Lesson 3 – Atoms and Ions

Objective:

  • to explain, using the periodic table, how and why elements combine to form compounds

Timeline:

  • 1 class

Review:

  1. What are the 3 subatomic particles?
  2. Where are they found in an atom?
  3. Draw a Lewis dot diagram for the element Lithium.

Background Information:

In the previous lesson, you learnt how to draw a Lewis dot diagram for an atom. Many atoms are very stable, we know this because we can find them in their natural state. For example, we can find gold, silver and sulfur in the environment. Other atoms are unstable, they form compounds, sometimes even violently.

Check out the alkalai metals. These are the first column on the periodic table (not counting Hydrogen). We also call this the first group of the periodic table.

What makes elements react? What is special about the alkalai metals that makes them react so violently? The answer is in the electrons.

Remember electron energy levels. The further down the rows (periods) you go on the periodic table, the more energy levels there are. The higher the energy level, the more energy the electrons have.

Periodic Table

Electrons cause elements to react. The more unstable the electrons are, the quicker the element will react. Energy levels that are full are very stable. Think of the dot diagram for Neon:

Neon dot diagramIt has 2 electrons in the first level, and 8 electrons in the second. Both are full. Neon doesn’t react with other elements because, in a sense, there is nothing to grab on to. The electrons are stable. In fact, this applies to the entire group, called the noble gases.

This is an important point: as you go down a group on the periodic table, the properties of the elements are the same.

Every other group has electrons missing from the outermost energy level. We call these outer electrons the valence electrons. Normally, when we draw Lewis dot diagrams,we only include the valence electrons. It is assumed the other energy levels are all filled. Here is more information and examples of Lewis structures.

So our Neon Lewis dot diagram would look like:

Proper neon dot diagram

Valence electrons are unstable. They want to be filled. If there are only a few electrons, the atom will probably lose them. If there are many electrons, the atom will probably gain more to fill the level.

When an atom loses or gains an electron, the charges become unbalanced. The atom becomes charged, we call this an ion. Ions can react to form compounds with other ions.

Let’s look at a couple examples.

Sodium (Na) metal has one valence electron. That single electron is very unstable. As soon as a sample of sodium is exposed, the valence electron is ripped away, leaving sodium ions.

Sodium ion

Chlorine (Cl) gas has 7 valence electrons. Chlorine wants to have a full energy level, and it’s only one electron away. It will grab up extra electrons in order to fill that gap. It becomes a chloride ion.

Chloride ion

Now, this sort of thing is interesting because people make all sorts of health claims about ions. Ever hear the term “anti-oxidant?” The idea is that those electrons that are ripped off (called “free radicals”) cause cells to age and break down. Giving them anti-oxidants is like the diagram of chlorine, above. It eats up those nasty free electrons and saves your poor cells. Of course, things are not quite that simple.

Ion Nomenclature

The periodic table is divided into metals and non-metals. You can see the division along the “staircase” on the table.

You can see there are a lot more metals than nonmetals.

When a metal becomes an ion, we keep the name the same: Sodium metal becomes Sodium ion.

When a non-metal becomes an ion, we change the ending to -ide. Chlorine gas becomes Chloride ion.

Activity:

Forming Ions (I have also seen these kits ready made that can be purchased)

1. Cut out large cardboard circles. You may wish to draw an inner circle for a nucleus, and at least 3 energy levels.

2. Using coloured paper, cut out a nucleus for an atom. You should write the symbol and the number of protons on the nucleus. Tape this onto the centre of a cardboard circle.

3. Put “electrons” on your atom by sticking pushpins or thumbtacks onto the correct energy level.

4. Draw an electron dot diagram of the atom.

5. Remove the valence electrons (or add more electrons) from your atom, to get to the nearest full energy level.

6. Draw an electron dot diagram of the ion. Indicate whether the ion is positive or negative. (Did you add more negative charges, or did you remove them?)

7. Write an equation representing the formation of the ion.

Ex: [Li] –> [Li+] + e-

or

[O] + 2e- –> [O2-]

Students should do this for at least 5 different atoms and ions. They should have different charges so that the students can experience how different types of ions form.

Self Check

1. How many valence electrons do the following elements have?

a) Oxygen

b) Calcium

c) Iodine

d) Carbon

2. How many energy levels do the following elements have?

a) Be

b) I

c) P

d) Cs

3. Take a look at the metals section of the periodic table. Do metals form positive ions or negative ions? What type of ions do non-metals form? What generalization can you make regarding the atomic structure of metals, and the atomic structure of non-metals?

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