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
• 40K 19
19 protons, 21 neutrons, 19 electrons

5. • Not all atoms of same element are identical
• 238U 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
1H or Hydrogen-1 or Hydrogen 1
2H or Hydrogen-2 or Deuterium
3H or Hydrogen-3 or Tritium

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

10. How many neutrons in each isotope?
• Strontium • 48
• Strontium • 50
• Strontium • 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.
= (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) = =