If you change the number of…

It will affect the… And the result will be a…

Protons

Electrons

Neutrons

Identity of the atom

completely different atom

Charge Ion

Mass Isotope

They are atoms of the same element that have different numbers of neutrons.

It’s just a change in the neutrizzle fo shizzle!!!

Hydrogen has 3 isotopesThere are 3 different “versions” of a

hydrogen atom

Note how the number ofprotons never change!!!

Hydrogen has 3 isotopesThere are 3 different “versions” of a

hydrogen atom

Having more neutrons increases the mass

Hydrogen has 3 isotopesThere are 3 different “versions” of a

hydrogen atom

Mass number

= 1

Mass number = 2

Mas

s nu

mbe

r

= 3

6

C12.01115

10

Ne

20.180

13

Al26.982

1

H1.00797

This atomic mass is the one number that best represents the mass of all three versions of

hydrogen.

1

H1.00797

No atom of hydrogen anywhere in the universe actually has this mass.

1

H1.00797

This number is a “weighted” average of the masses of every isotope of hydrogen

1. First, you have to know the mass of each isotope

2. Second, you have to know the percent abundance* of each isotope

*see next slide

Percent abundance is how often that particular isotope occurs in a sample of the element

Sample of Magnesium

A sample of magnesium collected

anywhere in the universe will contain

three isotopes of magnesium.

These are the percent abundances of each isotope of magnesium

Sample of Magnesium

78.90% of this magnesium will be the isotope named

“magnesium 24”

10.00% of this magnesium will be the isotope named

“magnesium 25”

11.10% of this magnesium will be the isotope named

“magnesium 26”

This data will be provided to you in problems or you will be given a table where you can look

it up.

Sample of Magnesium

78.90% of this magnesium will be the isotope named

“magnesium 24”

10.00% of this magnesium will be the isotope named

“magnesium 25”

11.10% of this magnesium will be the isotope named

“magnesium 26”

The formula(Mass of isotope #1)(percent abundance)

100

(Mass of isotope #2)(percent abundance)100

(Mass of isotope #3)(percent abundance)100

# protons # neutrons Mass (amu’s)

% abundance

Isotope 1(magnesium-24)

12 12 23.985045 78.90%

Isotope 2(magnesium-25)

12 13 24.985839 10.00%

Isotope 3(magnesium-26)

12 14 25.982595 11.10%

Using the formula for calculating a weighted average

23.985045(.7890) +24.985839(.1000)

+25.982595(.1110) 24.306852 amu’s

Since its a “weighted”average this number

has more influence sincethis mass occurs 79% of

the time

It’s NOT just a straightaverage where you addthe masses and divide

by 3

Note how the %abundances were

converted to decimals(just divide each by 100)

Isotope Mass % abundance

#1 49.946046 4.35

#2 51.940510 83.79

#3 52.940651 9.50

#4 53.938882 2.36

49.946046(0.0435) 51.940510(0.8379)

+52.940651(0.0950) 53.938882(0.0236) 51.995926 amu’s

Terms associated with isotopes

Atomic number = the number of protons

It’s the same as the element’s position on the periodic table

For example, carbon is the 6th element on the table, so its atomic number is 6 which means it has 6 protons (and 6 electrons if it’s a neutral atom)

Atomic mass = the “weighted” average mass of all the different

isotopes of an atom

This is the “weird” number on the periodic table that usually has a bunch of places past the decimal point

Mass number = the sum of protons + neutrons

This one kind of makes sense because the electrons essentially don’t have any mass

This HAS to be a whole number because you can’t have part of a proton or neutron

Mass number is NOT given on the periodic table

1. What is the mass number of a carbon atom that has 6 neutrons? 7 neutrons?

2. How many neutrons does a nitrogen atom have if its mass number is 14?

3. How many neutrons does a chlorine atom have if its mass number is 36?

4. What is the atomic number of neon?5. What is the atomic mass of neon?

1. 12 and 132. 73. 194. 105. 20.180 amu’s

A shorthand way to write isotopes

AXZ

“A” is the symbolfor Mass Number

(#protons + #neutrons)“Z” is thesymbol for

Atomic Number(# of protons)

“X” will be thesymbol of the

element

Yo! If you’re doin’ chem problems I feel bad for you

son….

I got 99 problems, and here’s the first one….HIT ME!!!!

Write aXz notation for an isotope that has 17 protons and 18 neutrons.

Write aXz notation for an isotope that has 17 protons and 18 neutrons.

35Cl17

Write aXz notation for an isotope that has 19 protons and 21 neutrons.

Write aXz notation for an isotope that has 19 protons and 21 neutrons.

40K19

Write aXz notation for an isotope that has 15 protons, 18 electrons,

and 16 neutrons.

Write aXz notation for an isotope that has 15 protons, 18 electrons,

and 16 neutrons.

31 P3- 15

This isotope is

also an ion!!!

How many protons, electrons, and neutrons are in this isotope?

41

Ca2+

20

How many protons, electrons, and neutrons are in this isotope?

41

Ca2+

20

20 protons, 18 electrons, 21 neutrons

How many protons, electrons, and neutrons?

1. 56Fe2+26

2. 27Al3+13

3. 79Se2-34

How many protons, electrons, and neutrons?

1. 56Fe2+26 26, 24, 30

2. 27Al3+13 13, 10, 14

3. 79Se2-34 34, 36, 45

Write aXz notation for:1. Isotope with 21 protons, 18 electrons,

and 24 neutrons

2. Isotope with 53 protons, 74 neutrons, and 54 electrons

Write aXz notation for:1. Isotope with 21 protons, 18 electrons,

and 24 neutrons

45Sc3+21

2. Isotope with 53 protons, 74 neutrons, and 54 electrons

127I-53

35Cl17 is the same as 3517Cl

Might also see “Cl-35” or “chlorine-35”