HISTORY OF THE ATOM

ATOMISM 5th century BC, Ancient Greece – Leucippus, Democritus

Both from the Ionian school of naturalistic philosophy

The earliest proponents of the concept of atomism

“All matter is composed of indivisible particles called atoms.”

Proposed the earliest views on the shapes and connectivity of atoms.

“They reasoned that the solidness of the material corresponded to the

shape of the atoms involved. Thus, iron atoms are solid and strong with

hooks that lock them into a solid; water atoms are smooth and slippery; salt

atoms, because of their taste, are sharp and pointed; and air atoms are

light and whirling, pervading all other materials.”

BILLIARD BALL MODEL – “CHEMICAL ATOMISM”

~1800, Great Britain – John Dalton Elements are made of tiny particles (small solid spheres)

called atoms. All atoms of a given element are identical. Compounds are composed of atoms in specific ratios. Chemical reactions are rearrangements of atoms

(mass is conserved).

DISCOVERY OF CATHODE RAYS ~1860, Germany A heated cathode of an electric circuit in a vacuum with a large

potential difference applied between the electrodes emitted a

strange beam that was thought to be an electromagnetic wave.

DISCOVERY OF THE ELECTRON

~1900, Great Britain – J. J. Thompson

Investigated cathode rays.

Can be deflected in their path

by magnetic and electric fields

→ carry a charge.

Concluded that these rays were

actually particles, later called

electrons.

Able to measure their charge-to-mass ratio .e

e

mQ

THOMSON’S PLUMB PUDDING MODEL

The atoms consist of a number of negatively electrified corpuscles

enclosed in a sphere of uniform positive electrification. Atoms are neutral → the total neg. charge = the total pos. charge.

~1910, USA – R. A. Millikan

Charged droplets of oil between two metal electrodes

State of mechanical equilibrium between the downward

gravitational force and the upward electric force

The charges were all multiples of a fundamental value,

calculated to be 1.602×10−19 C → charge is quantized.

MILLIKAN’S OIL DROP EXPERIMENT

RUTHERFORD’S GOLD FOIL EXPERIMENT ~1910, New Zealand/Great Britain – E. Rutherford Beam of alpha particles (helium atoms without electrons)

was directed onto a sheet of very thin gold foil.

RUTHERFORD’S GOLD FOIL EXPERIMENT

Expected Results Observed Results

Expected Results Observed Results

Thomson’s Model Rutherford’s Model

Conclusion: The atom is mostly empty space with a small, dense,

positively charged nucleus surrounded by negative electrons.

DISCOVERY OF THE PROTON ~1920, E. Rutherford Alpha particles were shot into nitrogen gas → scintillation detectors

showed the signatures of hydrogen nuclei. This hydrogen could only have come from the nitrogen → nitrogen

must contain hydrogen nuclei.

о The hydrogen nucleus is

present in other nuclei as

an elementary particle

named proton.

(Greek πρτον = first)

DISCOVERY OF THE NEUTRON ~1930, Great Britain – James Chadwick

Bombarded beryllium with alpha particles. Radiation emitted by beryllium stroke into paraffin wax. Protons shot out from the paraffin wax. What was in the “beryllium radiation”?

The radiation not affected by electric or magnetic field → neutral. Photons would be too “weak” to shoot out protons. These new neutral particles have approximately the mass of protons. Named neutrons.

DISCOVERY OF THE NEUTRON

ATOMIC NUCLEUS – SUMMARY

a very dense region at the center of an atom

consisting of nucleons: protons and neutrons

accounts for almost all the mass of an atom

~10-15 m in diameter

The number of protons

in an atomic nucleus identical to the charge number

of the nucleus uniquely identifies a chemical

element in a neutral atom, equal to the

number of electrons

The number of nucleons

in an atomic nucleus different for each different

isotope of a chemical element e.g., common has 6 n0

while radioactive has 8 n0

Atomic (Proton) Number Z

Mass (Nucleon) Number A

C126C146

XAZ

Chemical Element

Is composed of atoms with the same Z

Naturally a mixture of different isotopes

Nuclide

Is composed of atoms with the same Z and A

PERIODIC TABLE OF THE ELEMENTS

ELEMENTARY (?) PARTICLES Electron is not known to be made up of smaller particles Mass kg, charge C

BUT Protons and neutrons ARE known to be made up of QUARKS

(though these do not exist separated or isolated) Mass , charge C

31101.9 em19106.1 eQ

enp mmm 1800 19106.1 pQ

QUESTIONS

1) What is the charge of an electron?

2) What did Millikan find out in his experiment?

3) How was an atomic nucleus discovered?

4) What is the charge of a proton?

5) What is the charge of a neutron?

6) What is the difference between a chemical element and a nuclide?

7) What particles are atoms composed of?

8) What do you know about quarks?

UCHeH 23592

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