Periodic tablePeriodic table

Groups – Columns down Alkali MetalsAlkaline Earth MetalsHalogensNoble gases

Periods – Rows acrossLanthanidesActinides

Groups – Columns down Alkali MetalsAlkaline Earth MetalsHalogensNoble gases

Periods – Rows acrossLanthanidesActinides

Figure 02.16Figure 02.16

Diatomic GasesDiatomic Gases

Element Symbols Atomic Number=number of protons (+)the Z numberAtomic Mass=number of

protons and neutrons (+/-) the A number

Element Symbols Atomic Number=number of protons (+)the Z numberAtomic Mass=number of

protons and neutrons (+/-) the A number

IsotopesIsotopes

There are three different types of oxygen atoms shown above.

All have same # protons, but diff # neutrons, and diff mass #

Chemically alike because have identical # protons and electrons, which are subatomic particles responsible for chemical behavior.

There are three different types of oxygen atoms shown above.

All have same # protons, but diff # neutrons, and diff mass #

Chemically alike because have identical # protons and electrons, which are subatomic particles responsible for chemical behavior.

Symbol Practice ProblemsSymbol Practice Problems

Write the symbol for each of the three isotopes for oxygen (oxygen-16, oxygen-17 and oxygen-18) including atomic # and mass #

Three isotopes of chromium are Cr-50, Cr-52, and Cr-53. How many neutrons are in each isotope? Atomic number of Cr is 24.

Write the symbol for each of the three isotopes for oxygen (oxygen-16, oxygen-17 and oxygen-18) including atomic # and mass #

Three isotopes of chromium are Cr-50, Cr-52, and Cr-53. How many neutrons are in each isotope? Atomic number of Cr is 24.

Atomic Mass (amu)Atomic Mass (amu)

In nature, most elements occur as mixture of one or two isotopes.

Each isotope has a fixed mass and natural percent abundance.

Consider three isotopes of HydrogenHydrogen 1 (99.985%)Hydrogen 2 and 3 present in trace amounts.Look at atomic mass of H on periodic table

(1.0079)This is very close to the mass of hydrogen-1

(1.0078). The slight difference takes into account the

larger masses, but the smaller amounts of the other two isotopes of hydrogen.

In nature, most elements occur as mixture of one or two isotopes.

Each isotope has a fixed mass and natural percent abundance.

Consider three isotopes of HydrogenHydrogen 1 (99.985%)Hydrogen 2 and 3 present in trace amounts.Look at atomic mass of H on periodic table

(1.0079)This is very close to the mass of hydrogen-1

(1.0078). The slight difference takes into account the

larger masses, but the smaller amounts of the other two isotopes of hydrogen.

Atomic MassAtomic Mass

Atomic mass of an element is a weighted average mass of the atoms in a naturally occurring sample of an element.

Weighted average mass accounts for both mass and relative abundance of isotopes.

Atomic mass of an element is a weighted average mass of the atoms in a naturally occurring sample of an element.

Weighted average mass accounts for both mass and relative abundance of isotopes.

You can calculate the atomic mass of an element if you are given:The number of isotopesThe mass of each isotopeThe percent abundance of each isotope.

To calculate the atomic mass, multiply the mass of each isotope by its natural abundance (in decimal form) then add the products.

You can calculate the atomic mass of an element if you are given:The number of isotopesThe mass of each isotopeThe percent abundance of each isotope.

To calculate the atomic mass, multiply the mass of each isotope by its natural abundance (in decimal form) then add the products.

Atomic MassAtomic Mass

Example - Average Atomic Mass

Example - Average Atomic Mass

Copper has two stable isotopes: carbon-12 has abundance of 98.89% and carbon-13 has natural abundance of 1.11%. The mass of Carbon-12 is 12.00 amu and the mass of Carbon-13 is 13.003 amu. Calculate atomic mass of Carbon.

Copper has two stable isotopes: carbon-12 has abundance of 98.89% and carbon-13 has natural abundance of 1.11%. The mass of Carbon-12 is 12.00 amu and the mass of Carbon-13 is 13.003 amu. Calculate atomic mass of Carbon.

AnswerAnswer

Atomic mass of carbon = (12.00amu X .9889) + (13.003 X 0.0111) = 12.011 amu

Atomic mass of carbon = (12.00amu X .9889) + (13.003 X 0.0111) = 12.011 amu

ExampleExample

Calculate the atomic mass of bromine. The two isotopes of bromine have atomic masses and relative abundance of 78.92 amu (50.69%) and 80.92 amu (49.31%).

Calculate the atomic mass of bromine. The two isotopes of bromine have atomic masses and relative abundance of 78.92 amu (50.69%) and 80.92 amu (49.31%).

AnswerAnswer

Atomic mass of Bromine: (78.92 amu X .5069) + (80.92 amu X .4931) = 79.91 amu

Homework: Complete isotopes worksheet

Atomic mass of Bromine: (78.92 amu X .5069) + (80.92 amu X .4931) = 79.91 amu

Homework: Complete isotopes worksheet