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

What makes one element different from another? 4.3 Atomic Number What makes one element different from another? 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 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

for Conceptual Problem 4.1

How do you find the number of neutrons in an atom? 4.3 Mass Number How do you find the number of neutrons in an atom? 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.

4.1

4.1

for Sample Problem 4.1

Isotopes How do isotopes of an element differ? 4.3 Isotopes Isotopes How do isotopes of an element differ? 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?

for Conceptual Problem 4.2

4.3 Section Quiz 4.3.

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