Unit 3: Atomic Theory & Unit 3: Atomic Theory & StructureStructure

Section 2 – Distinguishing Among Atoms

IntroductionIntroduction

Just as apples come in different varieties, a chemical element can come in different “varieties” called isotopes.

Atomic NumberAtomic Number

Atomic number (Z)Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element.

Mass NumberMass NumberMass number Mass number is the number of protons and neutrons in the nucleus of an isotope.

Mass # = p+ + n0

8 8 1818

Arsenic 75 33 75

Phosphorus 15 3116

IsotopesIsotopesIsotopes are atoms of the same element having different masses due to varying numbers of neutrons.

Atomic Atomic MassesMasses

Atomic massAtomic mass is the average of all the naturally occurring isotopes of that element.

Carbon = 12.011

Mass of AtomsMass of Atoms

• One atomic mass unit (amu) is defined as 1/12th the mass of a carbon-12 atom.• One amu is nearly, but not exactly, equal to one proton and one neutron.

Mass of Atoms (cont.)Mass of Atoms (cont.)

• The atomic mass of an element is the weighted average mass of the isotopes of that element.

Copper-63The average is closer to 63 than it is to 65.

Using Atomic Mass to Determine the Using Atomic Mass to Determine the Relative Abundance of IsotopesRelative Abundance of Isotopes

The atomic mass of copper is 63.546 amu. Which of copper’s two isotopes is more abundant: copper-63 or copper-65?

boron-1110.81 is closer to 11 than it is to 10

Using Atomic Mass to Determine the Using Atomic Mass to Determine the Relative Abundance of IsotopesRelative Abundance of Isotopes

Boron has two isotopes: boron-10 and boron-11. Which is more abundant, given that the atomic mass of boron is 10.81?

Steps to Calculate Atomic MassSteps to Calculate Atomic Mass

1) Divide % abundance by 100 to make it a decimal.2) Multiply the mass of each isotope by the decimal

abundance.3) Add the products from part 2.

Example:

Isotope Mass (amu)

% Abundanc

e

Carbon-12 12.000 98.89

Carbon-13 13.003 1.11

Decimal abundance0.9889

0.0111

(12.000)(0.9889) =

11.87

(13.003)(0.0111) =

0.144

12.01 amu

HONORS

Calculating Atomic MassCalculating Atomic Mass

Element X has two natural isotopes. The isotope with a mass of 10.012 amu (10X) has a relative abundance of 19.91%. The isotope with a mass of 11.009 amu (11X) has a relative abundance of 80.09%. Calculate the atomic mass of this element.

for 10X: (10.012)(0.1991) =

1.993

For 11X: (11.009)(0.8009) =

8.817

10.810 amu

HONORS

Section QuizSection Quiz1. Isotopes of an element have

a. the same mass number.

b. different atomic numbers.

c. the same number of protons but different numbers of neutrons.

d. the same number of protons but different numbers of electrons.

Section QuizSection Quiz2. How many neutrons are in sulfur-33?

a.16 neutrons

b.33 neutrons

c.17 neutrons

d.32.06 neutrons

Section QuizSection Quiz3. If sulfur contained 90.0% sulfur-32 and 10.0%

sulfur-34, its atomic mass would be

a.32.2 amu

b.32.4 amu

c.33.0 amu

d.35.4 amu

HONORS