U2L2 - Lecture Notes - How Atoms Differ

Lesson 2
How Atoms Differ

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Unit 2
The Structure of the Atom

The number of protons and the mass number define
the type of atom.

Essential Questions

• How is the atomic number used to determine

the identity of an atom?

• Why are atomic masses not whole numbers?
• Given the mass number and atomic number,

how are the number of electrons, protons, and
neutrons in an atom calculated?

Atomic Number

•Each element contains a unique positive charge in their nucleus.

•The number of protons in the nucleus of an atom identifies the element
and is known as the element’s atomic number.

•Because all atoms are neutral, the number of protons and electrons in an
atom must be equal.

Atomic number = number of protons
= number of electrons

Multiple Choice

What is the atomic number?

• the number of protons

• the number of protons and neutrons

• the number of neutrons

• the number of atoms

Multiple Choice

What is the atomic number of this atom?

• 1

• 3

• 4

• 7

Multiple Choice

The fluorine atom has an atomic number of

• 19

• 9

Isotopes and Mass Number

•All atoms of a particular element have the same number of protons and
electrons but the number of neutrons in the nucleus can differ.

•Atoms with the same number of protons but different numbers of neutrons
are called isotopes.

•In nature, most elements are found as mixtures of isotopes. Usually, the
relative abundance of each isotope is constant.

–Example: In a banana, 93.26% is potassium-39, 6.73% is potassium-41 and
0.01% is potassium-40. In another banana or in a different source of
potassium, the percentage composition of the potassium isotopes will still be
the same.

•Isotopes containing more neutrons have a greater mass.

•Isotopes of an atom have the same chemical behavior.

How Atoms Differ
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Multiple Choice

What is an isotope? 

• A charged atom

• An atom with different amounts of neutrons

• An atom with different amounts of protons

• A neutral atom

Multiple Select

Which of the following can vary for atoms of the same element?

• number of protons

• number of electrons

• number of neutrons

Isotopes and Mass Number

•The mass number is the sum of the protons and neutrons in the nucleus.

Mass number = atomic number + number of neutrons

Multiple Choice

What is the mass number defined as?

• the number of protons

• the number of protons and neutrons

• the number of neutrons

• the number of protons and electrons

Multiple Choice

What is the mass number of this atom?

• 1

• 3

• 4

• 7

Multiple Choice

How many neutrons does oxygen have?

• 16

• 15.99

• 8

• Doesn't contain any neutrons

Mass of Atoms

•One atomic mass unit (amu) is defined as 1/12th the mass of a carbon-12
atom.

•One amu is nearly, but not exactly, equal to one proton and one neutron.

How Atoms Differ
Copyright © McGraw-Hill Education

Multiple Select

Which of the three sub-atomic particles is the lightest?

• the proton

• the electron

• the neutron

Mass of Atoms

•The atomic mass of an element is the weighted average mass of the
isotopes of that element.

•Because isotopes have different masses, the weighted average is not a
whole number.