Subatomic Heavyweights Isotopes 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 Li 3 6 Li 3 7 Li 3 8

How do isotopes differ? Physical properties Differ only in mass and related properties Density Chemically almost identical

Famous isotopes H 1 3

Famous isotopes uranium - 235

C 6 14 Famous isotopes Generated from N-14 in atmosphere by cosmic rays Absorbed in plants C-14 ½ life = 5800 yrs Date up < 60,000 yrs

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.

Which balls are protons, electrons and neutrons? boron atom 1 2 3 4 5 6 7 8 9 10 # protons # neutrons # electrons mass number

(cont.) 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

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?

stuff to know Vocabulary Mass number Isotope Atomic mass Skills 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 #’s Concepts What is a weighted average? How do isotopes account for the atomic weight? How do isotope phys. and chem. properties differ? stuff to know