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standards: 1e, 1hterms: 92mastering concept: 112 (34-46)

Homework Cornell notes: 4.2sec. assessment: 97(6-8)

Section 4-2Section 4.2 Defining the Atom (cont.)

atomcathode rayelectronnucleusprotonneutron

An atom is made of a nucleus containing protons and neutrons; electrons move around the nucleus.

Section 4-2The Atom

• The smallest particle of an element that retains the properties of the element is called an atom.

• An instrument called the scanning tunneling microscope (STM) allows individual atoms to be seen.

Section 4-2The Electron

• When an electric charge is applied, a ray of radiation travels from the cathode to the anode, called a cathode ray.

• Cathode rays are a stream of particles carrying a negative charge.

• The particles carrying a negative charge are known as electrons.

Section 4-2The Electron (cont.)

• This figure shows a typical cathode ray tube.

Section 4-2The Electron (cont.)

J.J. Thomson • measured the effects of both magnetic

and electric fields on the cathode ray to determine the charge-to-mass ratio of a charged particle, then compared it to known values.

• The mass of the charged particle was much less than a hydrogen atom, then the lightest known atom.

• Thomson received the Nobel Prize in 1906 for identifying the first subatomic particle—the electron

Section 4-2The Electron (cont.)

• In the early 1910s, Robert Millikan used the oil-drop apparatus shown below to determine the charge of an electron.

Section 4-2The Electron (cont.)

• Charges change in discrete amounts—1.602 10–19

coulombs, the charge of one electron (now equated to a single unit, 1–).

• With the electron’s charge and charge-to-mass ratio known, Millikan calculated the mass of a single electron.

the mass of a hydrogen atom

Section 4-2The Electron (cont.)

• Matter is neutral.• J.J. Thomson's plum pudding model of the

atom states that the atom is a uniform, positively changed sphere containing electrons.

Section 4-2The Nucleus

• In 1911, Ernest Rutherford studied how positively charged alpha particles interacted with solid matter.

• By aiming the particles at a thin sheet of gold foil, Rutherford expected the paths of the alpha particles to be only slightly altered by a collision with an electron.

Section 4-2The Nucleus (cont.)

• Although most of the alpha particles went through the gold foil, a few of them bounced back, some at large angles.

Section 4-2The Nucleus (cont.)

• Rutherford concluded that atoms are mostly empty space.

• Almost all of the atom's positive charge and almost all of its mass is contained in a dense region in the center of the atom called the nucleus.

• Electrons are held within the atom by their attraction to the positively charged nucleus.

Section 4-2The Nucleus (cont.)

• The repulsive force between the positively charged nucleus and positive alpha particles caused the deflections.

Section 4-2The Nucleus (cont.)

• Rutherford refined the model to include positively charged particles in the nucleus called protons.

• James Chadwick received the Nobel Prize in 1935 for discovering the existence of neutrons, neutral particles in the nucleus which accounts for the remainder of an atom’s mass.

Section 4-2The Nucleus (cont.)

• All atoms are made of three fundamental subatomic particles: the electron, the proton, and the neutron.

• Atoms are spherically shaped.

• Atoms are mostly empty space, and electrons travel around the nucleus held by an attraction to the positively charged nucleus.

Section 4-2The Nucleus (cont.)

• Scientists have determined that protons and neutrons are composed of subatomic particles called quarks.

Section 4-2The Nucleus (cont.)

• Chemical behavior can be explained by considering only an atom's electrons.

In 1839, Michael Faraday …

• Suggested the structure of atoms related to electricity somehow

• After many experiments – He proved that atoms contained particles that

have electrical charge

Benjamin Franklin …

• Lightning was a discharge of static electricity

• Electrical charges– Opposites attract– Same repel

Franklin (Cont.)…• Body that crosses a carpet picks up (-) charges

• These charges jump to a metal doorknob or another body in discharge of static electricity

• He concluded that lightning is the result of static electricity or sudden discharge or release of electricity between clouds or the ground

J.J. Thomson's Atomic Model • Watched the deflection of charges

in a cathode ray tube and put forth the idea that atoms are composed of positively charged protons and negatively charged electrons.

• said the atom is neutral• the atom must include an equal

amount of (+) charged particles

In 1897

J.J. Thomson Plum Pudding Model

• He assumed that negative charges are distributed (sprinkled) evenly throughout the atom’s positively charged atom like chocolate chips sprinkled throughout a blob of cookie dough.

Cathode Ray Tube …

• Called cathode ray tube because radiation comes streaming from cathode to anode

• Today, called television picture tube• Made of particle that carried a negative charge • Cathode ray could spin a small paddle of wheel

which suggested it was streams of particles

• Millikan, in 1909, calculated the mass of a single electron by examining the behavior of charged oil drop in an electric field.

• the electron has a mass of 9.11 x 10-28 gram (or 1/1837 the mass of the hydrogen atom).

• A proton weighs 1837 times the mass of a single electron.

In 1909, Rutherford concluded ….• Inside the nucleus are protons and neutrons

• all of the positive charge in an atom was concentrated in the center (nucleus)

• an atom is mostly empty space.

• nucleus contains most of the atom’s mass,

• and that the tiny, negatively charged electrons travel around this nucleus.

• The nucleus's size can be compared to a bee in a stadium

Scattered particles

Gold foilBeam of particles

Path of particles

Nucleus Gold foil

• Most particles passed through without deflection

• ~ 1 in 8000 particles were deflected• Foil scattered particles in all directions

He Explained …

• -Particles deflected because -particles are (+) and repelled by the (+) charged nucleus

He Concluded ….• -Particles pass through because most of

the atom is empty space• All of the atom’s (+) charge as well as its

mass is concentrated in the center of the atom

• He called it “the Nucleus”

Rutherford

Thomson

Mastering Concept 112 (34-46)

Mastering Concepts: 112 (34-46)34. Which subatomic particle was discovered by researchers working with cathode ray tubes? (4.2)34. the electron

Mastering Concepts: 112 (34-46)35. What experimental results led to the conclusion that electrons were part of all forms of matter? (4.2)

35. changing the type of electrode or the type of gas did not affect the ray produced

Mastering Concepts: 112 (34-46)• 36. What is the charge and mass of a

single electron? (4.2

• charge = - 1;• mass = 9.11X 10-28 g

Mastering Concepts: 112 (34-46)37. List the strengths and weaknesses of Rutherford’s nuclear model of the atom. (4.2)

37. Strengths: Rutherford’s model explained the results of the gold-foil experiment and why an atom is electrically neutral.

Weaknesses: The model could not account for the total mass of an atom or the arrangement of the electrons.

38. What particles are found in the nucleus of an atom? What is the net charge of the nucleus? (4.2)

38. protons and neutrons; net positive charge equal to the number of protons

39. Explain what keeps the electrons confined in the space surrounding the nucleus. (4.2)39. attraction to the positively charged nucleus

40. Describe the flow of a cathode ray inside a cathode ray tube. (4.2)

40. from cathode to anode

41. Which outdated atomic model could be likened to chocolate chip cookie dough? (4.2)

41. the plum pudding model

42. What caused the deflection of the alpha particles in Rutherford’s gold foil experiment? (4.2)42. The particles were deflected by the positively charged gold nuclei.

43. Which subatomic particles account for most all of an atom’s mass? (4.2)

43. protons and neutrons

44. How is an atom’s atomic number related to its number of protons? To its number of electrons? (4.2)

44. They are all equal.

45. What is the charge of the nucleus of element 89? (4.2)

45. 89+

46. Explain why atoms are electrically neutral. (4.2)

46. The number of positivelycharged protons equals the number of negatively charged electrons.