Isotopes - What are they?A Nuclear Chemistry Introduction

Isotopes - What are they?

• Find the number of protons, electrons, and neutrons for Nitrogen…

• What if one of these atoms had an extra neutron. Would it still be nitrogen? Why or why not?

• Why are the actual atomic masses decimals?

Isotopes

• While the number of protons for a given element never changes, the number of neutrons can change.

• An atom of the same element with a different number of neutrons is an ISOTOPE!

• If the number of neutrons changes, so does the mass. Different isotopes will have different mass numbers for the same element.

Isotopes

What element do these isotopes represent?

What about these two isotopes?

Writing symbols for isotopes• How do you tell the difference between different

types of the same element?

• The symbol of the element is always written first then the mass number

• Example: C-14– “C” is the symbol– “-14” tells you it is an isotope with a mass of 14– You cannot change the number of protons so you know that

this carbon atom has 2 more neutrons than usual

Radioactivity & Isotopes• What are radioactive elements?

– Isotopes with an unstable nucleus– The nuclei of these isotopes spontaneously

break apart, forming different elements– The number of protons and/or neutrons

change

• Why are some isotopes radioactive?– They just are, but do occur when there is a

large difference between the number of protons and neutrons in the nucleus

– We call them RADIOISOTOPES

•An atom is generally stable if the number of protons equals or is close to the number of neutrons in the nucleus.

•The further apart the two numbers the more unstable.

Stable vs. Unstable Isotopes

Atom (Isotope) Protons Neutrons

Carbon 12Carbon 12 66 66

Nitrogen 14Nitrogen 14 77 77

Oxygen 16Oxygen 16 88 88

Carbon 14 6 8

The line of stability

Isotopes on either side of The valley of stability areradioactive

Isotopes & sports• Floyd Landis stripped of his Tour de

France title

• Isotopes found in his blood came from synthetic testosterone.

Losing your Identity…

The number of protons (atomic number) changes

14

6

14

7

Radioactive Decay causes the nucleus of an atom to change. If the protons change it will change into a new element

• Radiometric Dating

Calculating the average atomic mass

• The abundance (percentage) of each individual isotope is multiplied by the mass of that isotope.

• All the individual abundances are added together

• This is the average mass (what is on the periodic table).

Calculating the average atomic mass

• Element “X”• Natural abundance

of isotope X• 10% = 4 amu• 30 % = 5 amu• 60 % = 6 amu

• 0.10 x 4 = 0.4 amu• 0.30 x 5 = 1.5 amu• 0.60 x 6 = 3.6 amu• 0.4 + 1.5 + 3.6 = 5.5• Average mass = 5.5

amu• This is the mass on

the periodic table

Practice

• Element Q• 25% = 15 amu• 10% = 16 amu• 10% = 17 amu• 55% = 18 amu

• Find the average mass

• 0.25 x 15 = 3.75• 0.10 x 16 =1.6• 0.10 x 17 = 1.7• 0.55 x 18 =9.9

• Total = 16.95• Average = 16.95 amu