ATOMIC THEORY

ATOMIC THEORY

Atomic Theorists and their Contributions

Atomic Theory: Timeline

•460 BC: Democritus

•1803: Dalton

•1897: Thomson

•1900: Planck

•1909: Millikan

•1911: Rutherford

•1913: Bohr

•1924: de Broglie

•1927: Heisenberg

Democritus

•460 BC

•another Greek philosopher

•student of Leucippus

•“atoms cannot be changed ordestructed and everything we

see is made of clusters of atoms”

•his ideas lacked experimental

support because scientific testing

was unknown at that time

Stamp issued by Greece on Sept. 26, 1983 to honor an International Conference on Democritus and his work

John Dalton

•1766-1844

•English schoolteacher

•developed the first useful atomic theoryof matter (in1803)

•Dalton’s atomic theory includes four mainassumptions…

Dalton’s Atomic Theory

1.All elements are composed of tiny indivisible (cannotbe separated) particles called atoms.

2.Atoms of the same element are identical. Atoms ofdifferent elements are different.

3.Atoms of different elements can physically mixtogether or can chemically combine with oneanother in simple whole number ratios to formcompounds.

4.Chemical reactions occur when atoms are separated,joined, or rearranged. Atoms of one element,however, are never changed into atoms of anotherelement as a result of a chemical reaction.

Dalton’s Table of Elementsand their Combinations

Sir J.J. Thomson

•English physicist

•discovered the electron (in 1897)

•used a cathode-ray tube to show thatthere are particles smaller than atoms(electrons)

•created the “plum pudding

model”

Thomson’s Plum Pudding Model

•atoms are made of asphere which ispositively charged, withnegatively-chargedelectrons “swimming” inthis sphere

•electrons = plums

•positive interior =pudding

Thomson’s Cathode Ray Experiment

Thomson used cathode-ray tubes in his experiments:

1. gases under low pressure were sealed into glass tubes fitted at bothends with metal disks called electrodes

2. the electrodes were connected to a source of high-voltage electricity

3. one electrode, the anode, became positively charged; the otherelectrode, the cathode, became negatively charged

4. a glowing beam formed between the electrodes – this beam, whichtraveled from the cathode to the anode is called the cathode ray

Thomson’s Experiment (cont.)

5.Thomson found that the cathode rays were attracted to the positively-charged metal plates – plates that were negatively-charged repelledthe cathode rays

6.Thomson knew that opposite charges attract and like charges repelSO he proposed that a cathode ray is a stream of tiny negatively-charged particles moving at high speed

7. these negatively-charged particles were then named electrons

he concluded that electrons must be parts of theatoms of all elements

by 1900, Thomson and others had determined that anelectron’s mass is about 1/2000 the mass of ahydrogen atom

Ernest Rutherford

•in 1911, discovered the atomic nucleus

•a New Zealand physicist

•“atoms have a positively-charged nucleus whichis surrounded by negatively-charged particles”

•suggested the so called planetary model, whichclaims electrons orbit the nucleus just like planetsorbit the Sun

•performed his famous experiment with a thin goldenfoil

Rutherford’s Experiment

•aimed a beam of alpha particles (+ charge) at a sheet ofgold foil surrounded by a fluorescent screen

•found that most of the particles passed through the foilwith no deflection at all

•a few particles were greatly deflected

CONCLUSION:

most of the alpha particles pass through the gold foilbecause the atom is mostly empty space

the mass and positive charge are concentrated in asmall region of the atom called the nucleus

particles that approach the nucleus closely are greatlydeflected

$gold foil experiment - Interpreting Rutherford\'s Gold Foil Experiment$

Robert Millikan

•found the quantity of charge carried by anelectron (in 1909)

•determined the ratio of the charge to the mass ofan electron

•found the values for electron charge andmass (in 1916)

•American scientist

•performed the famous

“oil-drop” experiment

Niels Bohr

•Danish physicist

•improved the “planetary model” by suggesting

that electrons only orbit the nucleus in

well-defined paths (in 1913)

•depicted the atom as a small, positively-charged nucleussurrounded by electrons in orbit

•“electrons travel in discrete orbits around thenucleus”

•electrons orbiting an atom can only exist at certainenergy levels (distances) from the nucleus, not atcontinuous levels as might be expected fromRutherford’s model

$\begin{figure} \begin{center} \leavevmode \epsfxsize=2.0 in \epsfbox{/export/home/fyde/randy/figs/fig28-1.eps}\end{center}\end{figure}$

Max Planck

•discovered the quantum nature of

energy (in 1900)

•“energy does not flow in a steadycontinuum, but is delivered in discretepackets called quanta”

•“quantum” is the amount of energy for anelectron to jump to an energy level

Werner Heisenberg

•discovered the “uncertainty principle”(in 1927) – we really don’t know whereelectrons are at any time

•electrons may exist in certain orbits (Bohr),but we cannot pinpoint where they will beat a certain time

•German physicist

Louis de Broglie

•discovered the wavenature of electrons –

(in 1924)

•electrons act not onlyas a particle, but as awave

•thus, electrons can havestable configurations inan orbit

$\begin{figure} \begin{center} \leavevmode \epsfysize=6 cm \epsfbox{figs/quantum-3.eps} \end{center} \end{figure}$