Isotopes and Mass Number

Atomic Number The number of protons in each atom identifies it as an atom of a

particular element Each atom has a unique number of protons and, in effect, the

number of protons cannot change for a particular atom The Periodic Table is organized by increasing atomic number

(left-to-right and top-to-bottom) All atoms are neutral, so the # protons = the # of electrons

because the number of positive charges equals the number of negative charges

Atomic # = # of protons = # electrons

For Helium, the atomic # is 2. So, weknow that Helium also has 2 protons and 2 electrons

Determining the State of Matter of a Particular Element Using the Periodic Table

The order of the elements on the Periodic Table is based on the number of protons in the nucleus.

How Atoms Differ

Protons are the only thing in an atom that does not change.

How many protons does Platinum have?

How many neutrons does Mercury have?

How many electrons does Lead have?

What similarities do these three elements share?

Proton, Neutron, Electron Practice

The number of protons in the nucleus is called the atomic number. Also since the positive charges must equal the negative charges to have a neutral atom, and all atoms are neutral, the atomic number will also equal the number of electrons in an atom.

What is the atomic number of this atom?

What is the element?

Isotopes

All atoms of a particular element have the same number of protons and electrons The number of neutrons may differ

In nature most elements are found as a mixture of isotopes

Don't be an isodope, learn about an isotope.

Isotopes and Mass

Isotopes differ in mass Isotopes of an atom have the same chemical

behavior. Why?

?

Mass Number

Identifies each of the isotopes of an element

10747Ag 109

47Ag

Mass number = top

Atomic number = bottom

# neutrons = mass # - atomic #

Isotopic Notation Worksheet

Symbol Isotope Name

Atomic # # p+ # n0 Mass # # e-

3H

20F

24Si

56 76

Banana Isotope

93.25% of K atoms have 20 neutrons 6.7302% of K atoms have 22 neutrons .0117% of K atoms have 21 neutrons

Potassium Isotope

Potassium-39 Potassium-40 Potassium-41

protons 19 19 19

neutrons 20 21 22

electrons 19 19 19

Mass of Individual Atoms

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

To determine the weighted average atomic mass of an element, calculate the mass contribution of each of the isotopes and add the values together

Weighted Average Atomic Mass

3517Cl 37

17ClAtomic mass: 34.969 amu Atomic mass: 36.966 amu

Percent abundance: 75.770% % abundance: 24.230%

Mass Contribution: Mass Contribution:

(34.969)(.75770)= 26.496 amu (36.966)(.24230)=8.957 amu

Weighted average atomic mass of chlorine =

(26.496 amu + 8.957 amu) = 35.453 amu

Steps to Solving Weighted Averages of Isotopes

1. Turn the % into a decimal for the % abundance.

2. Multiply the decimal by the atomic mass of each particular isotope.

3. Add the masses of the isotopes together.

4. Your answer will be in amu’s (atomic mass units).

You try one…..

Calculate the atomic mass of Magnesium. The three Magnesium isotopes have atomic masses and relative abundances of 23.985 amu (79.99%), 24.986 amu (10.00%), and 25.982 amu (11.01%).

Weighted Averages of Isotopes Practice I

1. What is the atomic mass of this sample of Silver?

107 Ag 106.905 amu 52 % 109 Ag 108.905 amu 48 %2. What is the average atomic mass of this

sample of Hydrogen? 1H 12 % 2H 48 % 3H 40 %

Weighted Averages of Isotopes Practice I Continued

3. Boron has two naturally occurring isotopes, Boron-10 (abundance 19.8%, mass = 10.013 amu, Boron –11 ( abundance 80.2%, mass = 11.009 amu). Calculate the average atomic mass of Boron.