Chapter 28: The Atom

PHYSICS Principles and

Problems

BIG IDEAAn atom consists of a nucleus of protons and neutrons surrounded by one or more electrons.

CHAPTER

28 The Atom

Section 28.1 Bohr’s Model of the Atom

Section 28.2 The Quantum Model of the Atom

Atomic History

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Early Atomic Theory– Leucippus and Democritus = matter is composed

of tiny, indivisible (atomos) particles in constant motion

– Aristotle = believed that you could keep on dividing matter infinitely =there was no “smallest” part of matter

Their simple concept of an atom

• Iron atoms are hard and stuck together with hooks• Clay atoms are composed of ball and socket joint that makes them

soft and flexible• Cheese atoms were squishy

Law of Conservation of Mass

Even if matter changes shape or form, it’s mass stays the same

Dalton's Atomic Theory

1) All matter is made of indivisible and indestructible particle called “atoms”

2) All atoms of a given element are identical in mass and properties

3) Compounds are formed by a combination of two or more different kinds of atoms.

4) A chemical reaction is a rearrangement of atoms.

Cathode Ray tube

Gas filled tubes with electrodes (negatively charged) with each end emit light when electrical current passes through them. Later on they discovered that the tube also emitted light with a positive electrode. A ray heading in the opposite direction. This means that there is also a positive charge in matter.

CHAPTER

28 JJ Thomson

Consist of negatively charged electrons. But are known to be neutrally charged.

Each negatively-charged electron was paired with a positively-charged particle that followed it everywhere within the atom.

• Rutherford directed beams of alpha particles (which are the nuclei of helium atoms and hence positively charged) at thin gold foil to test this model and noted how the alpha particles scattered from the foil.

• ..\..\..\..\..\..\..\Tess\videos in physics\Quantum Theory\Rutherford's Experiment_ Nuclear Atom.mp4

• View page 753

SECTION28.1

Light scattering experiment

• In this model the positive material is concentrated in a small but massive region called the nucleus. The negative particles (electrons) must be around the outside preventing one atom from trespassing on its neighbors space to complete this model.

SECTION28.1

New Model of an atom

How are electrons arranged in an atom?

Hydrogen gas in discharged chamber. If you shoot electrons on it, it will give off light. A diffraction grating can be used to analyze the light. A discrete units of light is seen instead of rainbow of light

How are electrons arranged in an atom?

• If we shine through the discharged chamber, some parts of it would be absorbed.

• Emission Spectra: some lights that get through (Emitted)

• Absorption spectra: some lights that did not get through (absorbed)

Bohr’s Model of the Atom

• Lower level: gives off a photon of light

• Higher level: absorbs a photon of light

A hydrogen Atom showing its first two energy level

NOTE: For atoms to behave in this manner, it requires certain colors of light. A right amount of light to give them enough energy.

• The energy absorbed and emitted in an atom are constant.

• Through the emission spectrum, we can look at what is given off in any element or molecule which will help us identify what type of element it is.

The spectrum

Light rays invisible to the naked eye

Longest wavelength: infrared rays

Shortest wavelength: Ultraviolet rays

Hitting a Hydrogen atom with red light will make it jump to a higher energy level, going back to a lower energy level will make it release same color of absorbed light

Incorrect color of light will not give an effect on the atom

A purple light will give it an energy to go on a higher energy level and wil release the same color of light that it absorbed as it goes down to a lower energy level.

Importance of Absorption and Emission Spectra: To identify unknown samples, mixture of gases in a sample

Revising Rutherford’s Model

Any charged particle that is moving will be giving off an electromagnetic radiation and at it does that, it is going to lose all its energy and spiral down at the middle.

• atoms have quantized amounts of energy= energy level.

• An atom with the smallest allowable amount of energy is said to be in the ground state.

• Any energy level above the ground state is called an excited state.

SECTION28.1

Quantized Energy

Bohr’s model: Quantized energy levels

• Answer: Bohr suggested that electromagnetic energy is emitted when the atom changes from one stationary state to another. When an atom absorbs a photon, the atom’s energy increases by an amount equal to that of the photon.

Ephoton = hf

SECTION28.1 Emission spectrum and Bohr’s model

• The change in energy of the atom equals the energy of the emitted photon. Ephoton = Eatom

or

Ephoton = Ef - Ei

SECTION28.1

Quantized Energy

Energy of an Emitted Photon

Formula• Energy of a Hydrogen Atom •The total energy of an atom with principal quantum number n is equal to the product of −13.6 eV and the inverse of n2.

•Both the electron’s orbital radius and the energy of the atom are quantized.

Electron Orbital Radius in Hydrogen

Problem solving

• Solve problem # 1 to 4 on page 762 and problem # 5 to 7 on page 763