Counting Atoms

3.3

Counting Atoms

• Very difficult to count

• Atomic Number – # of p+ of each atom of that element

• Whole numbers

• Elements arranged by atomic #

• Identifies the element

Isotopes

• Isotope – an atom of the same element with different # of neutrons

• Elements are mix of isotopes• Hydrogen has 3• Mass Number – total # of protons

and neutrons that make up the nucleus of an isotope

Designating Isotopes

• Hyphen notation = Hydrogen – 3• Nuclear symbol- U• 92 is atomic # (# of protons)• # of n0 = mass # - atomic #• How many p+,n0,e- r in Cl-37, Br-80?• What’s the notation for an isotope

with 15 e- and 15 n0?

238

92

Relative Atomic Mass

• Atomic Mass Standard is C-12

• Atomic Mass Unit – (amu) exactly 1/12 the mass of a carbon-12 atom

• Masses are relative to C-12 atoms.

• All masses are averages of amu’s b/c of isotopes

Average Atomic Mass• AAM – the weighted average of the

atomic masses of the naturally occurring isotopes of an element

• The mass of Cu-63 is 62.94 amu and the mass of Cu-65 is 64.93 amu. The abundance of Cu-63 is 69.17% and Cu-65 is 30.83%. Calculate the relative atomic mass of copper.

R.A.M. Problem• Chlorine exists as Cl-35 which has

a mass of 34.969 amu and makes up 75.8 % of Cl atoms. The rest of naturally occurring chlorine is Cl-37 with a mass of 36.996 amu. What is the average mass of chlorine?

R.A.M. Problem• Uranium-234 makes up 0.005% of

uranium atoms and has a mass of 234.041 amu. Uranium-235 makes up 0.720% and has a mass of 235.044 amu. Uranium-238 has a mass of 238.051 amu and makes up 99.275 %. What is the average atomic mass of Uranium?

R.A.M. Problem• Carbon-12 makes up 98.90% of

existing carbon. Carbon-13, with a mass of 13.003 amu, makes up 1.10%. Traces of carbon-14 also exist. What is the average atomic mass of carbon?

Relating Mass # to Atoms

• Mole – Amount of a substance that contains as many particles as there are atoms in exactly 12 g of C-12.

• Way of counting (like 1 dozen = 12)

• Avogadro’s number – the # of particles in 1 mole = 6.022 x 1023

Molar Mass

• Molar Mass – mass of 1 mole of a substance

• Expressed as g/mol

• For an element it’s = to atomic mass

• What’s the molar mass of 1 mole of Li, Hg, C, He, and Cl

Conversions with Molar Mass

• Convert …

2 mole of He to grams of He

3.5 mole of Cu to grams of Cu

100 grams of C to mole of C

250 grams of Br to mole of Br

Conversions with Avogadro’s #

• How many moles are in 3.01 x 1022 atoms of Ag?

• How many moles are in 4.97 x 1020 atoms of Cu?

• How many atoms are in 15 moles of C?

• How many atoms are in 4 moles of S

Practice

• Complete the section review on page 87 and do numbers 2-7.