Understanding Isotopes and Ions: Protons, Neutrons, and Electrons, Lecture notes of Chemistry

Download Understanding Isotopes and Ions: Protons, Neutrons, and Electrons and more Lecture notes Chemistry in PDF only on Docsity! D R . M IO Y T . H U Y N H | 2 0 1 9 CHEMICAL FOUNDATIONS B. ISOTOPES & IONS CHEMICAL FOUNDATIONS Objective: 01-B | 2ISOTOPES & IONS Understand what isotopes are Be able to determine the number of protons and neutrons in an isotope given the mass number (A) It is the atomic number (Z), or the number of protons, that defines an element. In other words, any atom of carbon (Z = 6) will always have 6 protons in its nucleus. But what about the number of neutrons? It turns out that it is possible for different atoms of the same element to have different numbers of neutrons, and therefore different mass numbers (A), which is the sum of the numbers of protons and neutrons. Such atoms are called isotopes. We often use either of the following notations to represent atoms and isotopes of an element (X): ! "X or "X As an example, let us consider the two isotopes of carbon shown below: carbon-12 (12C) and carbon-13 (13C). The difference between these two atoms is that 13C has one more neutron (gray sphere in the inset) 12C. ISOTOPES 6 protons 6 neutrons 6 protons 7 neutrons 𝟔 𝟏𝟐𝐂 𝟔 𝟏𝟑𝐂 CHEMICAL FOUNDATIONS Fundamental Concepts Problem-Solving Skills 01-B | 5ISOTOPES & IONS 1. Complete the following chart. 2. What is the symbol for the element with a –1 charge, 36 electrons, and 46 neutrons? 3. Boron exists in two stable isotopes: 10B (10.0129 amu) and 11B (11.00931 amu). If the atomic mass of boron is 10.811 amu, which is the best estimate for the natural abundances of each isotope? a) 40% 10B and 60% 11B b) 20% 10B and 80% 11B c) 60% 10B and 40% 11B 4. There are three stable isotopes of magnesium, with masses of 23.9850, 24.9858, and 25.9826 amu. If the average atomic mass of magnesium is 24.3050 amu and the natural abundance of the lightest isotope is 78.99%, what are the natural abundances of the other two isotopes? 5. A compound XCl2 contains an unknown ion (Xn+). If the ion Xn+ contains 10 electrons, what is the identity of X? 6. A compound contains three times as many atoms of bromine as an unknown metal M. Metal M contains 23 electrons and 31 neutrons when it forms the compound. What is the mass number of M? 7. Consider the following two isotopes: 18F– and 15N3–. a) Which isotopes have the same number of electrons as 17O2–? There may be more than one answer. b) Which isotopes have the same number of neutrons as 17O2–? There may be more than one answer. PRACTICE PROBLEMS Symbol: 5B CDZn 2C 54S 2C 54S4H # protons # neutrons # electrons Mass Number (A) CHEMICAL FOUNDATIONS 01-B | 6ISOTOPES & IONS Complete the following chart. See chart below. — answer — Objective: Be able to determine the number of protons and neutrons in an isotope given atomic symbol Objective: Be able to determine the number of electrons in an ion Recognize that the atomic symbol (!"X) gives us the mass number (A) as well as the atomic number (Z). For 5B CDZn, the atomic number is Z = 30, which means that there are 30 protons in the nucleus. There are also 30 electrons in the neutral atom since the charges from the protons (+1 each) must cancel out with the charges from the electrons (–1 each). Because the mass number (A = 64) is the sum of the number of protons and neutrons, we can determine there are 34 neutrons in the nucleus (64 = 30 + 𝑛). For 2C 54S4H, the atomic number is Z = 16, which means that there are 16 protons in the nucleus. There would be 16 electrons in the neutral atom, but since we have S2– there are two extra electrons, so 18 electrons in total. Because the mass number (A = 32) is the sum of the number of protons and neutrons, we can determine there are 16 neutrons in the nucleus (32 = 16 + 𝑛). PROBLEM 1 Symbol: 5B CDZn 2C 54S 2C 54S4H # protons 30 16 16 # neutrons 34 16 16 # electrons 30 16 18 Mass Number (A) 64 32 32 CHEMICAL FOUNDATIONS 01-B | 7ISOTOPES & IONS What is the symbol for the element with a –1 charge, 36 electrons, and 46 neutrons? — answer — Objective: Be able to determine the number of protons and neutrons in an isotope given atomic symbol Objective: Be able to determine the number of electrons in an ion First, recognize that we have an ion because there is an overall (net) negative (–1) charge on the unknown atom. Because we know the anion has 36 electrons, the neutral atom must have started with 35 electrons to begin with. Remember that electrons are negatively charged. Second, if the neutral atom had 35 electrons, then it must also have 35 protons in order to balance out the charges. Remember that protons are positively charged. This means that the atomic number is Z = 35, which is Br. Third, the mass number (A) is equal to the sum of the number of protons and neutrons: 𝐴 = 35 + 46 = 81. Finally, putting all this information together into a symbol: 5LM2BrH PROBLEM 2 CHEMICAL FOUNDATIONS 01-B | 10ISOTOPES & IONS A compound XCl2 contains an unknown ion (Xn+). If the ion Xn+ contains 10 electrons, what is the identity of X? — answer — Objective: Be able to determine the charge of an ion Objective: Be able to determine the identity of an atom from its atomic number (Z) First, recognize that the compound contains chloride anions: Cl–. Because there are two chloride anions in the compound, the charge of the X cation must be +2 or X2+ to make the compound neutral overall. Second, if X2+ cation contains a charge of +2, then the cation had to lose two electrons because each electron carries a –1 charge. Therefore, the neutral atom of X would have contained 12 electrons originally. Third, the neutral atom would need to have 12 protons, or an atomic number of Z = 12, which corresponds to Mg. The identity of X is Mg (or Mg2+). PROBLEM 5 CHEMICAL FOUNDATIONS 01-B | 11ISOTOPES & IONS A compound contains three times as many atoms of bromine as an unknown metal M. Metal M contains 23 electrons and 31 neutrons when it forms the compound. What is the mass number of M? — answer — Objective: Be able to determine the charge of an ion Objective: Be able to determine the identity of an atom from its atomic number (Z) First, recognize that the compound has this general formula: MBr3. Because there are three bromide anions (Br–) in the compound, the charge of the M cation must be +3 or M3+ to make the compound neutral overall. Second, if M3+ cation contains a charge of +3, then the cation had to lose three electrons because each electron carries a –1 charge. Therefore, the neutral atom of M would have contained 26 electrons originally. Third, the neutral atom would need to have 26 protons, or an atomic number of Z = 26, which corresponds to Fe. The mass number for Fe would be A = 26 + 31 = 57. PROBLEM 6 CHEMICAL FOUNDATIONS 01-B | 12ISOTOPES & IONS Consider the following two isotopes: 18F– and 15N3–. a) Which isotopes have the same number of electrons as 17O2–? There may be more than one answer. b) Which isotopes have the same number of neutrons as 17O2–? There may be more than one answer. — answer — Objective: Be able to determine the number of protons and neutrons in an isotope Objective: Be able to determine the number of electrons in an ion For each ion, let’s determine the number of protons, neutrons, and electrons. Therefore, both W 2MFH and Y 2LN5H contains the same number of electrons (10) as M 2YO4H . And, W 2MFH contain the same number of neutrons (9) as M 2YO4H . PROBLEM 7 Symbol: W 2MFH Y 2LN5H M 2YO4H # protons 9 7 8 # neutrons 9 8 9 # electrons 10 10 10 Mass Number (A) 18 15 17