2. The Modern Model The modern atom is composed of two regions: 1. Electron cloud- Most of the volume of an atom and is the region where the electron can be found (extranuclear) 2. The nucleus – with protons and neutrons so that it has a Positive charge and almost all the mass

3. Major Subatomic Particles Atoms are measured in picometers, 10 -12 meters Hydrogen atom, 32 pm radius Nucleus is tiny compared to atom If the atom were a stadium, the nucleus would be a marble Radius of the nucleus is on the order of 10 -15 m Density within the atom is near 10 14 g/cm 3 NameSymbolChargeRelative Mass (amu) Actual Mass (g) Electrone-e- 1/18409.11x10 -28 Protonp+p+ +111.67x10 -24 Neutronnono 011.67x10 -24

4. Nuclear Symbols Every element is given a corresponding symbol which is composed of 1 or 2 letters (first letter upper case, second lower), as well as the mass number and atomic number E A Z elemental symbol mass number atomic number

5. Locations on periodic table… Atomic Number = number of protons (p + ) in the nucleus Determines the type of atom Li atoms always have 3 protons in the nucleus, Hg always 80 Mass Number = number of protons + neutrons Electrons have a negligible contribution to overall mass In a neutral atom there is the same number of electrons (e - ) and protons (atomic number)

6. Find the Find the number of protons number of protons number of neutrons number of neutrons number of electrons number of electrons atomic number atomic number mass number mass number W 184 74 F 19 9 Br 80 35

7. If an element has an atomic number of 34 and a mass number of 78 what is the: If an element has an atomic number of 34 and a mass number of 78 what is the: number of protons in the atom? number of protons in the atom? number of neutrons in the atom? number of neutrons in the atom? number of electrons in the atom? number of electrons in the atom? complete symbol of the atom? complete symbol of the atom? If an element has 91 protons and 140 neutrons what is the: If an element has 91 protons and 140 neutrons what is the: atomic number? atomic number? mass number? mass number? number of electrons? number of electrons? complete symbol? complete symbol?

8. Isotopes Atoms of the same element can have different numbers of neutrons and therefore have different mass numbers The atoms of the same element that differ in the number of neutrons are called isotopes of that element H 1 1 Protium H 2 1 Deuterium H 3 1 Tritium

9. Naming Isotopes When naming, write the mass number after the name of the element Examples: carbon- 12 carbon -14 uranium-235

10. Average Atomic Mass Did you ever notice that the mass number on the periodic table is not a whole number? There are the decimal numbers on the periodic table. **This is because it is an average mass for all the isotopes of each atom

11. How heavy is an atom of oxygen? There are different kinds of oxygen atoms (different isotopes) 16 O, 17 O, 18 O We are more concerned with average atomic masses, rather than exact ones Based on abundance of each isotope found in nature We can’t use grams as the unit of measure because the numbers would be too small Instead we use Atomic Mass Units (amu) Standard amu is 1/12 the mass of a carbon-12 atom Each isotope has its own atomic mass

12. Calculating Averages You have five rocks, four with a mass of 50 g, and one with a mass of 60 g. What is the average mass of the rocks? Total mass = 4 x 50 g + 1 x 60 g = 260 g Average mass = (4 x 50 g + 1 x 60 g)/5 = (260 g)/5 = 52 g

13. 80% of the rocks were 50 grams 20% of the rocks were 60 grams What is the average mass of the rocks? Average mass = 0.8 x 50 + 0.2 x 60 = 40 + 12 = 52 g Average = (% as decimal) x (mass 1 ) + (% as decimal) x (mass 2 ) + (% as decimal) x (mass 3 ) + … Calculating Averages when given %:

14. 1.Calculate the av atomic mass of copper if copper has two isotopes 69.1% has a mass of 62.93 amu The rest (30.9%) has a mass of 64.93 amu 2.Magnesium has three isotopes 78.99% magnesium 24 with a mass of 23.9850 amu 10.00% magnesium 25 with a mass of 24.9858 amu The rest magnesium 26 with a mass of 25.9826 amu What is the atomic mass of magnesium? EXAMPLES:

15. If not told otherwise, the mass of the isotope is the mass number in amu The average atomic masses are not whole numbers because they are an average mass value Remember, the atomic masses the decimal numbers on the periodic table Average Atomic Masses

16. To do : Page 45 –Questions 1, 2, 3, 4 Page 46 –Questions 2, 3, 4, 5a