1. Atoms / Elements Different number of protons Protons found in nucleus # of protons = atomic number Since atom is electrically neutral: #protons = # electrons

2. Mass of atom concentrated in nucleus Nucleus contains protons & neutrons #protons + #neutrons = mass number

3. Composition Indicated by shorthand Mass# Symbol Atom# 12 C 6 Can use hyphen notation Name—mass# Carbon-12

4. Practice How many protons, neutrons, electrons in: 14 C 6 6 protons, 8 neutrons, 6 electrons 40 K 19 19 protons, 21 neutrons, 19 electrons

5. 238 U 92 92 protons,146 neutrons,92 electrons 99 Mo 42 42 protons, 57 neutrons, 42 electrons Not all atoms of same element are identical

7. Isotopes Atoms with same # of protons, but different # neutrons Behave exactly the same way chemically

8. Hydrogen Isotopes 1 H or Hydrogen-1 or Hydrogen 1 (Protium) 2 H or Hydrogen-2 or Deuterium 1 3 H or Hydrogen-3 or Tritium 1

9. Practice Write the symbol notation for: Strontium -86 Strontium -88 Strontium- 90

10. How many neutrons in each isotope? Strontium -86 48 Strontium -88 50 Strontium -90 52

11. Atomic Mass Extremely massive & difficult to work with Relative atomic mass based on standard reference isotope. Carbon—12 12 Atomic mass units (amu)

12. Mass proton = 1 amu Mass neutron = 1 amu Expectations: mass is whole number Not so: Elements in nature contain mixture of different isotopes

13. Atomic Mass Average atomic mass Based on mass of each isotope and its relative abundance. Average atomic mass = (isotope mass)(relative abundance) + (isotope mass)(relative abundance) + … Relative abundance = percent abundance /100

15. Carbon-Average Atomic Mass Avg. Atomic Mass =(12.000) (.9889) + (13.003) (.0111) =11.87+0.144 =12.014