The International School of Panama

General Chemistry

Chapter 4 notes – Part II

How atoms differ

  The atom is the smallest particle of an element that retains the properties of the element. However it is made of even smaller particles: protons, electrons and neutrons. (Protons and neutrons are made of even smaller particles called quarks). What makes the atoms of one element be different from the others? The amount of these particles.

Henry Moseley discovered that each element has a unique positive charge in its nucleus. This positive charge is due to the amount of protons. The amount of protons is therefore what identifies each element.

  Atomic Number (Z) : The number of protons in an atom.

  In the periodic table each element is in a box.

The box contains the following information:

Chemical name (0n top)

 Atomic number (Below the name)

  Chemical Symbol (Below the atomic number)          

  Average Atomic Mass  (At the bottom)  

Atoms are neutral therefore the number of protons is equal to the number of electrons.

Atoms are neutral therefore the number of protons is equal to the number of electrons.

Atomic number = Z = Number of protons = Number of electrons

Use the periodic table to complete the following table:

Isotopes: Atoms of the same element but with different mass. Isotopes have the same number of protons but different number of neutrons.

For example Potassium (K) has three different isotopes. All of them contain 19 protons and therefore 19 electrons, but each isotope has different number of neutrons: one isotope has 20, another 21 and a third isotope contains 22 neutrons. The three isotopes have different mass. How can we say that they are atoms of the same element? Because the atomic number (Z) is what identifies the element.

Mass Number (A): The sum of protons and neutrons y the nucleus of an atom. For example each isotope of potassium has the following mass numbers: 39, 40 and 41. (This number is the result of adding the 19 protons each isotope has plus 20, 21 and 22 neutrons respectively.

These isotopes can be represented by writing the mass number after the symbol of the name of the element:

K-39 , K-40,  K-41 or

Potassium-39, Potassium-40, Potassium-41

Another way to name these atoms is the following:

        Mass number          A  

           Atomic number     Z           SYMBOL           

  The number of neutrons is equal to A – Z

Use the periodic table to complete the following table:

An atomic mass unit, amu, is a unit used to measure the mass of an atom relative to a standard. The standard used is isotope 12 of carbon. The amu is 1/12 the mass of C-12.

The different isotopes of an element exit in different amounts. However the relative amount of each is always the same. Scientists use a tool called mass spectrometer to determine the percentage of abundance of each isotope. Potassium for example has the following percentages of abundance:

Will the average mass of potassium considering the relative abundance be closer to 39, 40 or 41?

Let’s calculate it:

0.932581 x 38.9637074   + 0.000117 x 39.9639992  + 0.067302 x 40.9618254 = 39.098  

Check the number below the symbol K in the periodic table… What did you find? The atomic mass given in the periodic table is the average of the masses of all the isotopes considering the relative abundance of each of them. This is called atomic mass.

Explain why the mass number is a whole number, while the atomic mass has decimals.

Do example problem on page 103 and problems 15 to 17 on page 104.