Answer Sheet Quarterly Review Questions

Transcription

1 Answer Sheet Quarterly Review Questions 1. Compared to the charge and mass of a proton, an electron has a. the same charge and a smaller mass b. the same charge and the same mass c. an opposite charge and a smaller mass d. an opposite charge and the same mass Explanation: Protons are positively charged and have a mass of amu; electrons are negatively charged and have a mass of amu. See Table O. 2. When alpha particles are used to bombard gold foil, most of the alpha particles pass through undeflected. This result indicates that most of the volume of a gold atom consists of. a. deuterons b. neutrons c. protons d. unoccupied space Explanation: Because most of the particles do not hit anything (pass right through), one can infer the atom is mostly empty space. See hyperlink for more details. 3. A proton has approximately the same mass as a. a neutron b. an alpha particle c. a beta particle d. an electron Explanation: A proton has the same mass as a neutron. An electron has the same mass as a beta particle. An alpha particle has a mass of 4: two protons plus two neutrons. See Ref. Table O. 4. When electrons in an atom in an excited state fall to lower energy levels, energy is a. absorbed, only b. released, only c. neither released nor absorbed d. both released and absorbed Explanation: The ground state is the lowest energy state. To raise the atom to an excited state, energy must be added. To fall back to the ground state (lowest energy), the energy it had absorbed must be released. 5. A neutron has approximately the same mass as a a. an alpha particle b. a beta particle c. an electron d. a proton Explanation: See Reference table O. Protons and neutrons each have a mass of 1 amu. 6. Which symbols represent atoms that are isotopes? a. C-14 and N-14 b. O-16 and O-18 c. I-131 and I-131 d. Rn-222 and Ra-222 Explanation: Isotopes are atoms of the same element (same atomic number) but with a different number of neutrons (different atomic mass). Only choice 2 is correct. Choice 1: C and N are not the same elements. Choice 3: both are the same element but there is no difference in mass. Therefore, they are not isotopes. Choice 4: not the same elements. 7. Which element's ionic radius is smaller than its atomic radius? a. neon b. nitrogen c. sodium d. sulfur Explanation: Elements that lose electrons form positive ions that are smaller than atoms of the same element. Therefore the ionic radius of these ions is smaller than its atomic radius. Only choice 3, sodium, is a metal that will lose electrons. Neon is stable and will neither lose nor gain electrons. N and S will tend to gain electrons, each having a larger ionic radius than atomic radius. 8. Which atom contains exactly 15 protons? a. P-32 b. S-32 c. O-15

2 d. N-15 Explanation: An atom with 15 protons also has an atomic number of 15. Refer to the Periodic Table for element 15 (phosphorus). 9. An ion with 5 protons, 6 neutrons, and a charge of 3+ has an atomic number of a. 5 b. 6 c. 8 d. 11 Explanation: The number of protons is the same as the atomic number. There are 5 protons so the atomic number is also What is the mass number of an atom which contains 28 protons, 28 electrons, and 34 neutrons? a. 28 b. 56 c. 62 d. 90 Explanation: Atomic mass is found by adding protons plus neutrons = 62. Electrons have a mass of nearly 0 so they are not counted in the formula for atomic mass. 11. Which three groups of the Periodic Table contain the most elements classified as metalloids (semimetals)? a. 1, 2, and 13 b. 2, 13, and 14 c. 14, 15, and 16 d. 16, 17, and 18 Explanation: Go to the Periodic Table and note the location of the stepped line dividing metals and nonmetals. Elements next to the left or right of this line are metalloids. Choice three includes 5 metalloids. 12. Which element has the highest first ionization energy? a. sodium b. aluminum c. calcium d. phosphorus Explanation: See Ref. Table S. Phosphorus (P) has a first ionization energy of 1012 kj/mol. 13. Which compound forms a colored aqueous solution? a. CaCl2 b. CrCl3 c. NaOH d. KBr Explanation: Colored aqueous solutions are a characteristic of transition compounds. The only transition compound in the above choices is CrCl3. Cr is a transition element. 14. When a metal atom combines with a nonmetal atom, the nonmetal atom will a. lose electrons and decrease in size b. lose electrons and increase in size c. gain electrons and decrease in size d. gain electrons and increase in size Explanation: Metals tend to lose electrons (becoming smaller ions) while nonmetals tend to gain electrons and become larger ions. 15. According to Reference Table S, which of the following elements has the smallest atomic radius? a. nickel b. cobalt c. calcium d. potassium Explanation: With a radius of 124, nickel has the smallest atomic radius of the elements given. 16. Which element in Group 15 has the strongest metallic character? a. Bi b. As c. P d. N Explanation: From left to right across the period, metallic character decreases. But it increases going down a group (last element in a group will have the most metallic character). Bi is the last element in group 15 so it is the most metallic. 17. Which halogens are gases at STP?

3 a. chlorine and fluorine b. chlorine and bromine c. iodine and fluorine d. iodine and bromine Explanation: Iodine is a solid and bromine is a liquid. The gaseous halogens are chlorine and fluorine. 18. When combining with nonmetallic atoms, metallic atoms generally will a. lose electrons and form negative ions b. lose electrons and form positive ions c. gain electrons and from negative ions d. gain electrons and form positive ions Explanation: Metals tend to lose electrons and form positive ions. A neutral atom of Li has 3 protons (+) and 3 electrons (-). If it loses an electron, the Li atom now has +3 and -2 or a net charge of +1. Metals also have low electronegativities and low ionization energies. 19. Which set of elements contains a metalloid? a. K, Mn, As, Ar b. Li, Mg, Ca, Kr c. Ba, Ag, Sn, Xe d. Fr, F, O, Rn Explanation: Go to the Periodic Table and note the location of the stepped line dividing metals and nonmetals. Elements next to the left or right of this line are metalloids. As (Arsenic) is a metalloid. 20. Atoms of elements in a group on the Periodic Table have similar chemical properties. This similarity is most closely related to the atoms' a. number of principal energy levels b. number of valence electrons c. atomic numbers d. atomic masses Explanation: Most of the time elements in the same group have the same number of valence electrons. Valence electrons are involved in bonding and this effects how elements react and determines their chemical properties. See also question #11, topic: Nuclear Chemistry. 21. As atoms of elements in Group 16 are considered in order from top to bottom, the electronegativity of each successive element a. decreases b. increases c. remains the same Explanation: See Ref. Table S. 22. The first ionization energy of an element is 736 kj per mole of atoms. An atom of this element in the ground state has a total of how many valence electrons? a. 1 b. 2 c. 3 d. 4 Explanation: Use Ref. Table S. 736 kj/mol is the value for Mg. Use the Periodic Table to find Mg. It has 2 valence electrons. 23. An atom of which of the following elements has the greatest ability to attract electrons? a. silicon b. sulfur c. nitrogen d. chlorine Explanation: See Ref. Table S. Electronegativity is a measure of the ability of an atom to attract electrons. The greater the ability to attract, the higher the electronegativity. Chlorine has the highest electronegativity of the choices given. 24. At STP, which substance is the best conductor of electricity? a. nitrogen b. neon c. sulfur d. silver Explanation: The best conductors of electricity are metals. Silver is a metal, the others are nonmetals. 25. Which metal is obtained commercially by the electrolysis of salt? a. Zn

4 b. K c. Fe d. Ag Explanation: See Ref. Table J. Metals, including K, listed near the top of Table J occur as compounds in nature. The pure, uncombined form of these metals results from commercial electrolysis of salt. Notice that these metals (Mg, Na, Ca, Ba, Cs, K, Rb and Li) are either alkali metals or alkaline earth metals, the most reactive metals. You would not expect to find these very active metals uncombined. Au can be found in nuggets and dust. (Remember the California gold rush and the silver mines in Nevada?) but you will not find "nuggets" of alkali or alkaline metals in nature. See also #16, this topic. 26. Which element can be found in nature in the free (uncombined) state? a. Ca b. Ba c. Au d. Al Explanation: Remember the Gold Rush? Gold is found as dust or nuggets uncombined in nature. See Ref. Table J. Metals listed near the bottom are rarely found combined with other elements (Au or gold is found at the bottom of the table). However the metals near the top of Table J occur as compounds in nature. The pure, uncombined form of these metals results from commercial processing. See also #15, this topic. 27. Which formula represents a molecular substance? a. CaO b. CO c. Li2O d. Al2O3 Explanation: Use Ref. Table S to determine the electronegativity difference of each of the choices. The difference between C (2.6) and O (3.4) is 0.9 or in the range of covalent bonding (covalent bonds have electronegativity differences of less than 1.7). The other three compounds have differences in the ionic bond range or over Which sequence of Group 18 elements demonstrates a gradual decrease in the strength of the Van der Waals forces? All the choices are elements in the liquid state. a. Ar, Kr, Ne, Xe b. Kr, Xe, Ar, Ne c. Ne, Ar, Kr, Xe d. Xe, Kr, Ar, Ne Explanation: Van der Waals forces are weak forces of attraction between molecules. These forces decrease as the molecule gets smaller and increase as the molecule increases. Look at the periodic table and notice how in answer 4, the elements are getting smaller. Therefore the Van der Waals forces would also be decreasing. 29. Which substance is an example of a network solid? a. nitrogen dioxide b. sulfur dioxide c. carbon dioxide d. silicon dioxide Explanation: Only silicon dioxide forms a network solid, a network of covalent bonds extending throughout the crystal without forming a molecule. 30. Which combination of atoms can form a polar covalent bond? a. H and H b. H and Br c. N and N d. Na and Br Explanation: See Reference Table S. The electronegativity difference between H (2.1) and Br (3.0) is 0.9. If the difference is less than 1.7 (but not zero, usually more than 0.4), a polar covalent bond is formed. A nonpolar covalent bond is formed when the difference is close to zero (usually up to 0.4). If the difference is more than 1.7, an ionic bond is formed. 31. A strontium atom differs from a strontium ion in that the atom has a greater

5 a. number of electrons b. number of protons c. atomic number d. mass number Explanation: Strontium has 2 valence electrons in its outer shell. It is easier to lose these 2 electrons than it is to gain 6 more electrons. Losing 2 electrons will give the strontium ion a charge of +2. Ions are charged atoms. An neutral atom has no charge because it has the same number of protons and electrons. An ion can have more or less electrons than an atom of the same element does. A strontium atom would have 2 more electrons than a strontium ion. 32. Which bond has the greatest ionic character? a. H---CL b. H---F c. H---O d. H---N Explanation: See Ref. Table S. H has an electronegativity of 2.1 and F has an electronegativity of 4.0. Subtract the two values and the difference is 1.9. Ionic bonds have differences of 1.7 or greater. Find the electronegativity differences for the other choices. H--F has the greatest difference and is the most ionic of the choices. 33. A compound differs from a mixture in that a compound always has a a. homogeneous composition b. maximum of two components c. minimum of three components d. heterogeneous composition Explanation: By definition compounds are homogenous. They cannot be separated by physical means. A mixture (salad for example) is not homogenous. 34. Which substance cannot be decomposed into simpler substances? a. ammonia b. aluminum c. methane d. methanol Explanation: Elements cannot be decomposed into simpler substances because an element by definition is one kind of substance, one kind of atom. The other choices are compounds-- composed of two or more elements. 35. Which statement describes a chemical property? a. Its crystals are a metallic gray. b. It dissolves in alcohol. c. It forms a violet-colored gas. d. It reacts with hydrogen to form a gas. Explanation: While all the answers are true for iodine, only choice 4 describes a chemical property. The other choices are physical properties 36. A student investigated the physical and chemical properties of a sample of unknown gas and then investigated the gas. Which statement represents a conclusion rather than an experimental observation? a. The gas is colorless. b. The gas is carbon dioxide. c. When the gas is bubbled in limewater, the liquid becomes cloudy. d. When placed in the gas, a flaming splint stops burning. Explanation: Color, turning limewater cloudy, and inability to support combustion (cause a flame to go out) are observations. Figuring out what compound has these characteristics involves making a hunch or adding up the evidence (conclusion). Sort of like Judge Judy on TV. She listens to the facts (observations) and then decides who is at fault (conclusion). Another example: there are muddy footprints on a clean floor (observation). Figuring out who made the footprints is a conclusion. Was it the dog? 37. To determine the density of an irregularly shaped object, a student immersed the object in 21.2 milliliters of H2O in a graduated cylinder, causing the level of the H2O to rise to 27.8 milliliters. If the object had a mass of 22.4 grams, what was the density of the object?

6 a g / ml b. 6.6 g / ml c. 3.0 g / ml d. 3.4 g/ ml Explanation: D = M / V. First find the volume of the object by subtracting 21.2 ml from 27.8 ml or 6.6 ml Step 2: D = M / V or D = 22.4 g / 6.6 ml = 3.4 g / ml See also Table T