Distinguishing Among Atoms 4.3 Just as apples come in different varieties, a chemical element can come in different “varieties” called isotopes.

4.3 Atomic Number Atomic Number Elements are different because they contain different numbers of protons. The atomic number of an element is the number of protons in the nucleus of an atom of that element.

4.3 Atomic Number

7 protons & 7 electrons

for Conceptual Problem 4.1 b) N f) 16 c) 5 g) 23 d) 5 h) 23

4.3 Mass Number The total number of protons and neutrons in an atom is called the mass number. The number of neutrons in an atom is the difference between the mass number and atomic number.

Au is the chemical symbol for gold. 4.3 Mass Number Au is the chemical symbol for gold. Mass number Atomic number Au is the chemical symbol for gold. Applying Concepts How many electrons does a gold atom have?

4.1 a) 4 p, 4 e-, 9-4 = 5 n b) 10 p, 10 e-, 20-10 = 10 n c) 11 p, 11 e-, 23-11 = 12 n

a) 16 – 8 = 8 n b) 32 – 16 = 16 n c) 108 – 47 = 61 n d) 80- 35 = 45 n for Sample Problem 4.1 a) 16 – 8 = 8 n b) 32 – 16 = 16 n c) 108 – 47 = 61 n d) 80- 35 = 45 n e) 207 – 82 = 125 n

4.3 Isotopes Isotopes are atoms that have the same number of protons but different numbers of neutrons. Because isotopes of an element have different numbers of neutrons, they also have different mass numbers.

4.3 Isotopes Despite these differences, isotopes are chemically alike because they have identical numbers of protons and electrons. Neon-20, neon-21, and neon-22 are three isotopes of neon, a gaseous element used in lighted signs. Comparing and Contrasting How are these isotopes different? How are they similar?

12 13 C C 6 6

for Conceptual Problem 4.2 chromium-50 50 - 24 = 26 n chromium-52 52 - 24 = 28 n chromium-53 53 – 24 = 29 n

4.3 Atomic Mass Atomic Mass It is useful to compare the relative masses of atoms to a standard reference isotope. Carbon- 12 is the standard reference isotope. Carbon-12 has a mass of exactly 12 atomic mass units. An atomic mass unit (amu) is defined as one twelfth of the mass of a carbon-12 atom.

Some Elements and Their Isotopes 4.3 Atomic Mass Some Elements and Their Isotopes

4.3 Atomic Mass The atomic mass of an element is a weighted average mass of the atoms in a naturally occurring sample of the element. A weighted average mass reflects both the mass and the relative abundance of the isotopes as they occur in nature.

Weighted Average Mass of a Chlorine Atom 4.3 Atomic Mass Weighted Average Mass of a Chlorine Atom Chlorine is a reactive element used to disinfect swimming pools. Chlorine occurs as two isotopes: chlorine-35 and chlorine-37. Because there is more chlorine-35 than chlorine-37, the atomic mass of chlorine, 35.453 amu, is closer to 35 than to 37. Evaluating How does a weighted average differ from an arithmetic mean?

Copper-63 The average is closer to 63 than it is to 65.

for Conceptual Problem 4.3 boron-11 10.81 is closer to 11 than it is to 10.

4.3 Atomic Mass To calculate the atomic mass of an element, multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products.

For example, carbon has two stable isotopes: 4.3 Atomic Mass For example, carbon has two stable isotopes: Carbon-12, which has a natural abundance of 98.89%, and Carbon-13, which has a natural abundance of 1.11%.

4.2

for Sample Problem 4.2 (78.92)(0.5069) + (80.92)(0.4931) = 40.00 + 39.90 = 79.90 amu

1. Isotopes of an element have the same mass number. 4.3 Section Quiz 1. Isotopes of an element have the same mass number. different atomic numbers. the same number of protons but different numbers of neutrons. the same number of protons but different numbers of electrons. ANS: C PTS: 1 REF: p. 112 OBJ: 4.3.1

2. How many neutrons are in sulfur-33? 16 neutrons 33 neutrons 4.3 Section Quiz 2. How many neutrons are in sulfur-33? 16 neutrons 33 neutrons 17 neutrons 32.06 neutrons ANS: C PTS: 1 REF: p. 111 OBJ: 4.3.2

4.3 Section Quiz 3. If sulfur contained 90.0% sulfur-32 and 10.0% sulfur-34, its atomic mass would be 32.2 amu. 32.4 amu. 33.0 amu. 35.4 amu. ANS: A PTS: 1 REF: p. 116 OBJ: 4.3.3