Isotopes

DrillYou completed problem #2 on the Structure

of the Atom WS yesterday.

Pull out that worksheet.We will go over #2.

ObjectivesiWBAT

determine the number of protons, neutrons and electrons for isotopes

Explain how isotopes are different from neutral atoms and ions.

Guided NotesPick up a copy of the “Isotopes” Guided

Notes WS

Isotopes are like twins

ISOTOPESatoms that have the same number of protons,

but different numbers of neutronsthe chemical properties of an element depend

primarily on its electrons and protonsisotopes of the same element, with different

numbers of neutrons have essentially the same chemical properties

ISOTOPESthe major difference between 2 isotopes is their mass

isotopes with more neutrons have higher mass than those with fewer neutrons

mass # = (isotope’s # of p+) + (isotope’s # of n0)

to identify an isotope you add the mass number after the element’s name (i.e. chlorine -37)

number of neutrons = mass # - atomic #

On Your NotesChlorine is used as an example.

We will look at two chlorine isotopes Chlorine – 35 Chlorine - 37

The “35” and “37” represent mass numbers.Do you remember how to determine a mass

number?

Determine the # of neutrons for each isotope of chlorineChlorine – 35 has 18 neutronsChlorine – 37 has 20 neutrons

Both isotopes have the same number of protons and electrons, but different numbers of neutrons.

Carbon-12The entire periodic table is based on the

carbon-12 isotope.

You can write the isotope as:

carbon-12 (the line is a dash not minus sign)or12 C (notice how the mass # is written at the top of the symbol)

ExampleMass number

p+ e- nº

Carbon-12 6 6 6

Carbon-14 6 6 8

2 extra neutrons

Protons don’t change

Electrons don’t change

Iodine-125What is the difference between this isotope of

iodine and the iodine shown on the periodic table?

How many neutrons does iodine-125 have?

PracticePick up a copy of the “Structure of the Atom”

WSAnswer question “5” only

Average Atomic MassTake a look at the Periodic Table.Notice that the atomic mass for most of the

elements is not a whole number.

This is because there are various isotopes for each element.

The average atomic mass of the element is calculated and that is what you see as the mass on the periodic table.

Avg Atomic MassPick up a copy of the

Isotopes and Average Atomic Mass WS.

Let’s look at the example in the box at the top of the page.

ExampleA sample of cesium is 75% 133 Cs, 20% 132 Cs and

5% 134 Cs. What is its average atomic mass?

Change the % into a decimal.Multiply the decimal times the mass #.Add all of the answers.

0.75 x 133 = 99.750.20 x 132 = 26.40.05 x 134 = 6.7

132.85 amu

You will sometimessee the mass withunits of amu“atomic mass units”

Your GradeWe use a math process like Average Atomic

Mass to calculate your grade.

50% assessments30% labs20% home/classwork

Your GradeIf you currently have…

89.4 % for assessments73.7 % for labs84.1 % for home/classwork

What is your overall grade for the marking period?

Your GradeAssessments 50% becomes 0.50Labs 30% becomes 0.30home/classwork20% becomes 0.20

Assessments 89.4 % x 0.50 = 44.7Labs 73.7 % x 0.30 = 22.11 home/classwork 84.1 % x 0.20 = 16.82 83.63

SummaryExplain the difference between a neutral

atom, ion and isotope.

Protons:

An atom’s identity is determined by the number of protons. If you change the number of protons, you change the element.

Neutrons:

Neutrons maintain stability. If you change the number of neutrons, you have an ISOTOPE.

Electrons:

Electrons determine an atom’s behavior. If you change the number of electrons, then you have an ION.

Summary