Subatomic HeavyweightsIsotopes of atoms

Isotopes???A chemist investigating a sample of lithium found that lithium atoms have different mass numbers. The chemist determined that the structures of the three types of atoms would be similar to the three drawings below.

• What is different about the three atoms?• What is the atomic number of each atom?• What is the mass number of each atom?• Do you think they are all lithium atoms? Explain

Write your answers on the back of this paper

What is an isotope?• Isotope – atoms with same protons but

different numbers of neutrons

3 p+

3 e-

3 n

3 p+

3 e-

4 n

3 p+

3 e-

5 n

How do we represent isotopes?

lithium-6 lithium-7 lithium-8

Li3

6 Li3

7 Li3

8

How do isotopes differ?

• Physical properties–Differ only in mass and related

properties–Density

• Chemically almost identical

Famous isotopes

H1

3

Famous isotopes

uranium - 235

Famous isotopesGenerated from N-14 in atmosphere by cosmic raysAbsorbed in plantsC-14 ½ life = 5800 yrsDate up < 60,000 yrs C6

14

How do they determine % of isotopes?

Mass spectrometer – separates atoms on the basis of mass (and charge)

Back to lithium…Atomic weight vs. mass number

• What is the atomic mass of lithium listed on your periodic table?

• Do you think the scientist found more lithium with three, four, or five neutrons?

• Why do you think the atomic mass of lithium is not a whole number?

93%7% <0.1%

The Big Question

• How do isotopes of an atom account for the atomic mass of an element?

Goal - Predict the isotopes of an element.

boron atom 1 2 3 4 5 6 7 8 9 10# protons# neutrons# electronsmass number

Which balls are protons, electrons and neutrons?

Element Chemical Symbol

Atomic Number

Atomic Mass

# of protons

# of electrons

# of neutrons

Boron B 5 or 6

Chlorine 17

Lithium 6.94

Vanadium V 23

Nitrogen 7

Magnesium

Argon Ar 39.9 18. 20. or 22

(cont.)

Making Sense• How are all the atoms of an element the same?

What doesn’t change?• How many protons and neutrons in lanthanum-57?• Explain why the atomic masses listed in the periodic

table are not usually whole numbers.• Is the atomic mass of carbon always the same

everywhere in the universe?• Do the physical and chemical properties of carbon

isotopes differ?

Vocabulary• Mass number• Isotope• Atomic massSkills• Know two ways to represent isotopes• Determine dominant isotope from atomic mass• Calculate atomic weight from isotope distribution• Determine # of neutrons from mass and atomic #’sConcepts• What is a weighted average?• How do isotopes account for the atomic weight?• How do isotope phys. and chem. properties differ?

stuff to know