Chemistry Chapter 4 Test

.What is the charge of the nucleus of element 89?

89 +

If a nucleus contains 12 protons, how many electrons are in the neutral atom? Explain.

12; protons are the only charged particles in the nucleus. To balance their positive charge, there must be the same number of electrons as protons.

If an atom has 18 electrons, how many protons does it have?

18 protons

If you had a balance that could determine the mass of a proton, how many electrons would you need to weigh on the same balance to measure the same mass as that of a single proton?

1836

For each chemical symbol, determine the number of protons and electrons an atom of the element contains. .V . Mn . Ir .S

23, 23 . 25, 25 . 77, 77 . 16, 16

How many electrons, protons, and neutrons are contained in each atom? . gallium-69 . luorine-23 . titanium-48 . tantalum-181

31 electrons, 31 protons, 38 neutrons . 9 electrons, 9 protons, 14 neutrons . 22 electrons, 22 protons, 26 neutrons . 73 electrons, 73 protons, 108 neutrons

How many protons and electrons are contained in an atom of element 44?

44 protons, 44 electrons

A carbon atom has a mass number of 12 and an atomic number of 6. How many neutrons does it have?

6 neutrons

Use the elements labeled in Figure 23 to explain the direction of a cathode ray inside a cathode-ray tube.

A cathode ray is composed of a stream of electrons that originate at the cathode and flow through the tube to the anode.

Describe a cathode-ray tube and how it operates.

A cathode-ray tube has a metal electrode at each end. The electrodes are connected to a power supply. When current flows, electrons are emitted from the cathode and travel through the tube to the anode.

How is a qualitative observation different from a quantitative observation? Give an example of each.

A qualitative observation does not involve measurement (water is hot). A quantitative observation involves measurement (the water is 42°C).

Approximately how many times greater is the diameter of an atom than the diameter of its nucleus? Knowing that most of an atom's mass is contained in the nucleus, what can you conclude about the density of the nucleus?

An atom's diameter is about 10,000 times the diameter of its nucleus. The density of the nucleus must be large.

If atoms are primarily composed of empty space, explain why you cannot pass your hand through a solid object.

Atoms are extremely small and very close together in comparison to our hand. Atoms in a solid object are bonded together by electrical forces— bonds that are not easily broken.

Determine what was wrong with Dalton's theory and provide the most recent version of the atomic structure.

Atoms are indivisible and atoms of the same element can have different mass. An atomic structure consists of subatomic particles: electrons, protons and neutrons.

State the main points of Dalton's atomic theory using your own words. Which parts of Dalton's theory were later found to be erroneous? Explain why.

Atoms are not indivisible and all atoms of an element are not identical.

Boron-10 and boron-11 are the naturally occurring isotopes of elemental boron. If boron has an atomic mass of 10.81 amu, which isotope occurs in greater abundance?

B-11 must occur in greater abundance because the atomic weight of bromine is much closer to the mass of B-11 than to the mass of B-10.

Who originally proposed the concept that matter is composed of tiny, indivisible particles?

Democritus

Distinguish between Democritus's ideas and Dalton's atomic theory.

Democritus's ideas: matter is composed of empty space through which atoms move; different kinds of atoms have different sizes and shapes; the movement, size, and shape of atoms result in unique properties of matter; changes in matter result from changes in the groupings of atoms. Dalton's atomic theory: matter is composed of atoms; different atoms combine in simple whole number ratios to form compounds; atoms cannot be created, destroyed, or divided.

How were scientific methods used to determine the model of the atom? Why is the model considered a theory?

Experiments were performed to explain observations and test hypotheses. It is a theory because it is still subject to modifications as more data become available.

Gallium, which has an atomic mass of 69.723 amu, has two naturally occurring isotopes, Ga-69 and Ga-71. Which isotope occurs in greater abundance? Explain.

Ga-69 must be more abundant because the atomic mass of gallium is closer to the mass of Ga-69 than the mass of Ga-71.

What was Aristotle's objection to the atomic theory?

He did not believe that atoms could move through empty space.

Explain why Democritus was unable to experimentally verify his ideas.

He had no scientific instruments to research matter at the atomic level.

Was Democritus's proposal of the existence of atoms based on scientific methods or ideas?

Ideas. He had no experimental evidence.

What is the approximate size of an atom?

It is in the range of 1010 m.

Whose work is credited with being the beginning of modern atomic theory?

John Dalton

Explain how Dalton's atomic theory offered a convincing explanation of the observation that mass is conserved in chemical reactions.

Mass is conserved because atoms cannot be created, divided, or destroyed. Chemical reactions involve only the separation and rearrangement of atoms.

Define matter and give two everyday examples.

Matter is anything that occupies space and has mass. For example: desk, chair.

What was Henry Moseley's contribution to the modern understanding of the atom?

Moseley discovered that each element contains a unique positive charge (or number of protons) in its nucleus. Thus, the number of protons in an atom's nucleus uniquely identifies it as an atom of a particular element.

What is radioactive decay?

Radioactive decay occurs when unstable nuclei spontaneously (without input of energy) lose energy by emitting radiation.

Explain how energy loss and nuclear stability are related to radioactive decay.

Radioactivity results when unstable nuclei emit energy in order to gain stability.

Briefly explain how Rutherford discovered the nucleus.

Rutherford aimed a beam of alpha particles at a thin foil of gold. He expected most of the alpha particles to pass through the gold atoms, confirming the plum pudding model. However, a few particles were deflected at very large angles, which led to the discovery of the positively charged nucleus.

How did the actual results of Rutherford's gold foil experiment differ from the results he expected?

Rutherford expected particles to be slightly deflected when passing through a gold foil. But some particles were deflected at very large angles.

What experiment led to the dispute of J. J. Thomson's plum pudding atomic model? Explain your answer.

Rutherford's thin gold foil experiment. His experimental results were inconsistent with the plum pudding atomic model.

What technique can be used to visualize individual atoms?

STM

What are the strengths and weaknesses of Rutherford's nuclear model of the atom?

Strengths: explained the results of the gold-foil experiment and why an atom is electrically neutral. Weaknesses: could not account for the total mass of an atom or the arrangement of the electrons.

Sulfur's average atomic mass is close to the whole number 32. Chlorine's average atomic mass is 35.453, which is not a whole number. Suggest a possible reason for this difference.

Sulfur has an isotope with a very high abundance. Chlorine has more than one isotope with a large percent abundance.

What caused the deflection of the alpha particles in Rutherford's gold foil experiment?

The alpha particles were deflected by the positively charged gold nuclei.

How was an electric field used to determine the charge of a cathode ray?

The cathode ray was attracted to the positive end of the magnet implying that the cathode ray is negative.

Is the charge of a nucleus positive, negative, or zero? The charge of an atom?

The nucleus is positively charged, whereas the atom is neutral.

Which is greater, the number of compounds or the number of elements? The number of elements or the number of isotopes? Explain.

The number of compounds is greater than the number of elements because compounds are combinations of elements and the elements can be combined in many ways. The number of isotopes is greater than the number of atoms because each element has only one type of atom but may have more than one isotope.

Explain why atoms are electrically neutral.

The number of protons equals the number of electrons.

Indium has two naturally occurring isotopes and an atomic mass of 114.818 amu. In-113 has a mass of 112.904 amu and an abundance of 4.3%. What is the identity and percent abundance of indium's other isotope?

The other isotope is In-115. Its percent abundance is 95.7%.

Why are some atoms radioactive?

The stability of atoms depends on their neutron-toproton ratio. When this ratio is either too large or too small the nucleus of an atom becomes unstable causing an atom to be radioactive.

An atom's nucleus has 92 protons and its mass number is 235. How many neutrons are in the nucleus? What is the name of the atom?

There are 143 neutrons. Uranium.

How is an atom's atomic number related to its number of protons? To its number of electrons?

They are all equal.

How did the charge distribution in the plum pudding model affect alpha particles passing through an atom?

They would only be slightly deflected.

Explain how J. J. Thomson's determination of the charge-to-mass ratio of the electron led to the conclusion that atoms were composed of subatomic particles.

Thomson showed that the electron's mass was less than the mass of hydrogen, the lightest atom. This showed that there were smaller subatomic particles. Atoms are divisible.

Magnesium constitutes about 2% of Earth's crust and has three naturally occurring isotopes. Suppose you analyze a mineral and determine that it contains the three isotopes in the following proportions: Mg-24 (abundance = 79%), Mg-25 (abundance = 10%), and Mg-26 (abundance = 11%). If your friend analyzes a different mineral containing magnesium, do you expect her to obtain the same relative abundances for each magnesium isotope? Explain your reasoning.

Yes. The isotopic abundance of an element is the same no matter where the element comes from.

Does the existence of isotopes contradict part of Dalton's original atomic theory? Explain.

Yes; not all atoms of an element are identical in mass.

An element has three naturally occurring isotopes. What other information must you know in order to calculate the element's atomic mass?

You also need to know the mass and percent abundance of each isotope.

Label the subatomic particles shown in Figure 22.

a. electron cloud b. protons c. neutron

Define alpha particle, beta particle, and gamma ray.

alpha particle: helium atom with a 2+ charge; beta particle: electron; gamma rays: high energy radiation

Define the atomic mass unit. What were the benefits of developing the atomic mass unit as a standard unit of mass?

amu = 1/12 of the mass of a C-12 atom; It's a relative standard that is closer in size to atomic and subatomic masses.

Describe how each type of radiation affects an atom's atomic number and mass number.

atomic number decreases by 2, mass number decreases by 4; : atomic number increases by 1, mass number unchanged; : atomic number and mass number are unchanged

Explain what keeps the electrons confined in the space surrounding the nucleus.

attraction to the positively charged nucleus

Why are electrons in a cathode-ray tube deflected by electric fields?

because they are charged

Discuss how radioactive atoms gain stability.

by emitting radiation or a particle

What experimental results led to the conclusion that electrons were part of all forms of matter?

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

How do isotopes of a given element differ? How are they similar?

differ: number of neutrons, masses; similar: chemical properties, number of protons and electrons

Arrange the following subatomic particles in order of increasing mass: neutron, electron, and proton.

electron < proton = neutron

How was the overall charge distributed in the plum pudding model?

evenly through a sphere

What is the mass number of potassium-39? What is the isotope's charge?

mass number = 39; charge = 0

How is the mass number related to the number of protons and neutrons an atom has?

mass number = number of p + number of n

What change in mass number occurs when a radioactive atom emits an alpha particle? A beta particle? A gamma particle?

mass number decreases by 4; , no change in mass number; , no change in mass number

An isotope of xenon has an atomic number of 54 and contains 77 neutrons. What is the xenon isotope's mass number?

mass number is 131

An isotope of mercury has 80 protons and 120 neutrons. What is the mass number of this isotope?

mass number is 200

What type of reaction involves changes in the nucleus of an atom?

nuclear reaction

How can you determine the number of neutrons in an atom if its mass number and its atomic number are known?

number of n0 = mass number - atomic number

Which particles account for most of an atom's mass?

protons and neutrons

What particles are found in the nucleus of an atom? What is the charge of the nucleus?

protons and neutrons; positive charge equal to the number of protons

Explain what must occur before a radioactive atom stops undergoing further radioactive decay.

stable, nonradioactive atom must be formed

What do the superscript and subscript in the notation represent?

superscript = mass number and subscript = atomic number

Which subatomic particle was discovered by researchers working with cathode-ray tubes?

the electron

What is the primary factor that determines whether a nucleus is stable or unstable?

the neutron-to-proton ratio