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

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Atomic Number What makes one element different from another? 4.3

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

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

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. Au is the chemical symbol for gold. Applying Concepts How many electrons does a gold atom have?

4.1

for Sample Problem 4.1

4.3 Isotopes Isotopes How do isotopes of an element differ?

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?

for Conceptual Problem 4.2

Atomic Mass How do you calculate the atomic mass of an element? 4.3

1 gram = 6.02 x 1023 amu’s (Avagadro’s number)
4.3 Atomic Mass It is useful to compare the relative masses of atoms to a standard reference isotope. Carbon-12 is the standard reference isotope. Cabon-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. It is based on the mass of a proton or neutron. 1 gram = 6.02 x 1023 amu’s (Avagadro’s number) 1 amu = 1.67 x grams

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, amu, is closer to 35 than to 37. Evaluating How does a weighted average differ from an arithmetic mean?

for Conceptual Problem 4.3

4.3 Atomic Mass To calculate the atomic mass of an element, multiply the mass of each isotope by its percent relative 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 relative abundance of %, and Carbon-13, which has a relative abundance of 1.11%.

The Periodic Table—A Preview
4.3 The Periodic Table—A Preview The Periodic Table—A Preview Why is a periodic table useful?

The Periodic Table—A Preview
4.3 The Periodic Table—A Preview A periodic table is an arrangement of elements in which the elements are separated into groups based on a set of repeating properties. A periodic table allows you to easily compare the properties of one element (or a group of elements) to another element (or group of elements).

The Periodic Table—A Preview
4.3 The Periodic Table—A Preview The Periodic Table Elements are arranged in the modern periodic table in order of atomic number. Interpreting Diagrams How many elements are in Period 2? In Group 2A?

The Periodic Table—A Preview
4.3 The Periodic Table—A Preview Each horizontal row of the periodic table is called a period. Within a given period, the properties of the elements vary as you move across it from element to element.

The Periodic Table—A Preview
4.3 The Periodic Table—A Preview A Period

The Periodic Table—A Preview
4.3 The Periodic Table—A Preview Each vertical column of the periodic table is called a group, or family. Elements within a group have similar chemical and physical properties.

The Periodic Table—A Preview
4.3 The Periodic Table—A Preview A Group or Family

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

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