# Sample Exercise 2.1 Illustrating the Size of an Atom

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1 Sample Exercise 2.1 Illustrating the Size of an Atom The diameter of a US penny is 19 mm. The diameter of a silver atom, by comparison, is only 2.88 Å. How many silver atoms could be arranged side by side in a straight line across the diameter of a penny? The unknown is the number of silver (Ag) atoms. We use the relationship 1 Ag atom = 2.88 Å as a conversion factor relating the number of atoms and distance. Thus, we can start with the diameter of the penny, first converting this distance into angstroms and then using the diameter of the Ag atom to convert distance to the number of Ag atoms: That is, 66 million silver atoms could sit side by side across a penny! The diameter of a carbon atom is 1.54 Å. (a) Express this diameter in picometers. (b) How many carbon atoms could be aligned side by side in a straight line across the width of a pencil line that is 0.20 mm wide? Answer: (a) 154 pm, (b) C atoms

2 Sample Exercise 2.2 Determining the Number of Subatomic Particles in Atoms How many protons, neutrons, and electrons are in (a) an atom of 197 Au; (b) an atom of strontium-90? (a) The superscript 197 is the mass number, the sum of the number of protons plus the number of neutrons. According to the list of elements given inside the front cover, gold has an atomic number of 79. Consequently, an atom of 197 Au has 79 protons, 79 electrons, and = 118 neutrons. (b) The atomic number of strontium (listed inside the front cover) is 38. Thus, all atoms of this element have 38 protons and 38 electrons. The strontium-90 isotope has = 52 neutrons. How many protons, neutrons, and electrons are in (a) a 138 Ba atom, (b) an atom of phosphorus-31? Answer: (a) 56 protons, 56 electrons, and 82 neutrons; (b) 15 protons, 15 electrons, and 16 neutrons.

3 Sample Exercise 2.3 Writing symbols for Atoms Magnesium has three isotopes, with mass numbers 24, 25, and 26. (a) Write the complete chemical symbol (superscript and subscript) for each of them. (b) How many neutrons are in an atom of each isotope? (a) Magnesium has atomic number 12, so all atoms of magnesium contain 12 protons and 12 electrons. The three isotopes are therefore represented by 12 Mg, 12Mg, and 12Mg. (b) The number of neutrons in each isotope is the mass number minus the number of protons. The numbers of neutrons in an atom of each isotope are therefore 12, 13, and 14, respectively. Give the complete chemical symbol for the atom that contains 82 protons, 82 electrons, and 126 neutrons. Answer: Pb

4 Sample Exercise 2.4 Calculating the Atomic Weight of an Element from Isotopic Abundances Naturally occurring chlorine is 75.78% 35 Cl, which has an atomic mass of amu, and 24.22% 37 Cl, which has an atomic mass of amu. Calculate the average atomic mass (that is, the atomic weight) of chlorine. We can calculate the average atomic mass by multiplying the abundance of each isotope by its atomic mass and summing these products. Because 75.78% = and 24.22% = , we have Average atomic mass = (0.7578)( amu) + (0.2422)( amu) = amu amu = amu This answer makes sense: The average atomic mass of Cl is between the masses of the two isotopes and is closer to the value of 35 Cl, which is the more abundant isotope. Three isotopes of silicon occur in nature: 28 Si (92.23%), which has an atomic mass of amu; 29 Si (4.68%), which has an atomic mass of amu; and 30 Si (3.09%), which has an atomic mass of amu. Calculate the atomic weight of silicon. Answer: amu

5 Sample Exercise 2.5 Using the Periodic Table Which two of the following elements would you expect to show the greatest similarity in chemical and physical properties: B, Ca, F, He, Mg, P? Elements that are in the same group of the periodic table are most likely to exhibit similar chemical and physical properties. We therefore expect that Ca and Mg should be most alike because they are in the same group (2A, the alkaline earth metals). Locate Na (sodium) and Br (bromine) on the periodic table. Give the atomic number of each, and label each a metal, metalloid, or nonmetal. Answer: Na, atomic number 11, is a metal; Br, atomic number 35, is a nonmetal.

6 Sample Exercise 2.6 Relating the Empirical and Molecular Formulas Write the empirical formulas for the following molecules: (a) glucose, a substance also known as either blood sugar or dextrose, whose molecular formula is C 6 H 12 O 6 ; (b) nitrous oxide, a substance used as an anesthetic and commonly called laughing gas, whose molecular formula is N 2 O. (a) The subscripts of an empirical formula are the smallest whole-number ratios. The smallest ratios are obtained by dividing each subscript by the largest common factor, in this case 6. The resultant empirical formula for glucose is CH 2 O. (b) Because the subscripts in N 2 O are already the lowest integral numbers, the empirical formula for nitrous oxide is the same as its molecular formula, N 2 O. Give the empirical formula for the substance called diborane, whose molecular formula is B 2 H 6. Answer: BH 3

7 Sample Exercise 2.7 Writing Chemical Symbols for Ions Give the chemical symbol, including mass number, for each of the following ions: (a) The ion with 22 protons, 26 neutrons, and 19 electrons; (b) the ion of sulfur that has 16 neutrons and 18 electrons. (a) The number of protons (22) is the atomic number of the element. By referring to a periodic table or list of elements, we see that the element with atomic number 22 is titanium (Ti). The mass number of this isotope of titanium is = 48 (the sum of the protons and neutrons). Because the ion has three more protons than electrons, it has a net charge of 3+. Thus, the symbol for the ion is 48 Ti 3+. (b) By referring to a periodic table or a table of elements, we see that sulfur (S) has an atomic number of 16. Thus, each atom or ion of sulfur must contain 16 protons. We are told that the ion also has 16 neutrons, meaning the mass number of the ion is = 32. Because the ion has 16 protons and 18 electrons, its net charge is. Thus, the symbol for the ion is 32 S. In general, we will focus on the net charges of ions and ignore their mass numbers unless the circumstances dictate that we specify a certain isotope. How many protons, neutrons, and electrons does the 79 Se ion possess? Answer: 34 protons, 45 neutrons, and 36 electrons

8 Sample Exercise 2.8 Predicting the Charges of Ions Predict the charge expected for the most stable ion of barium and for the most stable ion of oxygen. We will assume that these elements form ions that have the same number of electrons as the nearest noblegas atom. From the periodic table, we see that barium has atomic number 56. The nearest noble gas is xenon, atomic number 54. Barium can attain a stable arrangement of 54 electrons by losing two of its electrons, forming the Ba 2+ cation. Oxygen has atomic number 8. The nearest noble gas is neon, atomic number 10. Oxygen can attain this stable electron arrangement by gaining two electrons, thereby forming the O anion. Predict the charge expected for the most stable ion of (a) aluminum and (b) fluorine. Answer: (a) 3+; (b) 1

9 Sample Exercise 2.9 Identifying Ionic and Molecular Compounds Which of the following compounds would you expect to be ionic: N 2 O, Na 2 O, CaCl 2, SF 4? We would predict that Na 2 O and CaCl 2 are ionic compounds because they are composed of a metal combined with a nonmetal. The other two compounds, composed entirely of nonmetals, are predicted (correctly) to be molecular compounds. Which of the following compounds are molecular: CBr 4, FeS, P 4 O 6, PbF 2? Answer: CBr 4 and P 4 O 6

10 Sample Exercise 2.10 Using Ionic Charge to Write Empirical Formulas for Ionic Compounds What are the empirical formulas of the compounds formed by (a) Al 3+ and Cl ions, (b) Al 3+ and O ions, (c) Mg 2+ and NO 3 ions? (a) Three CL ions are required to balance the charge of one AL 3+ ion. Thus, the formula is AlCl 3. (b) Two AL 3+ ions are required to balance the charge of three O ions (that is, the total positive charge is 6+, and the total negative charge is 6 ). Thus, the formula is Al 2 O 3. (c) Two NO 3 ions are needed to balance the charge of one MG 2+. Thus, the formula is Mg(NO 3 ) 2. In this case the formula for the entire polyatomic ion NO 3 must be enclosed in parentheses so that it is clear that the subscript 2 applies to all the atoms of that ion. Write the empirical formulas for the compounds formed by the following ions: (a) Na + and PO 4 3, (b) Zn 2+ and SO 4, (c) Fe 3+ and CO 3. Answer: a) Na 3 PO 4, (b) ZnSO 4, (c) Fe 2 (CO 3 ) 3

11 Sample Exercise 2.11 Determining the Formula of an Oxyanion form Its Name Based on the formula for the sulfate ion, predict the formula for (a) the selenate ion and (b) the selenite ion. (Sulfur and selenium are both members of group 6A and form analogous oxyanions.) (a) The sulfate ion is SO 4. The analogous selenate ion is therefore SeO 4. (b) The ending -ite indicates an oxyanion with the same charge but one O atom fewer than the corresponding oxyanion that ends in -ate. Thus, the formula for the selenite ion is SeO 3. The formula for the bromate ion is analogous to that for the chlorate ion. Write the formula for the hypobromite and perbromate ions. Answer: BrO and BrO 4

12 Sample Exercise 2.12 Determining the Names of Ionic Compounds from Their Formulas Name the following compounds: (a) K 2 SO 4, (b) Ba(OH) 2, (c) FeCl 3. Each compound is ionic and is named using the guidelines we have already discussed. In naming ionic compounds, it is important to recognize polyatomic ions and to determine the charge of cations with variable charge. (a) The cation in this compound is K +, and the anion is SO 4. (If you thought the compound contained S and O ions, you failed to recognize the polyatomic sulfate ion.) Putting together the names of the ions, we have the name of the compound, potassium sulfate. (b) In this case the compound is composed of Ba 2+ and OH ions. Ba 2+ is the barium ion and OH is the hydroxide ion. Thus, the compound is called barium hydroxide. (c) You must determine the charge of Fe in this compound because an iron atom can form more than one cation. Because the compound contains three Cl ions, the cation must be Fe 3+ which is the iron(iii), or ferric, ion. The Cl ion is the chloride ion. Thus, the compound is iron(iii) chloride or ferric chloride. Name the following compounds: (a) NH 4 Br, (b) Cr 2 O 3, (c) Co(NO 3 ) 2. Answer: (a) ammonium bromide, (b) chromium(iii) oxide, (c) cobalt(ii) nitrate

13 Sample Exercise 2.13 Determining the Formulas of Ionic Compounds from Their Names Write the chemical formulas for the following compounds: (a) potassium sulfide, (b) calcium hydrogen carbonate, (c) nickel(ii) perchlorate. In going from the name of an ionic compound to its chemical formula, you must know the charges of the ions to determine the subscripts. (a) The potassium ion is K +, and the sulfide ion is S. Because ionic compounds are electrically neutral, two K + ons are required to balance the charge of one S ion, giving the empirical formula of the compound, K 2 S. (b) The calcium ion is Ca 2+. The carbonate ion is CO 3, so the hydrogen carbonate ion is HCO 3. Two HCO 3 ions are needed to balance the positive charge of Ca 2+, giving Ca(HCO 3 ) 2. (c) The nickel(ii) ion is Ni 2+. The perchlorate ion is ClO 4. Two ions are required to balance the charge on one Ni 2+ ion, giving Ni(ClO 4 ) 2. Give the chemical formula for (a) magnesium sulfate, (b) silver sulfide, (c) lead(ii) nitrate. Answer: (a) MgSO 4, (b) Ag 2 S, (c) Pb(NO 3 ) 2

14 Sample Exercise 2.14 Relating the Names and Formulas of Acids Name the following acids: (a) HCN, (b) HNO 3, (c) H 2 SO 4, (d) H 2 SO 3. (a) The anion from which this acid is derived is CN, the cyanide ion. Because this ion has an -ide ending, the acid is given a hydro- prefix and an -ic ending: hydrocyanic acid. Only water solutions of HCN are referred to as hydrocyanic acid: The pure compound, which is a gas under normal conditions, is called hydrogen cyanide. Both hydrocyanic acid and hydrogen cyanide are extremely toxic. (b) Because is the nitrate NO 3 ion, HNO 3 is called nitric acid (the -ate ending of the anion is replaced with an -ic ending in naming the acid). (c) Because SO 4 is the sulfate ion, H 2 SO 4 is called sulfuric acid. (d) Because SO 3 is the sulfite ion, H 2 SO 3 is sulfurous acid (the -ite ending of the anion is replaced with an -ous ending). Give the chemical formulas for (a) hydrobromic acid, (b) carbonic acid. Answer: (a) HBr, (b) H 2 CO 3

15 Sample Exercise 2.15 Relating the Names and Formulas of Binary Molecular Compounds Name the following compounds: (a) SO 2, (b) PCl 5, (c) N 2 O 3. The compounds consist entirely of nonmetals, so they are molecular rather than ionic. Using the prefixes in Table 2.6, we have (a) sulfur dioxide, (b) phosphorus pentachloride, and (c) dinitrogen trioxide. Give the chemical formula for (a) silicon tetrabromide, (b) disulfur dichloride. Answer: (a) SiBr 4, (b) S 2 Cl 2

16 Sample Exercise 2.16 Writing Structural and Molecular Formulas for Hydrocarbons Consider the alkane called pentane. (a) Assuming that the carbon atoms are in a straight line, write a structural formula for pentane. (b) What is the molecular formula for pentane? (a) Alkanes contain only carbon and hydrogen, and each carbon atom is attached to four other atoms. Because the name pentane contains the prefix penta- for five (Table 2.6), we can assume that pentane contains five carbon atoms bonded in a chain. If we then add enough hydrogen atoms to make four bonds to each carbon atom, we obtain the following structural formula: This form of pentane is often called n-pentane, where the n- stands for normal because all five carbon atoms are in one line in the structural formula. (b) Once the structural formula is written, we can determine the molecular formula by counting the atoms present. Thus, n-pentane has the formula C 5 H 12. Butane is the alkane with four carbon atoms. (a) What is the molecular formula of butane? (b) What are the name and molecular formula of an alcohol derived from butane? Answer: (a) C 4 H 10, (b) butanol, C 4 H 10 O or C 4 H 9 OH

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