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Atomic Structure
Name Period
Unit 2 – Worksheet 1 (Goals 1 – 3)
- Democritus, who lived in Greece during the 4 th^ century B.C., suggested that matter is made up of
tiny particles that cannot be divided. He called these particles atoms.
- Explain why the ideas of Democritus were not useful in a scientific sense.
The ideas of Democritus did not explain chemical behavior and lacked experimental support.
- The modern process of discovery about atoms began with the theories of an English school teacher named
John Dalton.
- Circle the letter of each sentence that is true about Dalton’s atomic theory.
a. All elements are composed of tiny, indivisible particles called atoms.
b. An element is composed of several types of atoms.
c. Atoms of different elements can physically mix together, or can chemically combine in simple whole number ratios to form compounds.
d. Chemical reactions occur when atoms are separated, joined, or rearranged; however, atoms of one element are never changed into atoms of another element when the atoms of elements A and B combine chemically.
- Which atomic particles carry a negative charge?
Electrons carry a negative charge.
- Thomson observed that the production of cathode rays did not depend on the kind of gas in the tube or the type of metal used in the electrodes. What conclusion did he draw from these observations?
Thomson concluded that electrons must be parts of the atoms of all elements.
- Explain Thomson’s Plum Pudding model of the atom.
The Plum Pudding model of the atom is a diffuse positively charged sphere with negatively charged particles dispersed throughout.
- How many units of positive charge remain if a hydrogen atom loses an electron? one
- The positively charged subatomic particle that remains when a hydrogen atom loses an electron is called a(n)
proton.
- Circle the letter of each sentence that is true about the nuclear theory of atoms suggested by Rutherford’s experimental results.
a. An atom is mostly empty space.
b. All the positive charge of an atom is concentrated in a small central region called the nucleus.
c. The nucleus is composed of protons.
d. The nucleus is large compared with the atom as a whole.
e. Nearly all the mass of an atom is in its nucleus.
- Describe Rutherford’s gold foil experiment and explain how his results improved upon Thomson’s Plum Pudding model of the atom.
Rutherford shot positively charged alpha particles at a very thin sheet of gold foil. Nearly all the alpha particles went through the gold foil (the atom is mostly empty space), relatively few of the alpha particles where deflected at wide angles (positively charged particles are present in the atom), and a very small number of alpha particles bounced straight back (there is a very dense region in the center of an atom).
Rutherford’s gold foil experiment led to the solar system model of the atom where electrons orbit a dense positive nucleus.
- Fill out the following table:
Name Symbol Charge Mass (amu)
electron e -^ -1 1/
proton p+^ +1 1
neutron n^0 0
- Would you expect two electrons to attract or repel each other? Why?
Two electrons would repel because electrons have a charge of negative 1 and like charges repel each other.
- What is the charge, positive or negative, of the nucleus of every atom?
The charge of the nucleus is positive because the nucleus is composed of protons and neutrons with a charge of positive 1 and zero, respectively. Therefore, the net charge of the nucleus is positive.
- Why is every atom electrically neutral?
Atoms are electrically neutral because they have the same number of electrons and protons.
- How can there be more than 1000 different atoms when there are only about 100 different elements?
Each element may have several different isotopes.
- How is the carbon-12 atom used to define atomic mass unit?
Atomic mass unit is 1/12 the mass of a carbon-12 atom.
- Distinguish among atomic mass and mass number.
atomic mass – weighted average of the masses of the isotopes of an element.
mass number – number of protons and the number of neutrons in the nucleus of an atom
- What data must you know about the isotopes of an element to calculate the atomic mass of the element?
In order to calculate the atomic mass of an element the mass and relative abundance of each isotope is needed.
- Element X has two naturally occurring isotopes. The isotope with mass 10.0 amu has a relative abundance of 20.0%. The isotope with a mass number of 11.0 amu has a relative abundance of 80.0%. Calculate the value of the atomic mass of element X. State the atomic number and true identity of element X.
10.0 amu x 0.200 + 11.0 amu x 0.800 = 10.8 amu
X = Boron w/ 5 protons
- The lithium found in a hearing aid battery has two naturally occurring isotopes. Lithium-6 has a mass of 6. amu and an abundance of 7.42%. Lithium-7 has a mass of 7.01 amu and an abundance of 92.58%. Calculate the atomic mass of lithium.
6.01 amu x 0.0742 + 7.01 amu x 0.9258 = 6.94 amu
Nuclear Chemistry
Name Period
Unit 2 – Worksheet 3 (Goals 6 – 8)
- Explain the difference between an isotope and a radioisotope.
isotope – stable p+^ to n^0 ratio
radioisotope – unstable p+^ to n^0 ratio, undergoes radioactive decay
- Complete the following table:
Type (Name) Alpha Beta Gamma Consists of 2 Protons and 2 Neutrons
Electron High-energy electromagnetic radiation Mass (amu) 4 1/1837 0
Symbol (^) ^ ^ ^ ^ γ
Penetrating Power (lo,hi,med)
lo med hi
Minimum shielding paper metal foil lead, complete
- Write the balanced nuclear equations for the following examples
a. Alpha emission of Po
214 84
→ (^) (^) +
b. Beta emission of Bi
210 83
→ (^) (^) +
c. Positron emission of 3370 As
^ →^ ^ +^
d. Neutron emission of Br
88 35
^ →^ ^ +^
e. Beta absorption of Sb
116 51
^ +^ ^ →^ +^
Unit 2 Review
Keys to Success:
- Review syllabus goals and key terms.
- Review class notes for Unit 3.
- Review all homework.
- Complete review and come in for extra help to clarify material if needed.
Key Terms: atom, electrons, neutrons, nucleus, protons, atomic number, mass number, atomic mass, atomic mass unit, isotope, radioisotope, alpha, beta, gamma, transmutation, fission, fusion, Dalton’s atomic theory,
The diagram above shows some of the results of Rutherford's gold foil experiment
- When Rutherford did his gold foil experiment most of the alpha particles went straight through the foil (path B). Some bounced straight back (path D). This was an astonishing result! From this Rutherford concluded that atoms a. are positively charged b. are mostly empty space c. have a very dense nucleus d. b and c
- In path A of Rutherford's experiment, the positively charged alpha particles changed their path without appearing to collide with a nucleus. Rutherford interpreted this to mean that atomic nuclei are a. positively charged, since positives repel each other b. negatively charged, since opposites attract c. are very dense d. are mostly empty space
- Who was the first person to suggest atoms, in the fourth century B. C.? a. Atomos b. Dalton c. Democritus d. Thomson
- Dalton’s atomic theory included what idea? a. All atoms of elements are the same size. b. Atoms of different elements always combine in one-to-one ratios. c. Atoms of the same element are always identical. d. Individual atoms can be seen with a microscope. e. Atoms of one element can be transformed into atoms of another element through chemical reactions.
- In the 1890's, J.J. Thomson did some crucial experiments using cathode ray tubes. The rays were attracted toward the positively charged end of the tube. He therefore concluded that whatever the ray was made of, it had to be a. positively charged b. negatively charged c. neutral in charge d. massive
- The atomic number of an element is the total number of which particles in the nucleus? a. neutrons b. protons c. electrons d. protons and electrons 7. The mass number of an element is equal to ____. a. the total number of electrons in the nucleus b. less than twice the atomic number c. a constant number for lighter elements d. the total number of protons and neutrons in the nucleus 8. Different elements have different numbers of ____. a. subatomic particles b. protons c. neutrons d. all of the above 9. How many neutrons are in the nucleus of iodine-131 (atomic number 53)? a. 53 b. 78 c. 127 d. 131 10. Uranium-235 atoms have how many protons (atomic number equals 92)? a. 92 b. 143 c. 235 d. 327
- In what ways are two isotopes of the same element different? Explain why isotopes of the same element have the same chemical behavior.
Isotopes of the same element have the same number of protons in their nucleus and a different number of neutrons. The isotopes have the same chemical behavior because they have they are chemically identical excluding the mass. Chemical behavior is primarily determined be the electrons.
- There are four naturally occurring isotopes of the element chromium. The relative abundance of each is: chromium-50 = 4.31%, chromium-52 = 83.76%, chromium-53 = 9.55%, and chromium-54 = 2.38%. Calculate the average atomic mass of chromium.
50 amu x 0.0431 + 52 amu x 0.8376 + 53 amu x 0.0955 + 54 amu x 0.0238 = 52.06 amu
- Explain why radioisotopes undergo radioactive decay.
Radioisotopes have an unstable p+/no^ ratio.
- Write a nuclear equation for the alpha decay of uranium-235.
(^) → (^) (^) +
- Write a nuclear equation for the beta decay of carbon-14.
(^) → (^) (^) +
- Three of the boxes in this diagram for Uranium-235 fission are missing some information. Write in the information required to make the boxes complete.
10 n---> 23592 U^10 n--->
10 n--->
10 n-------->
139 Rb
144Cs
B
Rb
10 n--->
23592 U 10 n--->
9436 Kr
