of the hydrogen atom; the single electron of the hydrogen can circle the nucleus only in allowed paths or orbits; when the electron is in each of these allowed orbits, the atom has a definite, fixed energy-- lowest total energy when it is in orbit closest to the nucleus; orbit is separated from the nuclues by a relatively large empty space where the electron can't exist

Erwin Schrodinger

contributed to wave mechanical model of atom; devised an equation that treated electrons moving around nuclei as waves that have only certain probability of being found at various distances from the nucleus; Schrodinger model: nucleus is surrounded by orbitals


a three-dimensional region about the nuclues in which a particular electron can be located; can be thought of as clouds showing the region of probable electron locations

Werner Heisenberg

contributed to wave mechanical model of atom; Heisenberg's uncertainty principle

Heisenberg's Uncertainty Principle

it is not possible to measure precisely both the velocity and the position of an electron at the same time

Quantum #s

numbers that specify the properties of atomic orbitals and of their electrons; there are 4 quantum #s used to describe the property of the orbital of a particular electron

Principal Quantum #

tells the distance from nucleus; symbolized by n; indicates the main energy levels surrounding a nucleus; values of n are whole #s only; as n increases, the distance of the main energy levels from the nucleus increases and their energy increases

Orbital Quantum #

symbolized by s (spherical), p (dumbbell), d (lobed) or f (complex shapes); indicates the shape of an orbital; in an nth main energy level, orbitals of n shapes are possible

Magnetic Quantum #

indicates the orientation of an orbital about the nucleus; S sublevel (no particular orientation), P sublevel (3 orientations), D sublevel (5 orientations), F sublevel (7 orientations)

Spin Quantum #

symbolized by +1/2 (clockwise) or -1/2 (counterclockwise); indicates two possible states of an electron in an orbital; each electron behaves as though it were a tiny bar magnet with a north and south pole; each electron spins either clockwise or counterclockwise

James Chadwick

British scientist; pupil of Rutherford; knew that mass had to be composed of more than just protons; discovered neutrons in 1932 because particles were not deflected by electric or magnetic fields so they were uncharged and had a mass the same size as that of the proton

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