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Download 03.03a Atomic Number, Mass Number, and Isotopes
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Chemistry Date: §03.03a Atomic Number, Most Important Idea(s): Name: KEY Mass Number, and Isotopes Background ATOMS: All atoms of the same element have the same number of protons: the number of protons determines the identity of the atom. For example, a carbon atom always has six protons. If it has seven protons, it’s nitrogen, not carbon. The number of protons is called the atomic number (Z). ISOTOPES: Although all atoms of an element have the same number of protons, they don’t all have the same number of neutrons. Isotopes are atoms of the same element that have different masses (resulting from the different numbers of neutrons). An analogy is the flavors of ice cream (even ones to which no specific flavor is added). Identifying an isotope requires knowing both the atomic number of the element (Z) and the mass number (A) of the isotope. The mass number is the total number of protons and neutrons that make up the nucleus of an isotope. For example, carbon-14, commonly used to date biological objects (up to approximately 50,000 years old), has six protons (Z=6) and eight neutrons. To determine the number of neutrons in an isotope: Mass Number = Atomic Number + Number of Neutrons For Carbon-14: 14 = 6 + 8 IONS: Just as isotopes have different numbers of neutrons, ions have different numbers of electrons. Because protons are positively charged, and electrons are negatively charged, atoms are considered neutral because the numbers of each are equal. However, ions have different numbers of protons and electrons. For example, a magnesium ion with 12 protons (because it is magnesium) but only ten electrons, the magnesium ion has a +2 charge (Mg2+). Similarly, then chlorine ion, Cl–, has 17 protons and 18 electrons. When the coefficient has no number and only the charge, we assume number is 1. (Similarly, we write H2O as such because it has two hydrogen atoms and one oxygen atom.) F:\2014-2015\330_ModChem\330_sections\330_03_Atoms_Hx\330.03.03a_Atomic'Number_Mass'Number_Isotopes_HW_KEY.docx (9/23/2014) Chemistry §03.03.03a Atoms, Isotopes, Ions p. 2 Questions 1. Atomic number versus mass number: A. Define mass number: Atomic number (#p+) + number of neutrons (#n0) B. Define atomic number: number of protons C. Which of these numbers (atomic number or mass number) can vary without changing the identity of the element? Explain. The atomic number. The number of protons determines the identify of the atom. 2. Define isotope. Isotopes are atoms of the same element that have different masses (resulting from the different numbers of neutrons). 3. Define ion. Ions are charged species of the same element. They have different numbers of electrons and protons. Where there are more electrons, the charge is negative; more protons, positive. A. Is an isotope always an atom? Explain. All atoms are isotopes. Given that an element has only one type of atom, then that one atom is considered to be an isotope. i. Is an atom always an isotope? Explain. (see above) 4. Given the following isotopes 31 16 X, 31 15 32 X , 16 X? A. Which two of the following are isotopes of the same element? (Write the entire isotopic symbol) 31 16 X 32 16 X B. Explain. because they have the same atomic numer (16) C. What is the name of the element for these two isotopes? sulfur 5. A. What makes isotopes different for the same element? different numbers of neutrons B. What makes ions different for the same element? ions have charges, atoms don’t C. What makes atoms different? the different number of protons (atomic number) 6. Fill in the table on the following page. Chemistry Atomic Mass, Mass Number, and Isotopes Element Atomic Symbol Isotope Symbol Atomic Number Mass Number Protons Oxygen O 16 8 O 8 16 8 Lithium Li 7 3 Li 3 7 Chromium Cr 24 24 Chromium Cr 24 Cobalt Co 59Co3+ Selenium Se Osmium Os Antimony Sb Electrons Net Charge 8 8 0 3 4 2 +1 52 24 28 18 +6 24 52 24 28 21 +3 27 59 27 32 24 +3 34 80 34 46 36 –2 76 192 76 116 74 +2 Sb 51 121 51 70 51 0 127 53 53 127 53 74 54 1– 52 3 Cr 80 3 34 Se 193 I 76 Number of Neutrons 6 Cr 52 Iodine p. 3 2 Os 121 51 I