Early Models of the Atom

Defining the Atom An atom is the smallest particle of an element that retains its identity in a chemical reaction. Philosophers and scientists have proposed many ideas on the structure of atoms.

Defining the Atom Democritus’s Atomic Philosophy How did Democritus describe atoms? Democritus

Democritus (460 B.C. - 370 B.C.) first to suggest the existence of atoms proposed all life and matter are made up of small particles called atoms believed that atoms were indivisible and indestructible

Democritus (460 B.C. - 370 B.C.) dismissed by Aristotle and Plato atom = Greek word “atomos” meaning indivisible dismissed by Aristotle and Plato idea was largely abandoned for 2000 years

John Dalton 1800 proposed modern atomic model transformed Democritus’ ideas on atoms into a scientific theory through using experimental methods

John Dalton based on his results, he came up with his atomic theory

Dalton’s Atomic Theory Elements are composed atoms. Atoms of the same element are identical. Atoms of different elements different. Atoms of different elements combine in simple, whole-number ratios to form compounds. In a chemical reaction, atoms are combined, separated or rearranged but never created, destroyed or changed.

Law of Multiple Proportions Dalton’s Atomic Theory (#4) whenever 2 elements combine to form two or more compounds, the mass of one element that combines with a given mass of the other is in the ratio of small whole numbers Water (H2O) Peroxide (H2O2)

Law of Conservation of Mass Dalton’s Atomic Theory (#5) mass can not be created nor destroyed in a physical change or chemical reaction

Law of Definite Proportions Dalton’s Atomic Theory in any chemical compound, the masses of the elements are always in the same proportions

Law of Definite Proportions mass ratio does not change no matter how much of the sample or how it was formed

Parts of the Atom Proton: positive charge Neutron: Neutral charge Electron: Negative charge Objectives Slide Menu

Discovery of the Electron 1897, discovered electrons Cathode Ray Tube stream of negatively charged particles J. J. Thomson Click on Picture to Access Website

Discovery of the Electron charge to mass ratio constant therefore electrons are part of all elements Millikan in 1916 electron = -1 charge Click on Picture to Access Website Click on Picture to Access Website

Protons discovered by Goldstein observed in cathode-ray tube (ray traveling in opposite direction) has a positive charge

Neutrons discovered by Chadwick in 1932 mass is equal to that of proton neutral charge

Plum Pudding Model J.J. Thomson electrons evenly distributed throughout positively charged material resembles a fruit cake disproved in 1909 by Rutherford

Gold-Foil Experiment

Gold-Foil Experiment 1911, Rutherford beam of alpha particles shot at Au foil should have passed through foil with slight deflection majority not deflected deflection at large angles Discovery of NUCLEUS (add in)

Rutherford’s Atomic Model atom is mostly empty space positive charge and most of mass is in a small central region called the nucleus nucleus contains both protons and neutrons

Rutherford’s Atomic Model electrons are around the nucleus and make up the bulk of the atom nucleus is tiny compared to size of atom

Size of Nucleus If an atom is the size of a football stadium, the nucleus would be the size of a marble on the 50 yard line. The rest of the stadium is the area where an electron can be found.

Atomic Number Elements differ because of the number of protons Atomic Number = number of protons Atomic Number identifies an element

Atomic Number Atoms are electrically neutral Number of protons must equal the number of electrons

Mass Number Mass number = number of protons + number of neutrons

Mass Number Example: Helium Atom Atomic Number = 2 therefore, Protons = 2 Neutrons = 2 Mass Number = 4

Mass Number Example: Oxygen Atom Atomic Number = 8 Mass Number = 16 Protons = 8 (= atomic number) Electrons = 8 Neutrons = mass number – atomic no. = 16 – 8 = 8

Nuclear Symbols

Isotopes same number of protons but different number of neutrons chemically alike different mass numbers

Isotopes Hydrogen 1 proton 1 electron Deuterium 1 proton 1 electron 1 neutron Tritium 1 proton 1 electron 2 neutron

Atomic Mass Unit 1/12 mass of a carbon-12 atom Carbon-12 chosen as reference abbreviated by amu

Atomic Mass weighted average mass of the atoms in a naturally occurring sample of the element determined based on relative abundance of isotopes mass of isotope x abundance = atomic mass

Atomic Mass 10X = 10.012 amu and relative abundance of 19.91% : 11X = 11.009 amu and relative abundance of 80.09%. Calculate the atomic mass of this element. 10X = 10.012 amu x 0.1991 = 1.993 amu 11X = 11.009 amu x 0.8009 = 8.817 amu atomic mass = 10.810 amu

Rutherford’s Atomic Model The one thing that this model could not explain is……. the chemical properties of elements.