the history of the atom
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Chapter 3. The History of the Atom. look at sugar; made of crystals grind them into finer powder, still sugar dissolved in water, they seem to disappear look underneath microscope, can’t see them, but still can taste them - PowerPoint PPT PresentationTRANSCRIPT
THE ATOM: PHILOSOPHICAL IDEA TO SCIENTIFIC THEORY
look at sugar; made of crystals grind them into finer powder, still sugar dissolved in water, they seem to
disappear look underneath microscope, can’t see
them, but still can taste them these observations led many
philosophers to ponder the fundamental nature of matter
400 BC Greek philosopher Democritus believed that matter was made of smaller particles he called ATOMS
atomos- Greek meaning indivisible
Aristotle did not believe in atoms
believed that matter was continuous and made of 4 elements: fire, water, air, and earth
due to his popularity, his theory was accepted for 2000 years
Alchemists were the next group to impact the atomic theory
main goal: find the smallest particle of matter through which they could change impure metals into gold & the fountain of youth
left a legacy of lab techniques still in use today
by late1700s, all chemists accepted modern definition of element-subst that can’t be further broken down by ordinary chemical means
also accepted that elements form compounds that have properties different from the elements that make them up
new balances brought about quantitative analysis of compounds
several laws were discovered:
law of conservation of mass-mass is neither created nor destroyed during ordinary chem rxns or physical changes
Sec 3-1 Pg 65-69
+ =Carbon, C Oxygen, O Carbon Monoxide, CO
Mass x Mass y Mass x + Mass y
= +Carbon, C Oxygen, O
Mass x Mass yCarbon Monoxide, CO
Mass x + Mass y
law of definite proportions- a chem compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or source of compound
Sec 3-1 Pg 65-69
+ =1 Carbon 1 Oxygen Carbon Monoxide
=Carbon Dioxide,
CO2
ALWAYS
+1 Carbon 2 Oxygen
ALWAYS
also known at this time: 2 elements sometimes will combine to form more than one compound
law of multiple proportions- if 2 or more different compounds are composed of the same 2 elements, then the ratio of the masses of the 2nd element combined with a certain mass of the 1st element is always a ratio of small whole numbers
JOHN DALTON in 1808, John Dalton, an English
schoolteacher, proposed explanation for laws: elements are composed of atoms that are going to combine in whole numbers
led to Dalton’s Atomic Theory
Sec 3-1 Pg 65-69
1.All matter is composed of extremely small particles called atoms
2.Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, & other properties
ELEMENT2
ELEMENT 3
ELEMENT 1
ELEMENT4
Sec 3-1 Pg 65-69
3.Atoms cannot be subdivided, created, or destroyed
4.atoms of different elements combine in simple whole # ratios to form chem compds
5. in chemical rxns, atoms are combined, separated, or rearranged
+ +
Democritus’s idea was turned into scientific theory that could be tested by experiments
not all Dalton’s theory have been proven to be true, but through the testing of other scientists over time the picture of the modern atom has evolved
ATOMIC STRUCTURE exceptions to Dalton’s theory: atoms are
divisible atoms consist of 2 regions:
nucleus- very small region in center of atom consists of neutrons (particles with no electric
charge) and protons (particles with positive charge) electron cloud- large region surrounding
nucleus consists of electrons (particles with negative charge)
1897 closed glass tube w/ low pressure gas
inside positive anode on one end; negative
cathode on other pass electricity from one end to other rays moved from cathode to anode Thompson suggested these rays were
streams of negative particles called electrons, e-
rays were deflected by negatively charged plates
studied the degree of deflection of rays and was able to determine the charge to mass ratio
ROBERT MILLIKAN in 1909, using
apparatus, he determined charge on e-
used atomizer to spray fine droplets of oil
e- attached to oil droplets which fell through hole into vacuum chamber
gravity caused droplets to fall down
by adjusting the electric charge on the plates in chamber, kept oil droplets suspended
calculated charges on droplets
using Thomson’s data and his own, he was able to calculate the mass of the e-
mass is 9.09 x 10-28 g e- mass is so minute that it is assigned
a mass value of 0 e- has a charge of -1
EUGEN GOLDSTEIN in 1886, a stream of positively charged
particles was observed moving from anode (+) to cathode (-)
called canal rays deflected by positive electric fields these were named protons, p+ determined that the electric charge
was opposite that of e-
ERNEST RUTHERFORD, et al famous Gold Foil experiment in 1910 bombarded gold foil w/ alpha particles
(+) from radioactive source fluorescent screen around foil would
determine how alpha particles scatter markings on screen were counted to
determine degree of deflection
most particles went through foil with little or no deflection
few reflected at large angles and some almost back toward source
analyzed mathematically
discovered: atoms mostly empty space positive core in center of atom
core contains mass of atom named core nucleus
HENRY MOSELEY in 1913,
determined magnitude of and charge in elements
studied X-rays produced in X-ray tubes w/ anodes of diff metals
results indicated that each element differed from preceding one by having 1 more positive charge in nucleus
charge was always same for a given element
X-ray spectra indicated # of protons
led to the atomic number atomic number is the number
of protons in nucleus of atom element’s identity is due to number of protons in it
JJ Thomson conducted exp w/ neon gas atoms
observed that there were 2 diff types of neon atoms; alike chemically, but diff in mass
named isotopes- atoms of the same element that differ in mass; have the same # of p, but varies in number of neutrons
JAMES CHADWICK in 1932, Chadwick bombarded
Be w/ high energy alpha particles
this produced some uncharged particles w/ mass ~equal to that of p+
named neutrons, n0 neutrons assigned mass of 1
HENRI BECQUEREL 1896, discovered uranium ore exposed
sealed photographic film Pierre & Marie Curie discovered rays
were given off by U & Ra these rays had an effect on charged
electroscope discovered radioactive substance
radioactivity- phenomenon of rays being produced spontaneously by unstable atomic nuclei
rays can be particles or energy or mixture of both