Learning 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.,

–Recognize discoveries from Dalton (atomic Recognize discoveries from Dalton (atomic theory), Thomson (the electron), Rutherford (the theory), Thomson (the electron), Rutherford (the nucleus), and Bohr (planetary model of atom) and nucleus), and Bohr (planetary model of atom) and understand how these discoveries lead to the understand how these discoveries lead to the modern theory.modern theory.

–Describe Rutherford’s “gold foil” experiment Describe Rutherford’s “gold foil” experiment that led to the discovery of the nuclear atom. that led to the discovery of the nuclear atom. Identify the major components (protons, Identify the major components (protons, neutrons, and electrons) of the nuclear atom and neutrons, and electrons) of the nuclear atom and explain how they interact.explain how they interact.

–Write the electron configurations for the first Write the electron configurations for the first twenty elements of the periodic table.twenty elements of the periodic table.

All matter is composed of atomsAtoms cannot be made or destroyedAll atoms of the same element are identicalDifferent elements have different types of atomsChemical reactions occur when atoms are rearrangedCompounds are formed from atoms of the constituent elements

Atomic theory proposed by John Dalton

Atom Definition

Atom

What is inside the atom?What is inside the atom?

Discovery 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.

Canal rays must be positive.Goldstein postulated the existence of a positive fundamental particle called the “proton”.

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

Thomson’s Experiment AndThomson’s Experiment AndDiscovery of ElectronsDiscovery of Electrons

Voltage source +-

Voltage source

Thomson’s ExperimentThomson’s Experiment

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

+

-

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

J.J. Thomson’s Model of Atom

• 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 anddiscovery 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

The alpha particles to pass through without The alpha particles to pass through without changing direction very much.changing direction very much.

What he got

He thought the mass was evenly distributed in the atom

The 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 ATOM

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 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

Parts of an AtomEach element has a different number of protons in its nucleus

Protons have positive chargeChange the number of protons change elements This is called nuclear physics

The element also has the same number of electrons

Electrons have negative chargeChange the number of electrons ionize the element This is called chemistry

Some elements also have neutrons Neutrons have no chargeThey are in the nuclei of atoms

p

n

e

Subatomic particles

Electron

Proton

Neutron

Name Symbol Charge

Actual mass (g)

e-

p+

n0

9.11 x 10-28

1.67 x 10-24

1.67 x 10-24

Bohr’s model• 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 Charge

Number 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.                                                                                  

Study Buddy Review

• Name three subatomic particles.

• Who is credited with discovering each particle?

• Describe the Rutherford Gold Foil Experiment.