The Atom

Lesson 2 - Isotopes

Different Forms of the Same Element

• In any specific element, the # of protons is always constant.

• Unlike the number of protons, the number of electrons and neutrons can vary within atoms of an element without changing the identity of the element.• Ex. Carbon (C) ALWAYS has 6 protons,

but it can have anywhere from 6-8 neutrons and 2-10 electrons

Isotopes• Isotopes are atoms of the same element

(same number of protons) but with different number of neutrons

• Carbon has three isotopes

Notice how the atomic # (# of protons) does NOT change but the mass number does.

Determining the mass number of isotopes

• The atomic mass on the periodic table is an average of all the known isotopes of each element.

• It is not the mass of any individual atom.

• To determine the mass number of a specific isotope you need to add the number of protons to the number of neutrons.

Practice • A lithium atom has 3 protons, 3

electrons, and 3 neutrons.• A =• A nitrogen atom has 8 neutrons• A =

• An unknown element has 92 protons and 143 neutrons.• Element = A = 235

15

6

U

Representing Isotopes• Option # 1

• U

• K

• C

• Option #2

• U – 235

• K - 40

• C – 14• Only the mass

number is listed, the atomic number can be found on the P.T.

92235

40

14

6

19

Finding average atomic mass

• To find the average atomic mass of an element you need two pieces of information:• The mass numbers of the different

isotopes• Neon has three: Ne - 20 , Ne - 21, Ne - 22

• The relative abundance of each isotope• Of all the neon measured, 90.60% is Ne-20,

0.200% is Ne-21, and 9.200% is Ne-22

Finding average atomic mass

• Even though it is the least massive, Ne- 20 accounts for the vast majority of Neon.

Isotope Mass Abundance Mass Contribution

Ne-20 20 amu 90.60% 18.12

Ne-21 21 amu 0.20% 0.042

Ne-22 22 amu 9.200% 2.024

Avg. mass = 20.19

Examples

• Gallium-69 has a relative abundance of 60.11% and Gallium-71 has a relative abundance of 39.89%. What is the average atomic mass of Gallium?

• Thallium has two stable isotopes, Thallium-203 and Thallium-205. Thallium-203 has a relative abundance of 29.52%.Thallium-205 has a relative abundance of 70.48%. What is the average atomic mass of Thallium?