FISIKA ATOM

Abdul Qodir

Standar Kompetensi

Menganalisis berbagai besaran fisis pada gejala kuantum dan batas-batas berlakunya relativitas Einstein dalam paradigma fisika modern.

Kompetensi Dasar

Mendeskripsikan perkembangan teori atom

Materi Pokok

Perkembangan Teori Atom Tingkat Energi Bilangan Kuantum Energi Ionisasi dan Afinitas

Elektron

Perkembangan Teori Atom

Democritus (460 – 370 BC) Aristotle (384 – 322 BC) Dalton (1766 – 1844) Thomson (1856 – 1940) Rutherford (1871 – 1937) Bohr (1885 – 1962)

Democritus (460-370 BC)

Proposed that matter is made of tiny particles and empty space

atoms = smallest part of matter different types of atom for each

type of matter

People considered Aristotle's opinions very important and if Aristotle thought the atomic idea had no merit, then most other people thought the same also.

For more than 2000 years nobody did anything to continue the explorations that the Greeks had started into the nature of matter.

In the 1800's an English chemist, John Dalton performed experiments with various chemicals that showed that matter, indeed, seem to consist of elementary lumpy particles (atoms).

http://antoine.fsu.umd.edu/chem/senese/101/atoms/dalton.shtml

Postulates

1. All matter consists of tiny particles.

2.Atoms are indestructible and unchangeable.

3.Elements are characterized by the mass of their atoms.

4.When elements react, their atoms combine in simple, whole-number ratios.

J. J. Thomson(1897)

Discovered the electron and proton

Vacuum Tube Experiment

Cathode–Ray Tube

Cathode Rays - JJ Thompson

Experiments performed by Thompson together with those of Robert Millikan were able to show that an electron has a mass of

9.1 x 10-28 g

The mass of one atom of the lightest element, hydrogen, is 1.7 x 10-24 g

An electron is approx 1/2000th the mass of a H atom!

Mass of an electron

So the electron appears to be an insignificant part of the atom - from a mass point of view!

How are electrons arranged inside an atom?

Thompson’s Plum Pudding Model of the Atom

electronDiffuse positivecharge

Ernest Rutherford (1871-1937)

Rutherford’s Gold Foil Experiment (1910)

Alpha particles (positively charged helium ions) from a radioactive source was directed toward a very thin gold foil.

A fluorescent screen was placed behind the Au foil to detect the scattering of alpha () particles.

Nucleus of the AtomRutherford (1871-1937)

P. 64Click on picture for movie

Rutherford’s Gold Foil Experiment (Observations)

Most of the -particles passed through the foil.

Many of the -particles deflected at various angles.

Surprisingly, a few particles were deflected back from the Au foil.

Nucleus of the Atom Continued

Rutherford’s Gold Foil Experiment (Conclusions)

Rutherford concluded that most of the mass of an atom is concentrated in a core, called the atomic nucleus.

The nucleus is positively charged. Most of the volume of the atom is

empty space.

RUTHERFORD’S MODEL

Rutherford knew that atoms consist of a compact positively charged nucleus, around which circulate negative electrons at a relatively large distance. The nucleus occupies less than one thousand million millionth of the atomic volume, but contains almost all of the atom's mass. If an atom had the size of the earth, the nucleus would have the size of a football stadium.

~ 10-10 m

nucleusMass 9 x 10Mass 9 x 10-31-31 kg kg Mass > 10Mass > 10-26-26 kg kg

electron

~ 1 – 7 x 10-15 m (1 – 7 fermi)

Microcosms of our solar system, atoms are dominantly empty space:

If an oxygen atom had a total radius of 100 km, the nucleus

would be a ~1 m diameter sphere in

the middle.

electron orbits

The Spacious Atom

In a simplistic model, electrons float around the nucleus in energy levels called shells.

As the number of

electrons increases,

they start to fill shells

farther out from the

nucleus.

In most cases,

electrons are lost or

gained only from the

outermost shell.

electron orbits

Electrons in Orbit

AtomAtom

NucleusNucleus

Shortfalls of Rutherford’s Model Did not explain where the atom’s

negatively charged electrons are located in the space surrounding its positively charged nucleus.

We know oppositely charged particles attract each other

What prevents the negative electrons from being drawn into the positive nucleus?

Niels Bohr

Niels Bohr (1885-1962), Danish scientist working with Rutherford

Proposed that electrons must have enough energy to keep them in constant motion around the nucleus

Analogous to the motion of the planets orbiting the sun

Planetary Model

The planets are attracted to the sun by gravitational force, they move with enough energy to remain in stable orbits around the sun.

Electrons have energy of motion that enables them to overcome the attraction for the positive nucleus

Atomic ModelsThomsonDalton

Rutherford

Bohr- Heisenberg