Chemist’s Dozen The Mole Ch 2 Chemical Formulas, Equations, and Reaction Yields

TP What is the value of significant figures? 1. 2. 3. 4. 5. 6. . . 392 to the correct number of • • • • • 2.88398 2.8840 2.884 2.9 3 2 Next: Chemist’s dozen: The mole; Ch 2: Chemical formulas, equations, and reaction yields 1 2 TP What is the value of significant figures? 1. 2. 3. 4. 5. 6. 3 Review: Significant figures Mass of atoms Isotopes and average atomic mass Atomic mass unit and Avogadro's number Atomic weight . . 392 to the correct number of Review: Significant figures 2.88398 2.8840 2.884 2.9 2 3 . . 394.9 392 392 395 394.9 392 392 3 4 1 TP What is the molecular mass of CS2? Atomic mass and molecular mass 1. 2. 3. 4. Atomic mass is the dimensionless ratio of the mass of a particular atom to the mass of a reference atom. The reference atom is the most abundant isotope we will discuss isotopes shortly of carbon that is assigned the atomic mass 12. This means the mass of an S atom is reference carbon atom. . 32.065 44.076 76.140 Something else 2.6721 times as heavy as the 5 5 6 TP What is the mass percent of oxygen in NO2? 1. 2. 3. 4. 5. Atom composition 46.7% 53.3% 69.6% 30.4% Something else Atoms consist of positively charged protons, negatively charged electrons, and neutral neutrons. The volume of the atom is due to its electron cloud. The mass of the atom is due to the protons and neutrons packed into a much smaller nucleus. The radius of the electron cloud 105 larger than that of the nucleus. The mass of the proton and neutron are about the same, but each nearly 2000 times as larger than that of the electron. 8 13 2 Atom composition Isotopes and average atomic mass Because atoms are electrically neutral, atoms contain equal numbers of electrons and protons. The composition of an atom is indicated as X, where X is the element symbol, atomic number 𝑍 is the number of protons, and the mass number 𝐴 is the number of protons plus the number of neutrons. All atoms of the same element have the same number of protons and electrons . As example, the isotopes of carbon are 12 neutrons, respectively. Different elements have different numbers of protons and electrons . Ne, Ne, and Ne, with 10, 11, and Atoms of an element can have different numbers of neutrons, and such atoms are called isotopes of the element. Because the electron cloud is the same for every isotope of an element, each isotope has the same chemical properties. 14 15 TP Chlorine has two isotopes, Cl and Cl, with atomic mass 34.969 and 36.966, respectively. The natural abundance of Cl is 75.78%. The average atomic mass of chlorine is … Isotopes and average atomic mass Sketch an atom of Ne, with 10 electrons, 10 protons, and 11 neutrons. 16 1. 2. 3. 4. 5. 34.969 34.969 36.966 /2 0.7578 34.969 1 36.966 Something else 0.7578 36.966 18 3 TP What is the least abundant isotope of magnesium? 1. 2. 3. Isotopes and average atomic mass Mg Mg More information needed To calculate abundances from the average atomic mass of an element, we must know its naturally occurring isotopes. For magnesium, there are three. 24.3050 𝑓 19 𝑓 Mg 𝑓 Mg 𝑓 Mg 0.7899, etc. 20 Element identity and atomic number Z Atoms of an element come in different “flavors” Atoms with the same number of protons … but with different numbers of neutrons … are chemically the same … but have different masses We call such different flavors of atoms of an element isotopes 39.0983 u is the average mass of the different kinds of atoms isotopes of K that are in a sample of K. Number of protons atomic number 𝑍 19 Relative atomic weight 39.0983 Where does the number 39.0983 come from? 22 23 4 Atomic mass unit u Atomic mass unit u 1 u defined to be exactly 1/12 mass of 1 atom of 12C Exactly 12 g of 12C contains 𝑁A number 6.02214076 1023 atoms Avogadro’s Therefore, the mass of one 12C atom is … 12 g/𝑁A And so, 1 u 1.99265 … 1/12 x 1.99265 24 10‒23 g 10‒23 g 1.66054 10‒24 g 25 Average mass of an atom of K Average mass of an atom of K Two isotopes: K-39 and K-41 Two isotopes: K-39 and K-41 K-39 peak at 38.9637 u, height 933 K-39 peak at 38.9637 u, height 933 K-41 peak at 40.9618 u, height 67 K-41 peak at 40.9618 u, height 67 Write and then evaluate the expression whose value is the average mass in u of an atom of K. The average mass in g of an atom of K is 𝑓 38.9637 u 𝑓 40.9618 u 39.098 u 39.098 u 39.098 1/12 12 g 1/𝑁A 39.098 g / 𝑁A 6.4923 10 23 g 26 27 5 Average mass of any atom Molar mass of any element The average mass of an atom of K is 39.098 g/𝑁A Molar mass is the mass of 𝑁A “average” atoms of an element. The average mass of an atom of Br is 79.904 g/𝑁A The average mass of an atom of K is 39.098 g/𝑁A The average mass of an atom of H is 1.008 g/𝑁A The molar mass of K is 𝑁A The average mass of any atom in g/𝑁A is the number given on the periodic table. The molar mass of Br is 79.904 g 39.098 g/𝑁A 39.098 g The molar mass of H is 1.008 g The molar mass of any element in g is the number given on the periodic table. 28 29 Atomic weight = magnitude of average mass The atomic weight of K is 39.098 no units! The atomic weight of Br is 79.904 The atomic weight of H is 1.008 The atomic weight of an element is the number given on the periodic table. 30 6