The History of the Atom

THE HISTORY OF THE ATOM

Chapter 3

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

Sec 3-1 Pg 65-69

+ =Carbon Oxygen Carbon Monoxide,

1:1

=Carbon Dioxide,

1:2

+Carbon Oxygen

1 1

1 2

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)

J.J.THOMSON

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-

mass of p+ is 2000x that of e-

protons have an assigned mass of 1

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

RUTHERFORD, BOHR, AND GEIGER

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

BECQUEREL AND THE CURIES