Isotopes

Isowhat?Man, you trippin’ nephew.

If you change the number of…

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

Protons

Electrons

Neutrons

If you change the number of…

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

Protons Identity of the atom

Completely different atom

Electrons Charge Ion

Neutrons Mass Isotope

What Are Isotopes?

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

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

An Example

• Hydrogen has 3 isotopes– There are 3 different “versions” of a hydrogen atom

1. 1 proton and 0 neutrons (mass number = 1)

2. 1 proton and 1 neutron (mass number = 2)

3. 1 proton and 2 neutrons (mass number = 3)

Note how the number ofprotons never change!!!

Ever Wonder Why The Massesof Atoms Are Such Weird

Numbers?• For example, carbon’s mass is 12.01115 amu

– amu means atomic mass unit– amu’s are a unit created specifically for measuring the

extremely small masses of atoms

• Another example, neon’s mass is 20.183 amu

Why All the Decimal Places?

• Atomic mass is the “weighted” average of all the different isotopes of that element.

Example: Magnesium• Has 3 isotopes

# protons # neutrons

Mass (amu’s)

% abundance

Isotope 1 12 12 23.985045 79%

Isotope 2 12 13 24.985839 10%

Isotope 3 12 14 25.982595 11%

% abundance??????

• Every isotope is found in nature in a fixed percentage.

• In other words, 79% of the magnesium atoms in any “scoop” of magnesium found anywhere in the universe will be magnesium atoms with 12 neutrons, 10% of the atoms in the scoop will have 13 neutrons, and 11% will have 14 neutrons.– These percentages are different for each

element…you just have to look them up to find out what they are.

Calculating Atomic Masses• For magnesium, the atomic mass is given

by…

23.985045(.79) +24.985839(.10)

+25.982595(.11) 24.30486 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 add

them up and divideby 3Note how the %

abundances were converted to decimals

(just divide each by 100)

The General Formula for Calculating Atomic Masses Is…

(mass of isotope 1)(% abundance/100)

+(mass of isotope 2)(% abundance/100)

+(mass of isotope 3)(% abundance/100)

+’keep going if there are more isotopes’

O.K. nephew…try these examples, then we can

chill‘till the next episode…

Calculate the Atomic Massof Carbon

Isotope Mass % abundance

#1 12.000000 98.90

#2 13.003355 1.10

For Carbon…

12.000000(98.90 / 100)

+13.003355(1.10 / 100)

Calculate the Atomic Massof Chromium

Isotope Mass % abundance

#1 49.946046 4.35

#2 51.940510 83.79

#3 52.940651 9.50

#4 53.938882 2.36

For Chromium…

49.946046(4.35/100) 51.940510(83.79/100)

+52.940651(9.50/100) 53.938882(2.36/100)

The Vocabulary Section

Terms associated with isotopes

A Few Important TermsYa Gotta Know

• 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)

A Few Important TermsYou Gotta Know

• 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

A Few Important TermsYou Gotta Know

• Mass number = the sum of protons + neutrons

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

– This HAS to be a whole number!!!!!– It has to be, because you can’t have part of a proton

or neutron– Mass number is NOT given on the periodic table

Answers

1. 12 and 13

2. 7

3. 19

4. 10

X Notation for Isotopes

A shorthand way to write isotopes

ZA

A Short Way to Write Isotopes

X

“A” is the symbolfor Mass Number

(#protons + #neutrons)

“X” will be thesymbol of the

element

Z

A

“Z” is the

symbol for

Atomic Number

(# of protons)

Jay - ZYo! If you’re doin’ chem

problems I feel bad for you son….

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

Problem #1

1. Write az X notation for an isotope that has

17 protons and 18 neutrons.

Problem #2

• Write azX notation for an isotope that has 19

protons and 21 neutrons.

Problem #3

• Write azX notation for an isotope that has

15 protons, 18 electrons, and 16 neutrons.

Answer: P3-

This isotope is

also an ion!!!

3115

Problem #4

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

•20

Ca2+41

Answer: 20 protons, 18 electrons, 21 neutrons

Problem #5

• How many protons, electrons, and neutrons are in:

1. 56Fe2+

2. 27Al3+

3. 79Se2-

26

13

34

Problem #5 Answers

1. 26, 24, 30

2. 13, 10, 14

3. 34, 36, 45

Problem #6

Write the proper notation for:

1. Isotope with 21 protons, 18 electrons, and 24 neutrons

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

Problem #6 Answers

1. 45Sc3+

2. 127I-

21

53

Different Forms of AX

• 3517Cl is the same as 35Cl17

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

Z