Radioactivity

Question 1

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Question 2

What discovery in 1834 suggested that matter was electrical in nature?

Answer 2

“Michael Faraday’s finding that chemical changes occur when an electric current is sent through certain solutions (electrolysis).”

Question 3

What carries the electric current through a solution in electrolysis?

Answer 3

“Atoms or ions in solution.”

Question 4

What did experiments with gas discharge tubes reveal?

Answer 4

“That gas under low pressure and high electric potential conducts electric current.”

Question 5

What are some of the models proposed to explain the structure of the atom?

Answer 5

“The Thomson

Question 6

What did the discovery of radioactivity by Henry Becquerel in 1891 show?

Answer 6

“That atoms are complex rather than ‘indivisible’.”

Question 7

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Question 8

Who is generally credited as the father of atomic theory?

Answer 8

“John Dalton (1766-1844).”

Question 9

What did the early Greek philosophers teach about matter?

Answer 9

“That it is composed of atoms and is finitely divisible.”

Question 10

How did John Dalton view the atom?

Answer 10

“As an ‘indestructible’

Question 11

What discovery challenged Dalton’s view of the atom as ‘indivisible’?

Answer 11

“The discovery of radioactivity by Henry Becquerel (1891).”

Question 12

What did radioactivity reveal about atoms?

Answer 12

“That they are not ‘indivisible’ or ‘indestructible’ and can disintegrate

Question 13

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Question 14

What did William Crookes’ discovery of cathode rays in 1895 reveal?

Answer 14

“That negatively charged electrons were components of the atom. “

Question 15

What was established about matter by 1900?

Answer 15

“That it consists of atoms. “

Question 16

What was known about the structure of the atom by 1900?

Answer 16

“That atoms contain electrons but are electrically neutral overall. “

Question 17

What did the electrical neutrality of the atom imply?

Answer 17

“That there must be positively charged components within the atom to balance the negatively charged electrons. “

Question 18

What was J.J. Thomson’s atomic model?

Answer 18

“A homogeneous sphere of positive charge with negatively charged electrons embedded within it. “

Question 19

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Question 20

What did J.J. Thomson determine about electrons?

Answer 20

“The ratio of their charge to mass (e/m) and found it to be identical for all cathode-ray particles.”

Question 21

What did Rutherford’s experiments contradict?

Answer 21

“Sir J.J. Thomson’s model.”

Question 22

What type of particles did Rutherford use in his experiments?

Answer 22

“Positively charged alpha (α) particles.”

Question 23

What did Rutherford observe when he directed alpha particles at a thin sheet of metal foil?

Answer 23

“Most of the alpha-particles passed through without deflection

Question 24

How did Rutherford explain the scattering of alpha-particles?

Answer 24

“As a repulsion from a heavy

Question 25

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Question 26

What did Rutherford propose in his planetary model of the atom?

Answer 26

"That the atom consists of a positively charged heavy core called the nucleus

Question 27

What is another name for Rutherford's model?

Answer 27

"The Nuclear Model"

Question 28

What does the nucleus contain?

Answer 28

"Protons and neutrons."

Question 29

What are protons?

Answer 29

"Positively charged particles found in the nucleus."

Question 30

What are neutrons?

Answer 30

"Neutrally charged particles found in the nucleus."

Question 31

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Question 32

What problem did Rutherford's model have according to Newtonian Physics?

Answer 32

"It predicted that the atom would collapse as electrons spiraled into the nucleus."

Question 33

Why would Rutherford's atom collapse?

Answer 33

"Because there is an attractive force between the oppositely charged nucleus and electrons

Question 34

What did experiments indicate about atoms in their normal state?

Answer 34

"That they neither collapsed nor emitted radiation."

Question 35

What were the two main difficulties of the Rutherford model?

Answer 35

"1. It predicted a continuous range of emitted light frequencies

Question 36

What did Neils Bohr propose to modify Rutherford's model?

Answer 36

"The idea of quantized energy states."

Question 37

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Question 38

What did Neils Bohr suggest in his model of the hydrogen atom?

Answer 38

"That the electron moves around the nucleus in specific circular orbits (energy levels) where the centrifugal force balances the electrostatic attraction

Question 39

What did Bohr call the possible orbits?

Answer 39

"Stationary states."

Question 40

How does the energy of an electron relate to its orbit's distance from the nucleus?

Answer 40

"The higher the electron's energy

Question 41

What is quantized in Bohr's model regarding electron energy?

Answer 41

"The energy of an electron

Question 42

How did Bohr describe the way electrons lose energy?

Answer 42

"In quantum 'jumps'

Question 43

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Question 44

What happens when an electron jumps from one stationary state to another of lower energy?

Answer 44

"A single photon of light is emitted."

Question 45

What is the formula for the energy of the emitted photon?

Answer 45

"hf = E_u - E_l (where h is Planck's constant

Question 46

What could Bohr's model account for that Rutherford's could not?

Answer 46

"The appearance of line spectrum rather than a continuous spectrum."

Question 47

According to Bohr's model, what happens when an electron absorbs energy?

Answer 47

"It transfers to a higher energy level (excitation)."

Question 48

What does an electron emit when it moves to a lower energy level?

Answer 48

"A photon."

Question 49

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Question 50

What was the third postulate of Bohr's model regarding angular momentum?

Answer 50

"That the angular momentum values of the electron are quantized

Question 51

What is the formula for the quantized angular momentum (L) in Bohr's model?

Answer 51

"L = n(h/2π)

Question 52

What is the integer 'n' called in the angular momentum formula?

Answer 52

"A quantum number."

Question 53

What is a key point in modern atomic theory first proposed by Neils Bohr?

Answer 53

"The quantization of angular momentum."

Question 54

What is the Bohr model also known as?

Answer 54

"The Bohr-Rutherford model."

Question 55

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Question 56

What is one of the great successes of Bohr's theory?

Answer 56

"It provides a model for why atoms emit line spectra and accurately predicts the wavelengths of emitted light for hydrogen."

Question 57

What does Bohr's theory offer an explanation for regarding spectra?

Answer 57

"Absorption spectra: photons of the right wavelength can knock an electron to a higher energy level."

Question 58

What is required for a photon to be absorbed by an electron in Bohr's model?

Answer 58

"It must have just the right energy to conserve energy."

Question 59

Why does a continuous spectrum passing through a gas have dark (absorption) lines?

Answer 59

"Because the gas absorbs photons of specific wavelengths

Question 60

How does Bohr's model ensure the stability of atoms?

Answer 60

"By stating that the ground state is the lowest state for an electron

Question 61

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Question 62

What did Bohr's model predict accurately for hydrogen?

Answer 62

"The ionization energy of 13.6 eV."

Question 63

What was the limitation of Bohr's theory?

Answer 63

"Though the idea of discrete energy levels applies to all atoms

Question 64

What is required to explain the spectra of more complex atoms?

Answer 64

"A more complex model than Bohr's."

Question 65

How does the Electron-Cloud Model visualize the atom?

Answer 65

"As a tiny nucleus of radius around 10^-15 m with electrons in rapid motion within a relatively large region around it

Question 66

In the Electron-Cloud Model, how is the electron viewed?

Answer 66

"Not as a ball revolving around the nucleus

Question 67

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Question 68

What do some chemists prefer to consider the electron as?

Answer 68

"A cloud of negative charges (electron cloud)

Question 69

What is the Electron-Cloud Model also known as?

Answer 69

"The cloud model"

Question 70

What does Fig. 9.6 illustrate?

Answer 70

"The electron density distribution of the electron cloud nearest the positive nucleus."

Question 71

Where is the electron most likely to be located according to the Electron-Cloud Model?

Answer 71

"Inside the spherical outline."

Question 72

What happens to the probability of finding the electron as the distance from the nucleus increases?

Answer 72

"The probability decreases rapidly."