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

#protons = # electrons

• Mass of atom concentrated in nucleus

• Nucleus contains protons & neutrons

• #protons + #neutrons = mass number

Composition• Indicated by shorthand• Mass# Symbol

Atom#12 C 6

• Can use hyphen notation• Name—mass#

Carbon-12

Practice• How many protons, neutrons, electrons in:• 14 C 6

6 protons, 8 neutrons, 6 electrons• 40K 19

19 protons, 21 neutrons, 19 electrons

• 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

Isotopes

• Atoms with same # of protons, but different # neutrons• Behave exactly the same way chemically

Hydrogen Isotopes

1H or Hydrogen-1 or Hydrogen 1

(Protium)2H or Hydrogen-2 or Deuterium 1

3H or Hydrogen-3 or Tritium1

Practice• Write the symbol notation for:

• Strontium -86• Strontium -88• Strontium- 90

How many neutrons in each isotope?

• Strontium -86 • 48• Strontium -88 • 50• Strontium -90 • 52

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

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

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

Carbon-Average Atomic Mass

• Avg. Atomic Mass

=(12.000) (.9889) + (13.003) (.0111)= 11.87 + 0.144= 12.014