Average Atomic MassUnstable nuclei and Radioactive Decay

What is an Isotope?

•Isotopes: atoms of the same element that have different masses.

•Isotopes of the same element all have the same number of protons and electrons but different number of neutrons.

•Isotopes are usually identified by specifying their mass number.

Naming Isotopes

•There are two methods for naming isotopes:▫1. Hyphen notation:

The mass number is written with a hyphen after the name of the element.

EX: Tritium (an isotope of hydrogen) is written as hydrogen-3

Naming Isotopes

▫2. Nuclear Symbol EX: Uranium-235 is written as 235

92U. The superscript indicates the mass number and the subscript indicates the atomic number.

Mass of Individual Atoms

•Masses of protons, neutrons, and electrons are so small and difficult to work with

•Scientists developed a method of measuring the mass of an atom relative to the mass of a specifically chosen atomic standard.

•Standard is the carbon-12 atom.

Mass of individual atoms, cont.

•The carbon-12 atom has a mass of 12 atomic mass units.

•So, one atomic mass unit (amu) is defined as 1/12 of the mass of a carbon-12 atom

Average atomic mass

•Definition: Weighted atomic masses of the isotopes of each element

•Each isotope has a known natural occurrence (percentage of that elements’ atoms)

•Example:▫A penny is a mixture of Copper – 63 (about

70%) and Copper – 65 (about 30%)▫So to find the mass of copper in the penny

you’d have to find a weighted average

Calculating Average Atomic Mass•Naturally occurring copper consists of:

▫69.17% Cu-63 (62.93 amu)▫30.83% Cu-65 (64.93 amu)

•Take the mass of each element and multiply by it’s % abundance. Then, add the two numbers together.

•Find the average atomic mass of Copper…

http://www.youtube.com/watch?v=xirPkCI1sMA

What if the identity of the element is not given?•An element has three main isotopes with

the following percent occurrences:▫#1: 19.99244 amu, 90.51%▫#2: 20.99395 amu, 0.27%▫#3: 21.99138 amu, 9.22%

•What is the element?

Radioactivity

•During a chemical reaction, only an atom’s electrons are involved. The nucleus of the atom remains unchanged.

•There are reactions that do involve an atom of one element changing into an atom of another element. These reactions, which involve a change in an atom’s nucleus, are called nuclear reactions.

Radioactivity, cont.

•In the late 1890’s, scientists noticed that some substances spontaneously emitted radiation in a process called radioactivity.

•The rays and particles emitted by a radioactive material were called radiation.

Radioactivity, cont.• Radioactive atoms undergo significant

changes that can alter their identities. • They emit radiation because their nuclei are

unstable. • They gain stability by losing energy when

they emit radiation. • This process is done spontaneously and is

known as radioactive decay. • They continue this process until they produce

stable nonradioactive atoms.

Types of Radiation

Alpha Radiation• Alpha radiation is made up of alpha

particles. Each alpha particle is made up of two protons and two neutrons and thus, has a +2 charge.

• An alpha particle is equivalent to a helium – 4 nucleus and is represented as 4

2He or α. • The alpha decay of radioactive radium-226 is

shown below.

226 88Ra → 222 86Rn + 42He

radium-226 radon-222 alpha particle

Beta Radiation• Beta radiation consists of fast moving

electrons called beta particles. Each beta particle is an electron with –1 charge.

• Beta particles are represented by the symbol 0

-1β. The beta decay of radioactive carbon-14 is shown below.

14 6C → 14 7N + 0-1β

carbon-14 nitrogen-14 beta particle

Gamma Radiation

•The third common type of radiation is gamma radiation or gamma rays.

•Gamma rays are high-energy radiation that possess no mass and have no charge.

•Gamma rays are denoted by the symbol 0

0γ. •Gamma rays usually accompany alpha and

beta radiation and account for most of the energy lost during the radioactive decay process.

Gamma Radiation example238 92U → 234 90Th + 4

2He + 2 0

0γ

uranium-238 thorium-234 alpha particle gamma rays

•Since gamma rays are massless, the emission of gammas rays by themselves cannot result in the formation of a new atom.

Balancing Nuclear Equations

213 At ---------> 4 He + ______85

2

______ ---------> 181 Ir + 4 He 77 2

66 Cu ----------> 66 Zn + ______

29 30

Fission and Fusion

Fission: one atom splitting apart to make two

Fusion: atoms coming together to make one atom

Fission Reaction

Fusion reaction

https://www.youtube.com/watch?v=Ezbyg2iNdQs

https://www.youtube.com/watch?v=pusKlK1L5To#t=29