Section 4.2—Atomic Structure

1. Section 4.2—Atomic Structure

2. What are atoms? Atom- smallest piece of matter that has the chemical properties of the element.

3. What’s in an atom? An atom is made of three sub-atomic particles Particle Location Mass Charge Proton Nucleus 1 amu = 1.6710-27 kg +1 Neutron Nucleus 1 amu = 1.6710-27 kg 0 Electron Outside the nucleus 0.00055 amu 9.1010-31 kg -1 1 amu (“atomic mass unit”) = 1.66  10-27 kg

4. What gives an atom its identity? What makes an atom “carbon” as opposed to “oxygen”? • Every atom has a different number of protons. • The number of protons determines the identity of the atom • The atomic number shows the number of protons. Atomic number = protons

5. The Nucleus & Mass • Since the nucleus has protons & neutrons, and the mass of each one is 1 amu… • The mass of the nucleus (in amu’s) is the number of protons + neutrons • Since electrons have relatively no mass (0.054% of one proton or neutron), we don’t need to worry about them when determining mass of an atom Mass # = protons + neutrons

6. Charges • Protons have a positive charge • Electrons have a negative charge • Neutrons have no charge • Overall charge = protons + (-1)×electrons Charge = protons - electrons

7. How do we show information about an element?

8. X A C Z # Element symbols Element Symbol 1 or 2 letters, found on the periodic table Charge # protons - # electrons (assumed to be “0” if blank) Mass number # protons + # neutrons Atomic number # of protons Number How many atoms do you have?

9. O 16 -2 8 Example: Element symbols Element Symbol O = Oxygen Charge -2 Mass number 16 Atomic number 8 Number Assumed to be “1” if blank

10. Let’s Practice Example: Fill in the missing values

11. Let’s Practice Remember: Atomic number is the identity Atomic number = protons Charge = proton - electrons Mass # = protons + neutrons Example: Fill in the missing values 12 25 12 13 10 Lead-208 208 0 82

12. Isotopes

13. What are isotopes? Isotopes- n. Atoms of the same element with a different number of neutrons Some isotopes are radioactive—but not all…many are quite stable!

14. Hydrogen-1 Hydrogen-2 Isotopes Example • If they have different number of neutrons, and neutrons have a mass of 1 amu… • Then isotopes of the same element will have different masses! • But because their protons are the same, they are the same element! Mass # = 2 amu Mass # = 1 amu

15. C C 12 13 Identifying Isotopes Isotopes can be differentiated by their different mass numbers in the element symbol Carbon-12 Carbon-13 Or by the mass number following their name.

16. Mass Number versus Atomic Mass Mass Number Average Atomic Mass # of protons + # of neutrons Average of actual masses Always a whole number Not a whole number For one specific isotope only Weighted average of all isotopes Is not found on the periodic table Is found on the periodic table

17. Calculating Average Atomic Mass Average atomic mass is a weighted average (it takes into account how often each isotope occurs). Actual mass (not mass number) “Sum of” Average atomic mass  ( )  Abundance of isotope Mass of isotope = What fraction of the time is that isotope present?

18. Example of Finding Avg Atomic Mass Example: Find the atomic mass of chlorine if Chlorine-35 has a mass of 34.969 amu and Chlorine-37 has a mass of 36.966 amu and is present 24.22% of the time. Remember that percents add up to 100. So they said the second isotope is present 24.22% of the time. This means that the first isotope is present 100-24.22 = 75.78% of the time This chart summarizes the information in the problem: = 35.45 amu (this is what’s on the periodic table for Cl!)