1. Section 4.2—Atomic Structure Objectives: determine the number of protons, neutrons, & electrons in atoms distinguish between the properties of protons, neutrons, & electrons define isotopes differentiate between mass number and average atomic mass calculate weighted average atomic masses

2. What are atoms? An atom is the smallest piece of an element that has all the chemical properties of that element.

3. What is in an atom? An atom is made of three sub-atomic particles ParticleLocation Nucleus Outside the nucleus Mass 1 amu (1.67  10 -27 kg) 0.00055 amu (9.10  10 -31 kg) Charge +1 0 Proton Neutron Electron

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 On a periodic table, the atomic number shows the number of 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 (called the mass number) 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. The mass number (protons + electrons) represents the mass of an atom.

6. Charges in an Atom Protons have a positive charge (+) Electrons have a negative charge (-) Neutrons have no charge Overall charge of an atom = 0 because the number of protons equals the number of electrons in any atom.

7. Ions An ion is any charged particle; specifically, if an atom LOSES or GAINS electrons, it will become an ion. If an atom loses electrons, there will be more protons than electrons. It will become a positively-charged ion. If an atom gains electrons, there will be more electrons than protons. It will become a negatively-charged ion.

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

9. Example: Element symbols O 16 -2 8 Charge -2 Element Symbol O = Oxygen Mass number 16 Atomic number 8

10. Another element symbol isotopes. It is possible for atoms to have the same number of protons but different numbers of neutrons. Such atoms are called isotopes. To make it easy to identify the various isotopes of an element, scientists add a number after the element’s name. mass number This number is the mass number - the sum of the number of protons and neutrons in the nucleus of an atom of the isotope.

11. Example 16 8 O 17 8 O 18 8 O

12. Let’s Practice Example: Fill in the missing values SymbolNameAtomic # Mass # ChargeProtonNeutronElectron Magnesium-25+2 8 3 Li 151618 53127

13. Isotopes

14. 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!

15. 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! Hydrogen-2 Hydrogen-1 Mass # = 1 amu Mass # = 2 amu

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

17. Isotopes Elements are found in nature as a mixture of all of their isotopes. percent abundance The percent abundance of the isotopes (percent of each isotope in a sample) is constant no matter where the sample is found. Isotopes have the same chemical behavior because chemical behavior is determined by the number of ELECTRONS an element has.

18. Atomic Mass atomic mass The atomic mass of an element is the weighted average mass of the isotopes of that element. Atomic mass, therefore, takes into account the percent abundance of each isotope as well as the mass of an atom of that isotope. The isotope in greater abundance will have a greater effect on determining the atomic mass.

19. Analyzing Atomic Mass Analyzing an element’s atomic mass can SOMETIMES give you a clue as to what the most abundant isotope for that element is. The atomic mass of fluorine is 18.998 amu. From this you can conclude that most fluorine occurs in the form of fluorine- 19.

20. Mass Number versus Atomic Mass Mass NumberAverage Atomic Mass # of protons + # of neutronsAverage of actual masses For one specific isotope only Is not found on the periodic table Weighted average of all isotopes Is found on the periodic table Always a whole numberNot a whole number

21. Calculating Atomic Mass 1 Multiply the % abundance of each isotope by its mass. (75%)(35) = 26.25 amu (25%)(37) = 9.25 amu 2 Add the mass contributions of all the isotopes together. 35.5 amu is the calculated atomic mass of chlorine. Chlorine-35 Chlorine-37

22. Practice Problems Calculate the atomic mass of lithium. IsotopePercent Abundance lithium-67.6% lithium -792.4%