Download Atomic Terms/People

Atomic Terms/People
Early Atomic Theory:
Greek philosopher, Democritus about 400 B. C. had a thought about the atom.
 He stated that the world is made of 2 things: empty space and tiny particles called atoms.
Aristotle shortly after stated
 that matter is continuous and not made of smaller particles
 the continuous substance known as hyle.
 This was believed until the 17th century
Isaac Newton and Robert Boyle
 Both believed in the 17th century that elements were made of atoms but had no proof.
John Dalton an English Chemist stated the 1st Atomic theory (1766-1844)
 All matter is composed of very small particles called atoms.
 That these atoms cannot be broken into smaller particles.
 That all atoms of an element were exactly alike and that atoms of different elements were quite
unlike.
 Also, that atoms can combine in simple ratios to form compounds.
 His ideas explained the laws of conservation of mass and definite proportions.
Antoine Lavoisier a French Chemist (1743-1794)
 Proposed the Law of Conservation of Mass: in ordinary chemical reactions,
matter can be changed in many ways, but it cannot be created or destroyed.
Joseph Lewis Proust a French Chemist (1766-1844)
 Proposed the Law of Definite Proportions: Specific substances always contain
elements in the same ratio by mass.
Dalton’s 2nd law (Law of Multiple Proportions): The ratio of masses of one element that combined with a
constant mass of another element can be expressed in small whole numbers.
 Dalton published his law of partial pressures of gases in 1802.
 Dalton later published his atomic theory. This was used to determine chemical formulas.
Dalton made the first table of atomic masses
o Later proven inaccurate.
Dalton changed his theory based on the Atomic Number: It now states that all atoms of an element contain
the same # of protons but they can have a different # of neutrons.
 Nuclide-Is a particular kind of atom containing a definite # of protons and neutrons.
o Ex. Protium: Hydrogen –1
 Nucleons-Are the particles that make up the atomic nucleus. (Protons and Neutrons)
 Atomic Mass Number-The total number of nucleons in an atom.
o A is the symbol
o Figure out the # of Neutrons -> A-Z(Atomic Mass)
J.J. Thomson, an English scientist (1853-1940)
 Research on cathode-ray tube
o Positive electrode-anode
o Negative electrode-cathode
o Cathode Ray: Occurs when a tube connected to high voltage electricity. The electrodes
(metal piece) are connected at each end of the tube and charged, causing rays appearing at
the cathode that travel toward the anode.
 Thomson was able to determine the charge of the electron to be
1.6 x 10-19 C.
1
o C is the symbol for coulomb-The SI unit of electrical charge.
 While working with neon atoms observed 2 kinds of neon. He discovered isotopes.
o Isotopes-have the same amount of protons but a different number of neutrons. (Unstable
atoms)
 Radioactivity-Are rays that occur spontaneously by unstable atomic nuclei.
Robert Millikan, an American scientist (1868-1953)
 In 1909, obtained the 1st accurate measurement of an electron’s charge.
o Used an apparatus which oil was sprayed from a brass atomizer, electrons were
transferred from the atomizer to the oil droplets.
 He determined the accurate measurement of electrons (e-) = 9.11 x 10-31 kg
 Thomson determined from Millikan's data that the Charge-to-Mass Ratio was
equivalent to 1.759 x 1011 C/kg
 Using Thomson and Millikan's data it was possible to calculate the actual mass of
an electron.
 Electron mass is only 1/1837 the mass of the lightest atom Hydrogen.
o Millikan's work with the cosmic ray led to the discovery of the meson.
 Meson-are composed of a quark and an anti-quark of complementary color.
E. Goldstein
 In 1885, used a modified cathode-ray to discover a beam of positive particles.
o Afterwards, Thomson discovered that rays traveling the opposite way as cathode were
positive and known as protons.
 He also calculated that a proton 1836 times that of electrons.
 Actual Mass of a proton: 1.673 X 10-24
 He would also proposed a model of the atom known as "plum pudding".
Henry Moseley a English scientist
 In 1913, studied X rays produced in X-ray tubes with anodes of different metals.
o Found that the wavelength of X-rays is characteristic of metal used as the anode which
depends on the number of protons in the nucleus.
 Led to the Atomic number of elements = # of protons
 Symbol Z represents of Atomic number
 Amount of Protons = # of Electron
Lord Rutherford an English physicist
He and his team (Niels Bohr, Hans Geiger, and Ernest Marsden) developed the modern concepts of the
atomic structure of an element.
 Under Rutherford's direction Geiger and Marsden conducted the Gold Foil Experiment.
o In doing so, Rutherford concluded:

that the atom is mostly empty space.

He also realized that there was a core to the atom known now as the nucleus.

And that the nucleus contained all the positive charged and almost all of the
mass of the atom.
o Geiger and Marsden concluded:

Most of the atom is empty space,

The diameter of most atoms is between 100 and 500 pm (1 x 10-12 )

The radii of the nuclei of atoms vary between 1.2 x 10-3 and 7.5 X 10-3 pm

The radius of the electron is about 2.82 x 10-3 pm
2

In small atoms the distance between the nucleus and the nearest electron is about
50 pm.
 Therefore, the nucleus occupies only about one trillionth (10-12) of the volume
an atom.
 In 1920, Rutherford predicted the existence of a 3rd particle of an atom-neutron
Walter Bothe in 1930 and Rutherford and co-worker James Chadwick in 1932-Proved existence of neutrons
 Actual mass of neutron: 1.675 x 10-24
 Later scientists realized protons, electrons, and neutrons have even smaller particles.
Bohr Atomic Model
Neil Bohr is responsible for the planetary model
First you need the Atomic number and/or the Atomic Mass from the Periodic Table:
 Atomic Number (Symbol A) is equal to the number of protons (inside the nucleus) –Positive charged
nucleus because of the positive charge of protons
 Amount of Protons is equal to the number of electrons-Electrons are negatively charge and form
clouds or levels around the nucleus
 Atomic Mass (Symbol Z) –Atomic number =neutrons Z-A=Neutrons (neutrons have no charge and
are found in the nucleus with the protons)
o Note: Atomic number + neutrons =Atomic mass
After you find PEN you can draw the model –Need to see in class
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