1 The Structure of the Atom. 2 Early Theories of Matter

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The Structure of the Atom

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Early Theories of Matter

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460-370 BC Democritus

Greek PhilosopherNamed atom

»smallest unit of matter

»means indivisible

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1807 John Dalton: Atomic Theory

Revived and revised Democritus’ ideas and began developing the modern atomic theory

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Dalton’s Atomic Theory

all elements are composed of tiny indivisible particles called atoms

atoms of same element alike. Each element is different from atoms of other elements

atoms of different elements combine in simple whole number ratios to form compounds

chemical reactions occur when atoms are rearranged

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Picture shows:

•Conservation of Mass

•Element combing in simple whole number ratios

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Subatomic Particles & the Nuclear Atom

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Discovering the Electron

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1879 William Crookes

investigated electrical discharge in gases

cathode ray tubeCathode rays are streams of

negatively charged particles.The particles are found in all

forms of matter

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1897 J.J. Thomson

determined nature of cathode ray determined charge to mass ratio of

electron Found that atoms were divisible -

Dalton & Democritus were wrong

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1901 J.J. Thomson

positive beam experimentsplum pudding model of

atom or chocolate-chip cookie dough model of the atom

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1909 Robert Milliken

determined charge of electronoil drop experimentwith Thomson’s charge to

mass ratio: able to determine the mass of e-

Mass of electron = 9.1x10-28 grams

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1911 Ernest Rutherford

discovered nucleus gold foil experiment disproved

plum pudding model small dense central part of atom

= nucleus (+) charge

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1920 Rutherford

Refined concept of nucleusConcluded that nucleus

contained positively charged particles called protons

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1932 James Chadwick

identified neutronsame mass as protonno charge

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How Atoms Differ

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NUCLEUS

1. Protons with (+) charge

2. Neutrons with no charge.

3. Protons & neutrons have about the same mass.

4. (+) charge is responsible for most of mass of atom (dense central part).

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ELECTRONS

move around nucleusresponsible for most of volume

of atom (-) chargenegligible mass

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ATOMIC NUMBER & MASS NUMBER

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ATOMIC NUMBER

# of protons in nucleus it identifies the element elements in Periodic Table are

listed in increasing order of atomic #

if atom is neutral: the # of protons equals # of electrons

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MASS NUMBER

sum of protons & neutrons in nucleus written as part of name: must be

given to you

Neon-20 mass #20 p + n = 20

atomic #10 p = 10

n = 10

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mass # p + n

Symbolatomic # p

? # neutrons

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Oxygen-17mass # 17atomic # 8

p + n = 17 p =_8_

9 n

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To calculate electrons for an ion you must look at the charge written in the upper right corner

To determine the number of electrons:» If the charge is positive then subtract that

number from the number of protons.» If the charge is negative then add that

number to the number of protons

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ISOTOPES

Atoms of the same element are not all identical - may differ in # of neutrons

Isotopes - atoms of the same elements (same # of protons), but different mass # (different # neutrons) and therefore different masses

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12 13 14

C C C 6 6 6

? # neutrons

6 7 8

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ATOMIC MASS

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ATOM

smallest unit of an element that can exist alone and still have the properties of that element

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Atomic Mass

Average Atomic Mass - weighted average of the masses of the naturally occuring isotopes» relative mass based on carbon-12 as the

standard» Carbon-12 is defined as having a mass of

exactly 12 amu atomic mass unit (amu) - 1/12 of the

mass of a carbon-12 atom

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Weighted Average Example

50% test 70

30% Lab 80

20% Daily 90

= 50%(70) + 30%(80) + 20%(90)

= .5(70) + .3(80) + .2(90)

= 35 + 24 + 18

= 77

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Isotopes of Hydrogen

H-1 protium 1.0078 99.985%

H-2 deuterium 2.0140 0.015%

H-3 tritium 3.0160 -------

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Calculating Average Atomic Mass

Multiply the percent (as a decimal #) by the mass and then add each together.

= 99.985% (1.0078 amu) + 0.015% (2.0140 amu)

= .99985 (1.0078 amu) + .00015 (2.0140 amu)

= 1.0076488 amu + 0.0003021 amu

= 1.00795 amu

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Example

chlorine - 35 75.8%

chlorine - 37 24.2%

Will the average atomic mass be closer to 35 or 37?

(35 because higher %)

75.8%(35) + 24.2%(37) =

35.5 amu