Masses of atoms

17.2

Warm up

Label the different parts of an atom.

Complete the table with the appropriate term or number.

Particle Proton Neutron Electron

Location

Charge

Mass

Objectives Compare between the atomic number and the mass number

of an atom

Calculate the number of electrons, protons, and neutrons in an atom given its mass and atomic numbers and vise verse

Explain the existence of the majority of the mass of an atom in its nucleus

Define isotopes

Explain why isotopes have different mass number and similar chemical properties

Define the average atomic mass

Calculate the average atomic mass of an element based on the natural abundances rate of its isotopes

Introduction

Can you guess what is the mass of your laptop in Kg?

Can you predict the mass of the laptop after you place a paperclip on top of it?

How can you compare the mass of the laptop with and without the paperclip to the mass of the atom and its particles?

Which of the following: laptop of paper clip refers to the mass of the electrons in an atom?

Atomic mass

Particle Mass (g) Mass in (Kg)

Protons

Neutrons

Electrons

Complete the following table

Atomic mass

The unit of measurement used for atomic particles is the atomic mass unit (amu)

Mass of p+ = mass of No = 1 amu

Mass of e- <<<<<<<<<<< mass of p+ or No

can you conclude where is the majority of the mass of an atom is located?

Atomic mass

Atomic mass is the mass of the atom (mass of protons and neutrons) in amu

Protons identify the elements

Each type of atom of an element has a different number of protons.

Compare number of P+ and e-

Application

Find, using the periodic table, which element has an atom with the following number of protons:

1- six protons

2- 8 protons

3- 12 protons

4- 20 protons

What is the atomic number of hydrogen, helium, argon, lithium, magnesium, chlorine, sulfur, phosphorous (use the periodic table)

Mass number

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

Number of neutrons= mass number – atomic number ( # of P+)

Atomic and mass number

Application

Find the Atomic number (Z) and the mass number (A) for the following.

Protons are colored red and neutrons are blue

Application

Element

Symbol Atomic number

# of protons

# of neutrons

Mass number

Average atomic mass

Boron 11

Carbon 12

Oxygen 16

Sodium 23

Copper 63

What is the difference and the similarity between the following pictures?

Isotopes Atoms of the same element

with different number of neutrons can have different properties.

Ex: C-12 and C-14

Isotopes are atoms of the same element that have the same atomic number

(number of protons) but different mass number (

number of neutrons)

Identifying isotopes An isotope can be

identified by writing the name or symbol of the atom followed by its mass number. For example:

Carbon-14 or C-14 &

Chlorine-35 or Cl-35

Application Naturally occurring elements of Boron have mass

number of 10 or 11. Calculate the number of neutrons of B-10, and B-11.

Uranium-238 has 92 protons. How many neutrons does it have?

ApplicationFind the number of protons, electrons and

neutrons for each isotope of Lithium.

Write the name or the symbol of the three isotopes of Lithium.

Application Naturally occurring carbon consists of three

isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms.

12C 13C 14C 6 6 6

#P _______ _______ _______

#N _______ _______ _______

#E _______ _______ _______

An atom of zinc has a mass number of 65.

A. Number of protons in the zinc atom

1) 30 2) 35 3) 65

B. Number of neutrons in the zinc atom

1) 30 2) 35 3) 65

C. What is the mass number of a zinc isotope

with 37 neutrons?

1) 37 2) 65 3) 67

Write the atomic symbols for atoms with the following:

A. 8 p+, 8 n, 8 e- ___________

B. 17p+, 20n, 17e- __________

C. 47p+, 60 n, 47 e- ___________

An atom has 14 protons and 20 neutrons.

A. Its atomic number is

1) 14 2) 16 3) 34

B. Its mass number is

1) 14 2) 16 3) 34

C. The element is

1) Si 2) Ca 3) Se

D. Another isotope of this element is

1) 34X 2) 34X 3) 36X

16 14 14

Atomic massNa22.99Listed on the periodic table

Gives the mass of “average” atom of

each element

Average atomic mass is based on all

the isotopes and their abundance %

Atomic mass is not a whole number

Application

Using the periodic table, specify the atomic mass of each element:

A. calcium __________

B. aluminum __________

C. lead __________

D. barium __________

E. iron __________

Calculating average

Calculate the weighted average of the student quiz scores if the following results were obtained: 5 students scored 80%; 19 students scored 90%

Calculating average atomic mass

The atomic mass of an element is the weighted average mass of all of its naturally occurring isotopes

Weighted average =

mass isotope1(%) + mass

isotope2(%) + …

100 100

Application Isotopes Mass of Isotope Abundance

24Mg =24.0 amu 78.70%

25Mg = 25.0 amu 10.13%

26Mg = 26.0 amu 11.17%

Atomic mass (average mass) Mg = 24.3 amu

Mg24.3

Application

Gallium is a metallic element found in small lasers used in compact disc players. In a sample of gallium, there is 60.2% of gallium-69 (68.9 amu) atoms and 39.8% of gallium-71 (70.9 amu) atoms. What is the average atomic mass of gallium?

Wrap up

What is meant by atomic number?

What is the mass number?

How can we calculate the number of protons, neutrons and electrons of an element?

What are isotopes?

How can we identify an isotope?

How can we calculate the average atomic mass of an element?

Home workSection review questions

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