User:Phaello/sandbox/Chemistry/SCI 1301P-A/March 6, 2009 Lesson

Monday March 16, 2009 SCI 1301P-A Lesson 1

As we discussed during the crush course in February, we said that the number of protons in an atom is always equal to the number of electrons in the same atom, and some of the things we noted were:

• Protons have a positive charge
• electrons have a negative charge
• That means that if an atom has 2 protons (+2), there will be 2 electrons (-2)
• so the atom is neutral – because the number of positive charges is the same with that of negative charges.

If you know the number of protons in an atom, automatically you will know the number of electrons. If you are told that an atom has 2 protons, you should be able to tell that the number of electrons is 2 – because the atom has to be neutral.

Again we mentioned that the electron shells around the nucleus can only take a certain number of electrons. Shell number 1 can take a maximum of 2 electrons; shell number 2, can take a maximum of 8 electrons, shell number 3 can take a maximum of 18. The formula used to determine these maximum numbers is 2n2, where n is the shell number. If you are told the number of protons in an atom, first that will help you to say how many electrons the atom has, then you should be able to find out how many shells the atom would have, and how the electrons are placed in the shells.

Remember that a shell is only stable when it has a maximum number of electrons. For instance, the first shell is most stable when it has 2 electrons. Only after it has filled, then the second shell can begin to fill.

Discussion
 Beginning with shell number 2, shells are most stable when they have 8 electrons in the outermost shell, this is known as the Octet Rule. Why does this rule begin with shell number 2 and not shell number 1? Discuss with your group mates.

Discussion
 Do you remember what an atomic number is? Discuss with your group mates and make sure you understand it.

If you know what an atomic number is, then you know what it tells you about an atom.

Atoms in the periodic table are arranged according to the atomic numbers. You have been provided with a copy of the periodic table, use it to do the following activity.

Activity
 Starting from Hydrogen up to Argon make neat drawings of the atoms showing protons in the nucleus and the shells around it, and fill electrons in the shells. I repeat: these must be neat drawings, nice circles should represent shells.

After making the drawings you should be able to see how many electrons each of the atoms has on the outermost shell.

The electrons on the outermost shell of an atom are called valence electrons.

Activity
 Answer the following questions What pattern, of valence electrons, do you notice as move from the left to the right across the periodic table for each of the periods 1 and 2? Looking at your drawings what do you think is the pattern of valence electrons as you move down the group in the periodic table – e.g. group I, group II, group III etc? What happens to the number of shells as you move down the group in the periodic table?

This work must be done within this hour of SCI 1301P-A lesson, collected by one person and handed in at the end of the hour to the Pure Sciences office.

Course: SCI 1301P-A Lecturer: P. Ntšonyane March 16, 2009