1. Section 4.3 How Atoms Differ

2. Objectives Explain the role of atomic number in determining the identity of an atom Define an isotope and explain why atomic masses are not whole numbers Calculate the number of electrons, protons, and neutrons in an atom given its mass number and atomic number

3. Atomic Number References the number of protons in the nucleus of an atom of a particular element Every element has a unique atomic number Atomic Number = # of protons = # of electrons IN A NEUTRAL ATOM!!!

5. Question If Pb has an atomic number of 82, how many protons are in the nucleus of every atom of Pb? How many electrons must be orbiting the nucleus of every neutral Pb atom?

6. Mass Number Total number of protons plus neutrons in the nucleus of an atom of an element Uses the number of neutrons in the most abundant form of the element

7. How many protons? How many electrons? How many neutrons?

8. Isotopes Atoms with identical atomic numbers but different mass numbers Number of Neutrons = Mass Number - # of protons

12. Mass of the Atom Expressed in the atomic mass unit (amu) 1 amu = 1.66054 x 10 -24 g 1 g = 6.02214 x 10 23 amu

13. Atomic Weight Determined by average atomic masses of the isotopes of an element Atomic Weight = Sum of multiplying the abundance of each isotope by its atomic mass

14. Isotopes: There are 23 known isotopes of krypton, 6 stable and 17 unstable. Natural krypton contains the 6 stable isotopes.

15. Atomic Weight Problem Naturally occurring chlorine is 75.78% 35 Cl, which has an atomic mass of 34.969 amu, and 24.22% 37 Cl, which has an atomic mass of 36.966 amu. Calculate the average atomic weight of chlorine.

16. Solution Average atomic weight = (0.7578)(34.969 amu) + (0.2422)(36.966 amu) = 35.45 amu