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### Isotopes

### IsotopesAt the conclusion of our time together, you should be able to:

### IsotopesLet’s see if you can:

### Average Atomic Mass Practice

Of the Atom

- Define an isotope
- Determine the number of protons, neutrons and electrons for an isotope
- Define average atomic mass
- Determine the average atomic mass for an element given the isotopes

Isotopes

Atoms with the same number of protons & electrons but a different number of neutrons.

They are the same element, are chemically identical and undergo the exact same chemical reactions

They have different masses (different mass number).

All isotopes are used to calculate average atomic mass (this mass is usually a decimal).

Most elements consist of a mixture of isotopes.

11B

10B

Isotopes- Atoms of the same element but different mass number.
- Boron-10 (10B) has 5 p and 5 n
- Boron-11 (11B) has 5 p and 6 n

- According to international convention all atomic weights derive from the isotope carbon-12.
- One atomic mass unit (amu) is exactly 1/12 of the mass of a C-12 atom.
- The natural atomic mass of an element is the average of the atomic masses of the isotopes:

15 Helpful Hints On The Lab Report from Mr. T’s Vast Lab Experience!!!

Hint #12. The probability of a given event occurring is inversely proportional to its desirability.

Schematic Diagram of a Mass Spectrometer

Neon Gas

Determining Average Atomic Mass

Because of the existence of isotopes, the mass of a collection of atoms has an average value.

Boron is 20%10B and 80%11B. That is, 11B makes up 80% of the boron in the earth’s crust.

For boron average atomic mass

= 0.20 (10 amu) + 0.80 (11 amu) = 10.8 amu

11B

10B

How to Determine Average Atomic Mass

Determine Relative Abundance –

% abundance divided by 100

Determine mass of each isotope and multiply relative abundance by this mass

(keep all digits your calculator gives you)

Determine Average Atomic Mass by adding up all the individual masses, round to “2” decimal places

#1 Nitrogen Example

- Because of the existence of isotopes, the mass of a collection of atoms has an average value.
- 14N = 99% abundant and 15N = 1%
- (0.99 x 14 amu) + (0.01 x 15 amu) =
Avg. Atomic mass of N = ______________

Avg. Atomic mass of Sb = ______________

14.01 amu

121.84 amu

Example Problem:

There are three naturally occurring isotopes of neon: Ne-20, 90.51%, 19.99244 amu;

Ne-21, 0.27%, 20.99395 amu;

Ne-22, 9.22%, 21.99138 amu. Calculate the average atomic mass of neon.

Atomic mass = (0.9051 x 19.99244 amu)

+ (0.0027 x 20.99395 amu) +

(0.0922 x 21.99138 amu)

Atomic mass = 18.10 amu + 0.057 amu

+ 2.03 amu =

20.18 amu

Isotopes & Average Atomic Mass

- Because of the existence of isotopes, the mass of a collection of atoms has an average value.
- 6Li = 7.5% abundant and 7Li = 92.5%
- Avg. Atomic mass of Li = ______________

- 28Si = 92.23%, 29Si = 4.67%, 30Si = 3.10%
- Avg. Atomic mass of Si = ______________

6.93 amu

28.11 amu

- Define an isotope
- Determine the number of protons, neutrons and electrons for an isotope
- Define average atomic mass
- Determine the average atomic mass for an element given the isotopes

Let’s see what you learned…

Get your clickers ready!!!

Isotopes?

Which of the following represent isotopes of the same element?

Which element?

234 X234 X235 X238 X

92939292

Uranium

Self-Check

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+ _______ _______ _______

#no _______ _______ _______

#e- _______ _______ _______

6

6

6

7

8

6

6

6

6

Self-Check Continued

An atom has 14 protons and 20 neutrons.

A. Its atomic number is

a) 14 b) 6 c) 34 d) 20

B. Its mass number is

a) 14 b)20 c) 16 d) 34

C. The element is

a) Si b) Ca c) Se d) C

D. Another isotope of this element is

a) 34X b) 34X c) 36X

16 14 14

Silicon has 3 isotopes with the following % abundances.

Si-28, 92.23%

Si-29, 4.67%

Si-30, 3.10%

Calculate silicon’s average atomic mass.

Element “X” has three naturally occurring isotopes. 78.70% of “X” atoms exist as X-24, 10.03% exist as X-25 and 11.17% exist as X-26. What is the average atomic mass of element “X” in amu’s?

- 24.00
- 24.29
- 24.30
- 24.99
- 25.00

Candium 78.70% of “X” atoms exist as X-24, 10.03% exist as X-25 and 11.17% exist as X-26. What is the average atomic mass of element “X” in amu’s? Lab

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