Mass of atom

Masses of Atoms

Lesson 2

How can we know the mass of a single atom?

Focus Question

average atomic mass

isotope

mass number

atomic number

New Vocabulary

mass: amount of matter in an object

Review Vocabulary

element and compound - Group sort

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Atomic Model - Labelled diagram

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Particle

Mass (g)

Proton

Neutron

Electron

The nucleus contains almost all of an atom’s
mass because protons and neutrons are far more
massive than electrons.

Atomic Mass

​1.6726X10^-24

​1.6726X10^-24

​9.1093X10^-28

The mass of proton is roughly the same of
neutron.

The mass of each is more than 1800 times
greater than the mass of electron

An electron mass is so small (ignored)

The unit of measurement used to quantify an
atoms mass is atomic mass unit (amu)

Atomic mass unit= is one twelfth of the mass of
an carbon atom

Atomic Mass

Element

Atomic
Number

Protons

Neutrons

Mass

Number

Boron

5

6

11

Copper

29

34

63

•

The number of protons in an
atom’s nucleus is equal to its
atomic number.

•

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

Atomic Mass

​5

​29

Multiple Choice

Which is the sum of the protons and neutrons in an atom?

• atomic number

• average atomic mass

• mass number

• atomic mass unit

Multiple Choice

How many neutrons does an atom of oxygen-15 have?

• 7

• 8

• 15

• 16

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Multiple Choice

What is the atomic number and the number of neutrons for an atom of uranium-238?

• an atomic number of 92 and 146 neutrons

• an atomic number of 92 and 238 neutrons

• an atomic number of 146 and 92 neutrons

• an atomic number of 238 and 92 neutrons

Atoms of the same element that have different
numbers of neutrons are called isotopes.

Isotopes

Isotopes name

Proton

Neutron

Electron

Boron-10

Boron-11

Complete the
following
table:

Isotopes

Atomic structure - Quiz

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Multiple Choice

How do isotopes of an element compare?

• They have different numbers of protons.

• They have different numbers of neutrons.

• They describe different elements.

• They have different atomic numbers.

Isotope

Abundance (%)

Boron-10

20

Boron-11

80

The average atomic mass of an element is the
weighted average mass of all naturally occurring
isotopes of an element, measured in atomic mass
units (amu), according to their natural abundances.

Isotopes

Isotopes

Isotopes

Multiple Choice

Chlorine has two isotopes. Chlorine-35 makes up about 76% of chlorine, and chlorine-37 makes up about 24%. What is the average atomic mass of chlorine?

• 36.5

• 36.0

• 35.5

• 35.0

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