Calculate the number of protons, neutrons, and electrons in an atom given its mass number and atomic number. Define an isotope and explain why atomic.

Calculate the number of protons, neutrons, and electrons in an atom given its mass number and atomic number.

Define an isotope and explain why atomic masses are not whole numbers.

To calculate how we get atomic masses of elements with isotopes.

1. Find the element on the periodic table. 2. Find the Atomic Number of the

element. 3. Atomic Number = # of protons in the

element.*Remember the # of protons of an atom

determines what element it is representing.

How many protons do the following elements have?1. Oxygen

8

2. Zinc 30

3. Bismuth 83

Atomic Mass = protons + neutrons If we know the # of protons from the

Atomic Number, then …

Atomic Mass - # of protons = # of neutrons

*Remember you should take the atomic mass on the periodic table and round it first

Oxygen1. Atomic Mass from the Periodic Table:

15.999 > 16

2. Subtract the # of protons (Atomic Number) from the Atomic Mass

16 – 8 = 8

*There are 8 neutrons in a neutral oxygen atom.

In a neutral atom, there must be the same number of positive and negative charges. Therefore…

# of protons = # of electron The # of electrons = Atomic Number.

Atoms that have the same number of protons but different numbers of neutrons.

They will have the same Atomic Number but different Atomic Masses.

In nature most elements are found as a mixture of isotopes.

No matter where a sample of an element is obtained, the relative abundance of each isotope is the constant.

Example: In a banana there is 93.25%

potassium with 20 neutrons, 6.7302% have 22 neutrons, and 0.0117% have 21 neutrons.

39

Potassuim-39 or K-39 or 19K

protons-19electrons-19 neutrons-20

*Both protons and neutrons are very close in mass to 1 amu.

*Atomic Mass Unit = 1/12 the mass of carbon-12

Masses of subatomic Particles

Particle Mass (amu

Electron 0.000549

Proton 1.007276

Neutron 1.008665

We take weighted averages of those elements based on the abundance of the isotopes.

Weighted Average = mass of isotope x percent abundance

(decimal form) Then add the values that each isotope

gives.

Element X has 2 natural isotopes. The isotopes with mass 10.012 amu has a relative abundance of 19.91%. The isotope with mass 11.009 amu has a relative abundance of 80.09%. What is this element?

1. Calculate the average atomic mass. 2. Compare this mass to the Periodic Table to find

your element.

Isotope A: mass = 10.012 amu @ 19.91%Isotope B: mass = 11.009 amu @ 80.09%

A: 10.012 x .1991 = 1.993

B: 11.009 x .8009 = 8.817

Now Add them: 1.993 + 8.817 = 10.81amu

Average Atomic Mass for Element X = 10.81 amu