• The number of protons in the nucleus of each atom
• Identifies element
• Periodic table – elements are arranged in order of increasing atomic #

• Atoms of the same element, with different numbers of neutrons
• Only the masses are different.
• Mass # = # protons plus the # neutrons

• Hyphen notation/Nuclear symbol
12
C
6
Carbon-12
13
C
6
Carbon-13

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.

One Oxygen-16 atom = 2.657 x 10 -23 g
• 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

Some examples:
Oxygen-16:
1 proton:
1 neutron:
1 electron:
15.994915 amu
1.007276 amu
1.008665 amu
0.00054896 amu
How can carbon-12 have a mass of 12 amu?

• 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
Carbon-12
Percent abundance
98.90 %
Atomic mass
(amu)
12
Carbon 13 1.10 % 13.003355
( 0 .
9890

12 amu )

( 0 .
0110

13 .
003355 amu )

12 .
0111 amu

Isotope
Oxygen 16
Oxygen-17
Oxygen-18
Copper-63
Copper-65
Uranium-234
Uranium-235
Uranium-238
Percent
Abundance
Atomic Mass
(amu)
99.762
0.038
0.200
69.17
30.83
15.994915
16.999131
17.999160
62.929599
64.927793
0.005
0.720
99.275
234.040947
235.043924
238.050784

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 10 23

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 mole Hydrogen:
1.00794 amu
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?

1. How many moles of carbon are in 2.25x10
22 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 10 9 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?