Atoms: The building blocks of

Matter

Counting Atoms

Atomic Number• The number of

protons in the nucleus of each atom

• Identifies element• Periodic table –

elements are arranged in order of increasing atomic #

Isotopes

• Atoms of the same element, with different numbers of neutrons

• Only the masses are different.• Mass # = # protons plus the # neutrons

Designating Isotopes• Hyphen notation/Nuclear symbol

C126 C136Carbon-12 Carbon-13

Sample Problems

1. How many protons, electrons, and neutrons are in an atom of Chlorine-37?

2. Write the nuclear symbol for oxygen-16.

3. Write the hyphen notation for the element whose atoms contain 7 electrons and 9 neutrons.

Atomic MassOne Oxygen-16 atom = 2.657 x 10-23g

• Masses of atoms are measured in atomic mass units (amu)

By definition: 1 Carbon-12 atom = 12 amu

so…. 1 amu = 1/12 mass of a Carbon-12 atom

Atomic MassSome examples:

Oxygen-16: 15.994915 amu

1 proton: 1.007276 amu

1 neutron: 1.008665 amu

1 electron: 0.00054896 amu

How can carbon-12 have a mass of 12 amu?

Average Atomic Mass• Atomic mass of carbon: 12.0111 amu (on p.t.)• The atomic masses on the periodic table are

weighted averages of the isotopes of each element.

Isotope Percent abundance

Atomic mass(amu)

Carbon-12 98.90 % 12

Carbon 13 1.10 % 13.003355

amuamuamu 0111.12)003355.130110.0()129890.0(

Isotope Percent Abundance

Atomic Mass (amu)

Oxygen 16Oxygen-17Oxygen-18

99.7620.0380.200

15.99491516.99913117.999160

Copper-63Copper-65

69.1730.83

62.92959964.927793

Uranium-234Uranium-235Uranium-238

0.0050.720

99.275

234.040947235.043924238.050784

The Mole

Mole – SI unit for amount

By definition…One mole equals the number of atoms in

exactly 12 grams of Carbon-12

Avogadro’s Number: 6.02 x 1023

Molar Mass

Molar Mass – the mass of one mole of a pure substance

• The molar mass of an element is numerically equal to the atomic mass.

1 atom Hydrogen: 1.00794 amu

1 mole Hydrogen: 1.00794 g

Sample Problems:

1. What is the mass in grams of 3.6 mol of the element Carbon?

2. What is the mass in grams of 0.733 mol of the element Chlorine?

3. How many moles of copper are in 3.22 g of copper?

4. How many moles of lithium are in 2.72 x 10-4 g of lithium?

Mole Conversions1. How many moles of carbon are in 2.25x1022 atoms

of carbon?

2. How many moles of oxygen are in 2,000,000 atoms of oxygen?

3. How many atoms of sodium are in 3.80 mol of sodium?

4. What is the mass in grams of 5.0 x 109 atoms of neon?

5. How many atoms of carbon are in 0.020 g of carbon?

6. What mass of silver contains the same number of atoms as 10.0g of boron?