Calculating Atomic Mass. Atoms are measured in picometers, 10 -12 meters Hydrogen atom, 32 pm radius Nucleus tiny compared to atom If the atom were a.

• Atoms are measured in picometers, 10-12 meters• Hydrogen atom, 32 pm radius

• Nucleus tiny compared to atom• If the atom were a stadium, the nucleus would be a marble

• Radius of the nucleus is on the order of 10-15 m• Density within the atom is near 1014 g/cm3

Name Symbol Charge Relative Mass (amu)

Actual Mass (g)

Electron e- -1 1/1840 9.11x10-28

Proton p+ +1 1 1.67x10-24

Neutron no 0 1 1.67x10-24

• Atomic Number (Z) = number of protons (p+) in the nucleus

• Determines the type of atom• Li atoms always have 3 protons in the nucleus, Hg always 80

• Mass Number (A) = number of protons + neutrons [Sum of p+ and nº]

• Electrons have a negligible contribution to overall mass

• In a neutral atom there is the same number of electrons (e-) and protons (atomic number)

• Every element is given a corresponding symbol which is composed of 1 or 2 letters (first letter upper case, second lower), as well as the mass number and atomic number

E A

Z

elemental elemental symbolsymbol

mass numbermass number

atomic atomic numbernumber

• Find the Find the • number of protonsnumber of protons• number of neutronsnumber of neutrons• number of electronsnumber of electrons• atomic numberatomic number• mass numbermass number

W184 74F

199 Br

80 35

• If an element has an atomic number of 34 and a If an element has an atomic number of 34 and a mass number of 78 what is the:mass number of 78 what is the:

• number of protons in the atom?number of protons in the atom?• number of neutrons in the atom?number of neutrons in the atom?• number of electrons in the atom?number of electrons in the atom?• complete symbol of the atom?complete symbol of the atom?

• If an element has 91 protons and 140 neutrons If an element has 91 protons and 140 neutrons what is the:what is the:

• atomic number?atomic number?• mass number?mass number?• number of electrons?number of electrons?• complete symbol?complete symbol?

• Atoms of the same element can have different numbers of neutrons and therefore have different mass numbers

• The atoms of the same element that differ in the number of neutrons are called isotopes of that element

• When naming, write the mass number after the name of the element

H11

Hydrogen-1

H21Hydrogen-2

H31

Hydrogen-3

•There are different kinds of oxygen atoms (different isotopes)

• 16O, 17O, 18O

• We are more concerned with average atomic masses, rather than exact ones

• Based on abundance of each isotope found in nature

• We can’t use grams as the unit of measure because the numbers would be too small

• Instead we use Atomic Mass Units (amu)• Standard amu is 1/12 the mass of a carbon-12 atom

• Each isotope has its own atomic mass

Average = (% as decimal) x (mass1) + (% as decimal) x

(mass2) + (% as decimal) x (mass3) + …

Try Again…THINK!!Your test and quiz mark is 30%. 10% is made

up of quiz, 20% is made up of tests

If you made 85%, 78% and 79% on your quizzes

If you made 87% on your only test…What would be your test quiz mark for your

report card?

• Calculate the atomic mass of copper if copper has two isotopes 69.1% has a mass of 62.93 amu The rest (30.9%) has a mass of 64.93 amu

• Magnesium has three isotopes 78.99% magnesium 24 with a mass of 23.9850 amu 10.00% magnesium 25 with a mass of 24.9858 amu The rest magnesium 26 with a mass of 25.9826 amu What is the atomic mass of magnesium?

Calculating Averages

• If not told otherwise, the mass of the isotope is the mass number in amu

• The average atomic masses are not whole numbers because they are an average mass value

• Remember, the atomic masses are the decimal numbers on the periodic table

Average Atomic Masses

Isotope WorksheetQuestion 2a:10B = 18.8 %, 11B = 81.2 %Avg = (10)(0.188) + (11)(0.812) = 10.812 amu

Skip 3f and 3g, unless you really want a challenge