1. Protons- determine the identity of an atom –Elements are different because they each have different #s of protons The atomic number of an element is the number of protons in the nucleus of an atom of that element –You can find this number by looking at your periodic table In a neutral atom, the number of protons is equal to the number of electrons 4.3 Distinguishing Among Atoms 19 55 16 23 B

2. The total number of protons and neutrons in an atom is called the mass number. –The number of neutrons in an atom is the difference between the mass number and atomic number. –How do I find the mass number? If given, add the protons and neutrons together If not, round the mass from the periodic table to the nearest whole number. This will provide the mass number of the most common isotope. 4.3 Distinguishing Among Atoms P = N = E = 445 10 111211

3. 4.3 Distinguishing Among Atoms

4. N = 8 16 61 45

5. Nucleus- contains most of the mass of an atom –Protons & Neutrons contain most of an atom’s mass Mass of proton = mass of neutron = 1amu –Electrons- 1/2000 th of the mass of a proton 4.3 Distinguishing Among Atoms

6. Grams are not used to measure mass of atoms because atoms are too small –Amu is used instead –Amu = atomic mass unit Amu – 1/12 th the mass of a carbon-12 atom, which contains 6 protons and 6 neutrons 4.3 Distinguishing Among Atoms

7. Just as apples come in different varieties, a chemical element can come in different “varieties” called isotopes. 4.3 Distinguishing Among Atoms

8. Isotopes are atoms that have the same number of protons but different numbers of neutrons. –Because isotopes of an element have different numbers of neutrons, they also have different mass numbers. Despite these differences, isotopes are chemically alike because they have identical numbers of protons and electrons. 4.3 Distinguishing Among Atoms

9. To indicate the identity of an isotope, write the element name – mass # –Carbon-12 Carbon-14 –Isotopes can also be indicated using the method we saw earlier 4.3 Distinguishing Among Atoms

10. Cr-50 N = Cr-52 N = Cr-53 N = 26 28 29

11. 4.3 Distinguishing Among Atoms Cu-63 is more abundant because the average mass is closest to the mass of Cu-63.

12. Average atomic mass- a weighted average of the masses of the isotopes of an element Average atomic mass = (% x mass) + (% x mass) + (% x mass)…. Example for Carbon: 4.3 Distinguishing Among Atoms mass numberexact weightpercent abundance 1212.0000 amu98.90 1313.003355 amu1.10 (12.0000amu) (0.9890) + (13.003355amu) (0.0110) = 12.011 amu

13. C has an average atomic mass of 12.011 amu. This is closer to 12 than to 13, which means most isotopes must be carbon- 12. This matches our known data, which say that 98.90% of carbon atoms are C- 12. 4.3 Distinguishing Among Atoms

14. How do we know the mass of isotopes?? Use a mass spectrometer 4.3 Distinguishing Among Atoms

15. A periodic table is an arrangement of elements in which the elements are separated into groups based on a set of repeating properties. –A periodic table allows you to easily compare the properties of one element (or a group of elements) to another element (or group of elements). 4.3 Distinguishing Among Atoms

16. Each horizontal row of the periodic table is called a period. Within a given period, the properties of the elements vary as you move across it from element to element. –This occurs because they have a different number of outer shell (or valence) electrons 4.3 Distinguishing Among Atoms

17. Each vertical column of the periodic table is called a group, or family. Elements within a group have similar chemical and physical properties. –This occurs because they have the same number of valence (or outer shell) electrons 4.3 Distinguishing Among Atoms

18. A Group or Family 4.3 Distinguishing Among Atoms