1. Atomic Number & Mass Number All atoms consist of protons and electrons- most also have neutrons Protons & neutrons- in the small dense nucleus- similar masses Electrons- occupy the space surrounding the nucleus (1/2000 mass of P & N) According to the atomic theory, atoms of different elements are different- elements differ in the number of PROTONS THEIR ATOMS CONTAIN.

2. Atomic Number & Mass Number Atomic number is the number of protons in an atom… it identifies the element, THE NUMBER OF PROTONS DOES NOT CHANGE IN NORMAL CHEMICAL REACTIONS For an atom to be electrically neutral, the atomic number also identifies the number of electrons. Mass number is the number of particles in the nucleus- thus the number of protons + neutrons Mass Number (amu) = P + N

3. Mass number - atomic number = number of neutrons For Oxygen- 16 - 8 = 8 neutrons 8 protons (atomic #) 8 electrons 8 neutrons

4. Mass number can vary among atoms of the same element. Atomic number identifies an element- but isotopes are atoms of the same element with different masses (number of neutrons) Isotopes are represented with the elemental symbol- 1 H- protium- 1 proton/1 electron/0 neutrons 2 H- deuterium- 1 proton/1 electron/1 neutron 3 H- tritium - 1 proton/1 electron/2 neutron

5. Isotopic Symbols Lithium has two isotopes- 6 Li and 7 Li or Li-6 and Li-7 The mass number written on top- atomic number on bottom

6. Isotopes All elements have at least one isotope- some are radioactive, most are not. Hydrogen has three isotopes, two of them- H- 1 and H-2 are present in your drinking water along with three isotopes of oxygen (O-16, O- 17 and O-18) none of which are radioactive or dangerous.

7. Atomic Mass The atomic mass on the PT is a weighted average of the atomic mass of an element’s isotopes. To find the AVERAGE ATOMIC MASS of an atom, we take into account all of the isotopes that exist and the percentage of each type. The calculation of the average atomic mass is a WEIGHTED AVERAGE. Average atomic mass = ∑ (mass of isotope x relative abundance)

8. Average Atomic Mass Example Isotope Name Isotope Mass(amu) % Abundance Silver-107 106.9050951.86 Silver-109 108.90470 remainder Average atomic mass = ∑ (mass of isotope x relative abundance) Average At. Mass (Ag)= (106.90509 x.5186) + (108.90470 x.4814) Silver Average Atomic Mass = 107.8677 or 107.9 amu HW- Read pp. 234-235 #1,2 p. 236