Learning StandardsLearning Standards

• Atomic Structure

Broad Concept: Atomic models are used to explain atoms and help us understand the interaction of elements and compounds observed on a macroscopic scale.

WWHAT HAT ISIS ANAN ATOATOMM ? ?

An atom a fundamental piece of matter. (Matter is anything that can be touched physically.) Everything in the universe (except energy) is made of matter, and, so, everything in the universe is made of atoms.

An atom itself is made up of three tiny kinds of particles called subatomic particles: protons, neutrons, and electrons. The protons and the neutrons make up the center of the atom called the nucleus and the electrons fly around above the nucleus in a small cloud. The electrons carry a negative charge and the protons carry a positive charge. In a normal (neutral) atom the number of protons and the number of electrons are equal. Often, but not always, the number of neutrons is the same, too.

WWhat is inside the ato

hat is inside the atomm??

Discovery of ProtonsDiscovery of Protons

• Eugene Goldstein noted streams of positively charged particles in cathode rays in 1886.–Particles move in opposite direction of

cathode rays. –Called “Canal Rays” because they passed

through holes (channels or canals) drilled through the negative electrode.

1.)Canal rays must be positive.2.)Goldstein postulated the

existence of a positive fundamental particle called the “proton”.

Passing an electric current makes a beam appear to move from the negative to the positive end.

TThomson’s Experiment Andhomson’s Experiment AndDiscovery of ElectronsDiscovery of Electrons

Voltage source +-

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field he By adding an electric field he found that the moving pieces found that the moving pieces were negative.were negative.

+

-

The electron was The electron was discovered in 1897 by discovered in 1897 by Thomson. He imagined Thomson. He imagined the atom as a “raisin the atom as a “raisin pudding” with electrons pudding” with electrons stuck in a cake of stuck in a cake of positive charge.positive charge.

J.J. Thomson’s Model of J.J. Thomson’s Model of AtomAtom

• Plum Pudding Model, 1896

• Thought an atom was like plum pudding– Dough was cloud– Raisins were

electrons– Didn’t know about

neutrons at this time

Rutherford’s experiment andRutherford’s experiment anddiscovery of nucleusdiscovery of nucleus

• English physicist Ernest Rutherford (1911)

• Shot alpha particles at fluorescent screen.

• When an alpha particle hits a fluorescent screen, it glows.

Lead block

Uranium

Gold Foil

Fluorescent Screen

What He Expected

He ExpectedHe Expected

TThe alpha particles to pass through he alpha particles to pass through without changing direction very much.without changing direction very much.

He thought the mass was evenly distributed in the atom

The Nuclear AtomThe Nuclear Atom

Since some particles were deflected at large angles, Thomson’s model could not be correct.

How Rutherford explained results…..

• Atom is mostly empty space.

• Small dense, positive piece at center is (NUCLEUS)

RUTHERFORD’S MODEL OF AN RUTHERFORD’S MODEL OF AN ATOMATOM

In 1912, Rutherford discovered the nucleus by doing scattering experiments. He concluded the atom was mostly empty space, with a large dense body at the center, and electrons which orbited the nucleus like planets orbit the Sun.

The model created by Rutherford had still some serious discordance. According to the classic science, electron moving around the nucleus should emit an electromagnetic wave. Electron should than move not by the circle but helical and finally collide with the nucleus. But atom is stable.

                                                                                                                           

Rutherford also realized that the nucleus must contain both neutral and positively charged particles. The neutron was then discovered in 1932 by Chadwick.

Bohr’s Model of the AtomBohr’s Model of the Atom• Similar to

Rutherford’s model• Thought atom was

mostly empty space• Neils Bohr, 1913

– Nucleus in center is dense, positively charge

– Electrons revolve around the nucleus.

Following Rutherford’s planetary model of the atom, it was realized that the attraction between the electrons and the protons should make the atom unstableBohr proposed a model in which the electrons would stably occupy fixed orbits, as long as these orbits had special quantized locations

Subatomic Subatomic ParticlesParticles

Electron

Proton

Neutron

Name Symbol Charge

Actual mass (g)

e-e-

p+p+

n0n0

9.11 x 10-28

1.67 x 10-24

1.67 x 10-24

BBoohhrr’’s s mmooddeell• Electrons move around the nucleus at

stable orbits without emitting radiation.

• Electron in one of these stable orbit has a definite energy.

• Energy is radiated only when electrons make transitions from high energy orbit to a low energy orbit.

In the Bohr model, the electron can change orbits, accompanied by the absorption or emission of a photon of a specific color of light.

Bohr’s Model

1913 – Atomic Model with Fixed Orbits proposed –

The Atom has No ChargeNumber of Protons= Number of Electrons

An easy way to calculate the total number of electrons that can be held by a given energy level is to use the formula 2*n2 ,      

Principle energy level ( n )

Maximum number of electrons 2*n2

1 2

2 8

3 18

4 32

5 50

6 72

7 98

The most electrons possible in the first shell are 2. After the first shell is filled, the second shell starts filling up, according to the number of positive charges in the nucleus. The most allowed in the second shell is 8 electrons. Then the third shell starts to fill.