1) Atomic Theory

7/30/2019 1) Atomic Theory

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What is matter? States of matter

Gas: indefinite volume and shape; compressible Liquid: indefinite shape but with definite volume Solid: rigid; with definite shape and volume

Pure Substances Has distinct properties and composition

Does not vary from sample to sample May be elements or compounds


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What is matter? Elements

Composed of onlyone type of atom

Element

Approximate

% by weight

Oxygen 46.6

Silicon 27.7

Table 1. Abundance of elements in Earths crust

decomposed tosimpler substances

~114 elements areknown

Aluminum 8.1

Iron 5.0

Calcium 3.6

Sodium 2.8

Potassium 2.6

Magnesium 2.1

All others 1.5


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Brown, LeMay Jr., Bursten, Murphy. 2009.


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The Atomic Theory of Matter Early Greek philosophers; Democritus (460-370 BCE):

material world is made up of indivisible particles calledatomos

Plato and Aristotle: no indivisible particles exist

o n a on s a om c eory cen ury1. Each element is composed of extremely small particlescalled atoms

2. All atoms of a given element are identical

3. Atoms of an element are not changed into different typesof atoms by chemical reactions4. Compounds are formed when atoms of more than one

element combine; a given compound always has thesame relative number and kinds of atoms


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Atomic Theory of Matter Atoms are the basic building blocks of matter.

The law of definite composition states that the relativenumbers and kinds of atoms in a given compound areconstant.

The law of conservation of mass states that the total mass

present after the chemical reaction is the same as the totalmass before the reaction. The law of multiple proportions states that when two

elements form more than one compound, the masses of one

element in these compounds for a fixed mass of the otherelement are in ratios of small whole numbers.


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The Development of the Atomic StructureRadioactivity

Henri Becquerel: uranium mineral spontaneouslyemits radiation

Marie and Pierre Curie: conducted studies to

Ernest Rutherford: three types of radiation (, , radiations)


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In 1903, Henri Becquerel, Marie Currie and Pierre Currie

shared the Nobel Prize in Physics.


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Cathode Rays A cathode ray is radiation produced when ahigh voltage is passed through a partially

evacuated tube Joseph John Thomson discovered that different

- ,were emitted

Ratio of electrons electrical charge to its mass:1.76 x 108 C/gram


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Oil Drop ExperimentMillikan oil-drop experiment

Robert Millikan observed how a charged droplet ofoil falls in the presence and absence of an electricfield

Experimental value of electron charge: 1.60 x 10-19 C

Electron mass: (1.60 x 10-19 C)/(1.76 x 108C/gram)= 9.10 x 10-28 g


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Oil Drop Experiment


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How are the particles arranged?

Thomsons plum-pudding model of the atom


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The Gold Foil Experiment


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The Gold Foil Experiment


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The Gold Foil Experiment Majority of an atoms mass and positive

charge are localized in a very small, denseregion called the nucleus

which electrons move around the nucleus Neutrons James Chadwick


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The Modern View of Atomic StructureParticle Charge Mass (amu)

Proton Positive (1+) 1.0073

Neutron None (neutral) 1.0087

Electron Negative (1-) 5.486 x 10-4

Subatomic particles affect chemical behavior Neutral atoms have no net charge Atomic mass unit (amu) = 1.66054 x 10-24 g Atoms are extremely small (1-5 ), the nuclei are even

smaller (and denser! ~1013-1014 g/cm3)


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Isotopes, Atomic Numbers and Mass

Numbers Atoms of the same element that differ in the number

of neutron (and mass) are called isotopes. An atom of a specific isotope is called a nuclide.

atom.

Mass number (A) is the number of protons plus the

number of neutrons in an atom.


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Isotopes, Atomic Numbers and Mass

NumbersSymbol # of protons # of electrons # of neutrons

11C 6 6 512C 6 6 6

13C 6 6 7

How many protons, neutrons, and electrons

are in an atom of197Au?

14C 6 6 8

Ans: 79 protons, 118 neutrons and 79 electrons


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Isotones, Isobars, Isoelectronic atoms Isotones

Atoms with the same number of neutrons Isobars Atoms with the same number of nucleons

Isoelectronic atoms Atoms with the same number of electrons

11H, 21H, 31H

5826Fe, 5827Ni

7632Ce, 7634Se

3717Cl, 3919K

3517Cl, 3717 Cl Li+, He

S-2, Ar


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The Atomic Mass Scale Mass of 12C atom= 12 amu Atomic mass unit (amu)=1.66054 x 10-24 gAverage Atomic Masses

Most elements occur in nature as a mixture of

isotopes To determine the average atomic mass of anelement, masses of various isotopes and their

relative abundances are used

Ex. For C, 98.93% exists as 12C (12 amu) and 1.07% exists as

13C (13.00335 amu).


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Exercises1. Naturally occurring magnesium has the ff isotopicabundances:24

Mg 78.99% 23.9850425Mg 10.00% 24.9858426Mg 11.01% 25.98259

2. The atomic weight of chlorine is reported as 35.5,yet no atom of chlorine has the mass of 35.5 amu.Explain.

3. How many protons, neutrons, and electrons are inthe ff atoms: (a) 195Pt (b)238U (c)85Rb (d)128Xe (e)60Ni

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1) Atomic Theory