Atomic Mass and Mass Number

Module 3 - Physical Science

Review

• The atomic number is the number of protons in the element.

• The atomic mass is the number of protons and neutrons in one atom of that element.

• All elements are electrically neutral, meaning they have ZERO NET CHARGE. This means that the number of protons (positive charges) are equal to the number of neutrons (negative charges)

Review

• The element Helium (He) has an Atomic Number of 2. This means that all Helium atoms have exactly two protons.

• Since the atom is electrically neutral, that means the atom also has exactly two electrons.

• How do we determine how many neutrons there are in an atom?

Review

• Atomic Mass = Protons + Neutrons

• We round the atomic mass to the nearest whole number, 4

• Since we already determined that Helium has two protons, that means: 4=2+Neutrons

• There must be 2 Neutrons in one atom of Helium

Let's Practice Together!

• What is the Atomic Number?

• What is the Atomic Mass?

• How many protons are in a Lithium Atom?

• How many electrons are in a Lithium Atom?

• What is the rounded Atomic Mass?

• How many neutrons are in a Lithium Atom?

Let's Practice Together!

• What is the Atomic Number? 3

• What is the Atomic Mass? 6.941

• How many protons are in a Lithium Atom? 3

• How many electrons are in a Lithium Atom? 3

• What is the rounded Atomic Mass? 7

• How many neutrons are in a Lithium Atom? 7-3=4