Atomic Structure – Part II

Signor Rinno D. Montales

Ernest Rutherford (1871-1937) British physicist, believed in the plum

pudding model of the atom. With his associate Hans Geiger(1882-1945)

a German physicist, studied the alpha particles emitted by radium which was isolated by Marie and Pierre Curie.

Alpha particles are found to be helium atoms with their electrons removed, positively charged and mass of 2500 times that of the electron.

a–Particleemitter

SlitDetecting screen

(a)

Gold foil

Rutherford’s Experimental Design

a–Particleemitter

SlitDetecting screen

(a)

Gold foil

a–Particleemitter

SlitDetecting screen

(a)

Gold foil

Lead block

Uranium

Gold Foil

Florescent Screen

Rutherford’s Hypotheses

The alpha particles to pass through without changing direction very much

Because most of the mass of the atom (positive charges) were spread. Alone they were not enough to stop the alpha particles

If the Thomson model were correct, all the alpha particles, travelling at high speeds and massive, would have passed through the metal foil undeflected or only slightly deflected

What he expected

Because

Because, he thought the mass was evenly distributed in the atom

Because, he thought the mass was evenly distributed in the atom

What he got

They observed that although majority of the alpha particles passed through undeflected, some were only slightly deflected, some were scattered by more than 90 degrees and a few by nearly 180 degrees or almost completely turned back

Atom consists of a very small nucleus surrounded by electrons. Rutherfordestimated the radius at 10-12 to 10-13 cm compared to radius of the atom of about 10-8 cm The nucleus contains most of the mass of the atom and all of its positive charge. Alpha particles are deflected bynucleus it if they get close enough at each

other

How he explained it

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Ernest Rutherford’s Model

(Nuclear Model of an Atom)

Bohr’s Model

Electrons move in circular orbits around the nucleus

Adopted Planck’s idea that energies are quantized

Three Phenomena Black-body radiation

Photoelectric effect

Emission spectra

ELECTROMAGNETIC SPECTRUM

The waves in the spectrum all travel at same speed through a vacuum but differ in the frequency and, therefore, wavelength.

LINE SPECTRUM Spectrum containing radiation of specific

wavelengths

Energy states of a Hydrogen Atom When a sample of gaseous H atoms is excited,

different atoms absorb different quantities of energy

Each atom has one electron, but so many atoms are present that all the energy levels (orbit) are populated by electrons

Ground state – lowest energy level (n = 1) Excited state - higher energy level (n= 2…)

When dropped from n = 3 orbit (second excited state) = infrared series or lines were emitted by photons –PASCHEN SERIES

When dropped from n = 2 orbit (first excited state) = visible series or lines – BALMER SERIES

When dropped from n = 1 orbit (ground state) = ultraviolet series or lines – LYMAN SERIES

An electron could jump from one allowed energy state to another by emitting or absorbing photons whose energy corresponds exactly to the energy difference between the two states.

ΔE = Ef – Ei

Line spectra are produced because these energy changes correspond to photons of specific wavelengths

The PARTICLE Nature of Light Blackbody Radiation Light given off by hot objects

Wavelength distribution of the radiation depends on temperature

“red-hot” object being cooler than a “white-hot” one

MAX PLANCK ( 1858-1947) Energy can be released or absorbed by atoms

only in discrete “chunks” of some minimum size.

Quantum – “fixed amount”, smallest amount of energy that can be mitted or absorbed as electromagnetic radiation.

Hot glowing object could emit (or absorb) only certain quantities of energy

E = hvE = energy of radiation

v= frequency

h= Planck’s constant ( 6.63 x 10-34 joule-seconds)

Hot object’s radiation is emitted by the atoms contained within it.

The atom itself can have only certain quantities of energy.

The energy is quantized- values are restricted only in certain quantities

CONTINUOUS QUANTIZED

Photoelectric Effect Emission of electrons from metal surfaces on

which light shines

ALBERT EINSTEIN (1905) Used Planck’s quantum theory to explain the

photoelectric effect

Radiant energy striking the metal surface is a stream of energy packets

PHOTON Behaves like a particle Has an energy proportional to

Energy of photon= E = hv

A photon transfers its energy to an electron in the metal.

A certain amount of energy is required to overcome the attractive forces that hold it within the metal.

Three postulates of Bohr Only orbits of certain radii, corresponding to certain

energies, are permitted for electrons in an atom called STATIONARY STATES. – fixed circular orbit

An electron in a permitted orbit has a specific energy and is in an “allowed” energy state. Electron will not radiate energy while moving within an orbit.

Energy is only emitted or absorbed by an electron as it changes from one allowed energy state to another. This energy is emitted or absorbed as a photon.

Ephoton = Estate A – Estate B = hv