ISOTOPES

Chemistry

I. Why Atoms Differ?

A. Atoms of different elements have their own unique structures

1. Because they have different structures they have different

properties

2. An atom of hydrogen in a water molecule is very different from an atom of sodium in salt.

B. How we know atoms are different

1. Each element has a different number of protons

2. The number of protons is called the atomic number

3. Since all atoms are neutral – the atomic number also means the number of electrons

C. The subatomic particle that is different is the neutrons

1. Since each element has a specific number of protons (which is the same as the atomic number) then any

change in atomic mass is caused by

the change in the number of

neutrons

• Neutrons exist to stabilize the nucleus – without them, the nucleus would consist of nothing but positively-charged protons in close proximity to one another.

• Because there are different ways of stabilizing the protons, there are different isotopes.

4. Each element has a different atomic number

5. Ex. Hydrogen has 1 proton and 1 electron = so the atomic

number is 1

6. Ex. Uranium has 92 protons = so has an atomic number of 92

7. The atomic number for an element NEVER changes

V. IsotopesA. Isotopes are atoms of the same

element with different atomic mass numbers.

C126Mass #

Atomic #

Nuclear symbol:

Hyphen notation: carbon -12

Isotopes

© Addison-Wesley Publishing Company, Inc.

B. Ex. of isotope• Chlorine-37

–atomic #:

–mass #:

–# of protons:

–# of electrons:

–# of neutrons:

–Name:

17

37

17

17

20Cl37

17Chlorine-37

III. Isotopes

A. Atoms of an element will always have the same atomic number1. But they do not always have the same atomic mass number2. They always have the same atomic number because they always have the same number of protons3. But, atoms don’t always have the same number of neutrons

B. Atoms that have the same number of protons but a different number of neutrons are called isotopes

1. Different number of neutrons give elements different mass numbers

Ex. Hydrogen has a mass number of 1 on average

But hydrogen also has two isotopes….– So it also has a mass number of 2– And a mass number of 3

2H13H1

1H1

3. Although isotopes have different number of neutrons these atoms still have very similar properties of the element since number of protons and electrons are the same

4. But because the isotopes are slightly different that changes their properties slightly

5. Note – the actual mass of an atom as measured by mass spectrometer is still incredibly small

6. More useful to compare relative masses of atoms using a reference isotope as standard (carbon-12)

7. An atomic mass unit (amu) = 1/12 that of carbon-12

8. The atomic mass unit listed in the periodic table is usually the weighted average mass of all isotopes of an element based on percent / relative abundance (or on some that are very unstable it is the most common isotope)

Abundance

• A factor that is most common, occurs naturally, most plentifully

Percent Abundance

• Percent abundance is parts per hundred

• The individual percent abundances add up to 100%

Relative Abundance

• Relative abundance is parts per one, or the decimal form of the percent

• The individual relative abundances add up to 1 

• Relative abundance tells you the decimal fraction of particles

Percent Abundance & Relative Abundance

Weighted average best represents atomic mass because it considers differences

in mass and abundance among the particles (isotopes)

Useful Isotopes

Carbon - 14 for dating artifacts Cobalt – 60 for cancer radiation

treatment Iodine – 131 for treating thyroid

disordersAmericium – 241 for smoke alarms

IV. Finding the Number of Neutrons

A. To find the number of neutrons an atom has

1. First you have to know the number of protons – that’s the atomic number

2. Then you have to know the atomic mass number

3. When you subtract the atomic number from the mass number rounded to nearest whole number

4. You get the number of neutrons

Mass Number = Atomic No. + No. of Neutrons

Mass Number = No. of Protons + No. of Neutrons

5. Ex. Chlorine 37has an atomic number of 17has a mass number of 37

so 37-17 = 20this chlorine atom has 20 neutrons

Ex. Chlorine 35has an atomic number of 17has an atomic mass number of 35

35 – 17 = 18this atom of chlorine has 18 neutrons

Review Key Points

• Define isotopes

• Describe characteristics of isotopes

• Give 3 examples of the uses of isotopes

Independent Practice

• Mass of Candium Lab – link

• Practice Sets – – Isotopes 1 link– Isotopes 2 link