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All matter is made up of fundamental building blocks called atoms. The properties of matter are determined by the structure of the atoms within it. Within each atom are elementary particles: protons, neutrons, and electrons. Scattering experiments show that the nucleus is small, massive, and positively charged. The orbit of the electron around the nucleus is maintained by the electric force between the negatively charged electrons and the positively charged protons in the nucleus. All atoms of a particular element have the same atomic number (and hence have the same number of protons and electrons), but they may be different isotopes depending on the number of neutrons in the nucleus. The atomic mass number of an isotope is the sum of the number of protons and neutrons. 
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atom, electron, proton, neutron, atomic number, elementary charge, nucleus, isotopes, mass number
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Review problems and questions |
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- Describe the evidence that demonstrates why we know the nucleus of an atom is small, is massive, and contains positive charge.
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The evidence for the small, massive, and positively charged nucleus comes from the Rutherford scattering experiment.
Small: Only a small fraction of the alpha particles shot at the gold foil were deflected; therefore, the nucleus is so small that only an occasional alpha particle interacts with it.
Positively charged: The nucleus scatters particles away from it, including nearly backward for the occasional alpha particle, which means that it has the same charge (positive) as the alpha particle.
Massive: The nucleus can scatter alpha particles strongly, including nearly backward. But even the highest energy alpha particles do not move the gold nuclei; therefore, the gold nucleus must be very massive. 
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- Describe the history of at least three changes in our understanding of the properties of the atom since 1850.
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Answers will vary but will likely include some of the following points:- J. J. Thomson’s discovery that cathode rays were made of tiny negatively charged particles provided the first hint that atoms had an internal structure. He hypothesized that these electrons came from inside of atoms.
- Rutherford’s scattering experiment showed that the nucleus was dense but also composed of many positively charged protons, which should repel each other.
- The neutron was discovered in 1932 by James Chadwick, indicating that the force holding the nucleus together must work for both positive particles (protons) and neutral particles (neutrons).
- Hideki Yukawa proposed a model for the nuclear force in 1934.
- The existence of quarks was proposed in 1964 by Murray Gell-Mann and George Zweig as the particles whose interactions are at the heart of the strong nuclear force.
- Quarks were experimentally confirmed in 1968 at the Stanford Linear Accelerator Center.
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- What property of atoms determines the overall organization of the periodic table of the elements?
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The periodic table of the elements is organized overall by the atomic number Z, which is the number of protons in the nucleus.
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- Compare the three primary elementary particles (protons, neutrons, and electrons) on the basis of their location within an atom, relative mass, and charge.
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Location: Protons and neutrons are in the nucleus, whereas electrons orbit around the nucleus.
Relative mass: Protons and neutrons have nearly the same mass; both are 1,836 times more massive than the electron.
Charge: Neutrons have no net charge (i.e., they are neutral); protons have a positive charge (1.6×10−19 C), while electrons have a negative charge equal in magnitude to that of the proton.
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- An element has a mass number A = 84 and a neutron number N = 48. Which element is this?
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Answer: krypton
Solution: To find the element we must find the atomic number Z.
From the equation A = N + Z we find that Z = A − N = 84 − 48 ⇒ Z = 36.
From the periodic table we see that the element with atomic number 36 is krypton (Kr).
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- An atom is made up of 19 protons and 20 neutrons. What is the total mass of this atom?
- 6.528×10−29 kg
- 6.528×10−26 kg
- 6.528×10−27 kg
- 3.350×10−26 kg
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b is correct. It is calculated from: 19 × (1.673×10−27 kg) + 20 × (1.675×10−27 kg) = 6.528×10−26 kg
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- The proton is approximately 1,836 times more massive than the electron. If we calculate the mass of an atom by considering only the mass of the protons and the neutrons, what is the percent error introduced in the calculation?
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Answer: 0.027%
Solution: First let’s assume that the mass of the neutron is equal to the mass of the proton, mp. Also, assume that there is approximately one neutron for every proton. Since Z is the number of protons in an atom, the ratio of its mass without electrons to its mass with electrons is The fractional error is then 100% − 99.973% = 0.027%.
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