2. Atoms & Elements

In the gold foil experiment,

Bromine has two naturally occurring isotopes; 79Br (mass of 78.918) and 81Br (mass of 80.916). If the atomic weight of bromine is 79.904, predict the mass spectrum of a sample of bromine atoms. (LO 2.17) (a)

(b)

(c)

(d)

Discovering which of the three subatomic particles proved to be the most difficult—the proton, neutron, or electron? Why?

What fraction of the a particles in Rutherford's gold foil experiment are scattered at large angles? Assume the gold foil is two layers thick, as shown in Figure 2.9, and that the approximate diameters of a gold atom and its nucleus are 270 pm and 1.0 * 10–2 pm, respectively. Hint: Calculate the cross sectional area occupied by the nucleus as a fraction of that occupied by the atom. Assume that the gold nuclei in each layer are offset from each other.

Consider the mass spectrometer shown in Figure 2.11. Determine whether each of the following statements is true or false. If false, correct the statement to make it true: (b) The height of each peak in the mass spectrum is inversely proportional to the mass of that isotope.

Consider the mass spectrometer shown in Figure 2.11. Determine whether each of the following statements is true or false. If false, correct the statement to make it true: (a) The paths of neutral (uncharged) atoms are not affected by the magnet.

Mass spectrometry is more often applied to molecules than to atoms. We will see in Chapter 3 that the molecular weight of a molecule is the sum of the atomic weights of the atoms in the molecule. The mass spectrum of H2 is taken under conditions that prevent decomposition into H atoms. The two naturally occurring isotopes of hydrogen are 1H (atomic mass = 1.00783 u; abundance 99.9885%) and 2H 1atomic mass = 2.01410 u; abundance 0.0115%2. (c) Which peak will be the largest, and which the smallest?

Mass spectrometry is more often applied to molecules than to atoms. We will see in Chapter 3 that the molecular weight of a molecule is the sum of the atomic weights of the atoms in the molecule. The mass spectrum of H2 is taken under conditions that prevent decomposition into H atoms. The two naturally occurring isotopes of hydrogen are 1H (atomic mass = 1.00783 u; abundance 99.9885%) and 2H 1atomic mass = 2.01410 u; abundance 0.0115%2. (a) How many peaks will the mass spectrum have?

Which statements are consistent with Rutherford's nuclear theory as it was originally stated? Why? a. The volume of an atom is mostly empty space. b. The nucleus of an atom is small compared to the size of the atom. c. Neutral lithium atoms contain more neutrons than protons. d. Neutral lithium atoms contain more protons than electrons.

Which statements are inconsistent with Rutherford's nuclear theory as it was originally stated? Why? a. Since electrons are smaller than protons, and since a hydrogen atom contains only one proton and one electron, it must follow that the volume of a hydrogen atom is mostly due to the proton. b. A nitrogen atom has 7 protons in its nucleus and 7 electrons outside of its nucleus. c. A phosphorus atom has 15 protons in its nucleus and 150 electrons outside of its nucleus. d. The majority of the mass of a fluorine atom is due to its 9 electrons.

The atomic mass of fluorine is 18.998 amu, and its mass spectrum shows a large peak at this mass. The atomic mass of chlorine is 35.45 amu, yet the mass spectrum of chlorine does not show a peak at this mass. Explain the difference.

Use the mass spectrum of europium to determine the atomic mass of europium.

Use the mass spectrum of rubidium to determine the atomic mass of rubidium.

The natural abundance of 3He is 0.000137%. (c) Based on your answer to part (b), what would need to be the precision of a mass spectrometer that is able to differentiate between peaks that are due to 3He+ and 3H+?

What discovery about atomic structure was made from the results of Rutherford's gold foil experiment?

Prior to Rutherford's gold foil experi-ment, the 'plum pudding' model of the atom represented atomic structure. In this model, the atom is composed of elec-trons interspersed within a positive cloud of charge. If this were the correct model of the atom, predict how the results of Rutherford's experiment would have been different. (a) The alpha particles would pass right through the gold foil with little to no deflection. (b) Most of the alpha particles would be deflected back toward the source. (c) Most of the alpha particles would be absorbed by the atom and neither pass through nor be deflected from the gold foil.

There are two different isotopes of bromine atoms. Under normal conditions, elemental bromine consists of Br2 molecules, and the mass of a Br2 molecule is the sum of the masses of the two atoms in the molecule. The mass spectrum of Br2 consists of three peaks: Mass (u) Relative Size 157.836 0.2569 159.834 0.4999 161.832 0.2431 (e) Calculate the abundances of the two isotopes. Calculate the abundance of the heavier isotope.

There are two different isotopes of bromine atoms. Under normal conditions, elemental bromine consists of Br2 molecules, and the mass of a Br2 molecule is the sum of the masses of the two atoms in the molecule. The mass spectrum of Br2 consists of three peaks: Mass (u) Relative Size 157.836 0.2569 159.834 0.4999 161.832 0.2431 (a) What is the origin of second (mass of 159.834 amu) peak (of what isotopes does each consist)?

There are two different isotopes of bromine atoms. Under normal conditions, elemental bromine consists of Br2 molecules, and the mass of a Br2 molecule is the sum of the masses of the two atoms in the molecule. The mass spectrum of Br2 consists of three peaks: Mass (u) Relative Size 157.836 0.2569 159.834 0.4999 161.832 0.2431 (c) Determine the average molecular mass of a Br2 molecule.

There are two different isotopes of bromine atoms. Under normal conditions, elemental bromine consists of Br2 molecules, and the mass of a Br2 molecule is the sum of the masses of the two atoms in the molecule. The mass spectrum of Br2 consists of three peaks: Mass (u) Relative Size 157.836 0.2569 159.834 0.4999 161.832 0.2431 (a) What is the origin of first (mass of 157.836 amu) peak (of what isotopes does each consist)?

It is common in mass spectrometry to assume that the mass of a cation is the same as that of its parent atom. (b) What percentage of the mass of an 1H atom does the electron represent?

Use the mass spectrum of lead to estimate the atomic mass of lead. Estimate the mass and percent intensity values from the graph to three significant figures.

Use the mass spectrum of mercury to estimate the atomic mass of mercury. Estimate the masses and percent intensity values from the graph to three significant figures.

The molecular weight of ethylene glycol is 62.0689 when calculated using the atomic weights found in a standard periodic table, yet the molecular weight determined experimentally by high-resolution mass spectrometry is 62.0368. Explain the discrepancy.

Refer to Figure 2.10 showing a schematic illustration of a mass spectrometer. (b) Compare two ions with a +1 charge traveling through the curved, evacuated tube in the mass spectrometer. Will a heavier ion or lighter ion be deflected to a greater degree by the magnetic field?

Refer to Figure 2.10 showing a schematic illustration of a mass spectrometer. (a) What is the purpose of bombarding the gaseous atoms with an electron beam?

Use the data from the mass spectrum of a sample of an element to calculate the element's atomic weight. Identify the element.

Use the data from the mass spectrum of a sample of an element to calculate the element's atomic weight. Identify the element.

Rutherford's experiment with alpha particle scattering by gold foil established that