4.3 Atomic Number

Atomic Number

What makes one element different from another?

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.

Atomic Number4.3

for Conceptual Problem 4.1

Mass Number

Mass Number

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

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.

4.3

Mass Number

Au is the chemical symbol for gold.

4.3

4.1

for Sample Problem 4.1

Isotopes

Isotopes

How do isotopes of an element differ?

4.3

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.

Isotopes

Despite these differences, isotopes are chemically alike because they have identical numbers of protons and electrons.

4.3

for Conceptual Problem 4.2

Atomic Mass

Atomic Mass

How do you calculate the atomic mass of an element?

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 10-24 grams

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.

4.3

Atomic Mass

Weighted Average Mass of a Chlorine Atom

4.3

for Conceptual Problem 4.3

for Conceptual Problem 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.

4.3

Atomic Mass

For example, carbon has two stable isotopes:

• Carbon-12, which has a relative abundance of 98.89%, and

• Carbon-13, which has a relative abundance of 1.11%.

4.3

The Periodic Table—A Preview

The Periodic Table—A Preview

Why is a periodic table useful?

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).

4.3

The Periodic Table—A Preview

The Periodic Table

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.

4.3

The Periodic Table—A Preview

A Period

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.

4.3

The Periodic Table—A Preview

A Group or Family

4.3

4.3 Section Quiz

1. Isotopes of an element have

a. the same mass number.

b. different atomic numbers.

c. the same number of protons but different numbers of neutrons.

d. the same number of protons but different numbers of electrons.

4.3 Section Quiz

2. How many neutrons are in sulfur-33?

a. 16 neutrons

b. 33 neutrons

c. 17 neutrons

d. 32.06 neutrons

4.3 Section Quiz

3. If sulfur contained 90.0% sulfur-32 and 10.0% sulfur-34, its atomic mass would be

a. 32.2 amu.

b. 32.4 amu.

c. 33.0 amu.

d. 35.4 amu.

END OF SHOW