CHAPTER CHAPTER 22

HISTORY OF THE ATOMHISTORY OF THE ATOM

1808 John Dalton

suggested that all matter was made up of

tiny spheres that were able to bounce around

with perfect elasticity and called them

ATOMSATOMS

8 X2Y16 X 8 Y+

DALTONS ATOMIC THEORY

Subatomic Particles

Particle Mass (g)

Charge (Coulombs)

Charge (units)

Electron (e-) 9.1 x 10-28 -1.6 x 10-19 -1

Proton (p) 1.67 x 10-24 +1.6 x 10-19 +1

Neutron (n) 1.67 x 10-24 0 0

mass p = mass n = 1840 x mass e-

HISTORY OF THE ATOMHISTORY OF THE ATOM

1898 Joseph John Thompson

found that atoms could sometimes eject a far

smaller negative particle which he called an

ELECTRONELECTRON

J.J. Thomson, measured mass/charge of e-

(1906 Nobel Prize in Physics)

A = alphaB = gammaC = beta

gold foil

helium nuclei

CHARGE OF AN ELECTRON

Millikan oil drop experiment

HISTORY OF THE ATOMHISTORY OF THE ATOM

1910 Ernest Rutherford

oversaw Geiger and Marsden carrying out his

famous experiment.

they fired Helium nuclei at a piece of gold foil

which was only a few atoms thick.

they found that although most of them

passed through. About 1 in 10,000 hit

Rutherford’s experiment.

Plum Pudding model of an atom.

Results of foil experiment if Plum Pudding model had been

correct.

Actual Results.

A nuclear atom viewed in cross section.

atomic radius ~ 100 pm = 1 x 10-10 m

nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m

Rutherford’s Model of the Atom

Atomic Structure

Atoms are composed of-protons – positively charged particles-neutrons – neutral particles-electrons – negatively charged particles

Protons and neutrons are located in the nucleus. Electrons are found in orbitals surrounding the nucleus.

HELIUM ATOM

+N

N

+-

-

proton

electron neutron

Shell

Atomic StructureAtomic StructureEvery different atom has a

characteristic number of protons in the nucleus.

atomic number = number of protons

Atoms with the same atomic number have the same chemical properties and belong to the same element.

Atomic StructureAtomic StructureEach proton and neutron has a mass

of approximately 1 dalton.

The sum of protons and neutrons is the atom’s atomic mass.

Isotopes – atoms of the same element that have different atomic mass numbers due to different numbers of neutrons.

ATOMIC STRUCTUREATOMIC STRUCTURE

the number of protons in an atom

the number of protons and neutrons in an atom

22

44Atomic mass

Atomic number

number of electrons = number of protons

ATOMIC NUMBER (Z) = number of protons in nucleus

MASS NUMBER (A) = number of protons + number of neutrons

= atomic number (Z) + number of neutrons

ISOTOPS are atoms of the same element (X) with different numbers of neutrons in the nucleus

XAZ

H11 H (D)

21 H (T)

31

U23592 U238

92

Mass Number

Atomic NumberElement Symbol

Atomic Structure

Atomic StructureAtomic Structure

Two isotopes of sodium.

HISTORY OF THE ATOMHISTORY OF THE ATOM

1913 Niels Bohr

studied under Rutherford at the Victoria

University in Manchester.

Bohr refined Rutherford's idea by adding

that the electrons were in orbits. Rather

like planets orbiting the sun. With each

orbit only able to contain a set number of

electrons.

MULTIELECTRON ATOMSMULTIELECTRON ATOMS

ELECTRONS IN ORBIT ABOUT ELECTRONS IN ORBIT ABOUT

THE NUCLEUSTHE NUCLEUS

An ion is formed when an atom, or group of atoms, has a net positive or negative charge (why?).If a neutral atom looses one or more electronsit becomes a cation.

If a neutral atom gains one or more electronsit becomes an anion.

Na11 protons11 electrons Na+ 11 protons

10 electrons

Cl17 protons17 electrons Cl-

17 protons18 electrons