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Group Activity: Atomic Structure
Part II

Here is a link to a page relevant lessons on the same topics as are covered in this activity.

Here is a YouTube Playlist of some videos that show the periodic nature of chemical reactions that elements in the same groups undergo.

Introduction

You know that atoms are made of protons, neutrons, and electrons. The protons and neutrons belong in the middle of the atom: the nucleus. The electrons whip around the nucleus at various distances. These distances are strictly prescribed:

Electrons can only have whole-number multiples of a certain basic amount of energy called the quantum energy. They can be found in shells around the nucleus that have limits on how many electrons can fit in each shell. Each shell is known as an energy level and each shell is farther away from the nucleus than the last. The farther away from the nucleus an electron shell is, the higher in energy it is. The chart shows the maximum number of electrons in each shell.

Shell No. No. of Electrons
1 2
2 8
3 8
This table is only completely accurate for atoms from hydrogen (H) through calcium (Ca). For atoms with more than 20 protons the electron structure gets more complicated.

A chart showing the protons, neutrons and electrons present in each of the first ten elements appears below. Notice that for He and Ne the chart indicates that they haved filled shells. Helium and neon are two members of that group of elements called the Noble Gases. These elements almost never react with other elements to form compounds.

The outermost shell of electrons in the electron structure of an atom is called the valence shell. The valence electrons which are found in the valence shell are very important because they determine the properties and reactivity of an element. When atoms react chemically they typically react in such a way that when they are done, they have filled their valence shells. This can be accomplished by losing electrons to become an cation, by gaining electrons to become an anion, or by sharing electrons in what is called a covalent bond. Losing or gaining electrons, or forming a covalent bond are ways in which atoms minimize energy. Energy minimization is a key principle in our understanding of the physical world. For example, objects roll downhill, dissipating their gravitational potential energy as kinetic energy. Bits of metals move toward magnets, losing their magnetic potential energy and turning it into kinetic energy. When fuel burns, it releases chemical energy as heat and light: ashes have less energy in them than wood.

Electron.Shell.Structures.H-Ne (20K)

A cation (CAT-eye-on) is an atom or molecule with a positive charge and comes from losing electrons. An anion (AN-eye-on) is an atom or molecule with a negative charge and comes from gaining electrons. A covalent bond is when two atoms are bound together by the fact that they are sharing their valence electrons so that each atom in the bond has a full valence shell. Atoms tend to lose or gain the proper number of electrons so that their valence shell is filled. You will use this information to predict the probable charges of atoms.

One of the major reasons the periodic table is such a valuable tool is that it shows how the various elements are related to each other. If you have not already done so, ask your teacher to show you a periodic table colored by the groups of related elements. Color your own periodic table to match. The groups come about because elements in the same column have the same number of valence electrons. Even though Br has 18 more electrons than Cl the two elements have similar properties because in each element there are seven electrons in the outermost, valence, shell of electrons. All halogens have seven valence electrons.



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Questions and Problems

Fill in the following table with the number of electrons in each shell. Identify the valence shell of each element by circling the number of electrons you write down in the valence shell. Also, place a star in the boxes where the outermost shell is a filled shell.

Atoms of an element that are electrically neutral have an overall charge of zero. That means that for an electrically neutral atom, the number of electrons equals the number of protons. For example, a neutral atom of carbon (6 p+) has six electrons.

Element No. of p+ Total
No. of e-		 Shell 1 Shell 2 Shell 3 Shell 4
H 1 1 1 – – –
He            
Li            
Be            
B            
C            
N            
O            
F            
Ne            
Na            
Mg            
Al            
Si            
P            
S            
Cl            
Ar            
K            
Ca            
  1. Elements can be grouped by the number of valence electrons they have. List elements with the same number of valence electrons below:
    1. Elements with 1 valence electron:
    2. 2 valence electrons:
    3. 3 valence electrons:
    4. 4 valence electrons:
    5. 5 valence electrons:
    6. 6 valence electrons:
    7. 7 valence electrons:
    8. 8 valence electrons:
  2. Which elements in the list have a full valence shell?
  3. If you look at the list of elements that have one valence electron you will notice something about how they are arranged on your periodic table. Describe that arrangement.
  4. What do all elements with the same number of valence electrons have in common when you look at their positions on the periodic table?


  1. Using the patterns you discovered about the number of valence electrons for elements on the periodic table, predict the number of valence electrons found in atoms of cesium (Cs), barium (Ba), tin (Sn), arsenic (As), iodine (I), and xenon (Xe).


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Cations and Anions

Atoms are not always neutral and when they are not, they are called ions. Atoms can become ions only by gaining or losing electrons. In chemical reactions atoms never gain or lose protons or neutrons. If an atom gains an electron it becomes an anion. If it loses an electron it becomes a cation. You symbolize this by using the atomic symbol followed by a superscript showing the charge. Some cations: Na+, Mg2+. Some anions: O2–, F–. The number is shown first, then the charge.

To calculate the charge on an atom you can use the following formula:
(# of p+) – (# of e–) = overall charge

So if a neutral carbon atom loses one electron this equation would read 6 – 5 = +1 and its symbol becomes C+. If a C atom gains four electrons the equation would read 6 – 10 = –4 and its symbol becomes C4–. The key to understanding how to find the charge on an ion is to always remember that an atom of any element always has the same number of protons.

Write the ionic symbol and calculate the charge on the following atoms. Write whether the ion is a cation or an anion.

  1. H | gained 1 e-
  2. F | gained 1 e-
  3. C | lost 3 e-
  4. O | gained 1 e-
  5. Si | lost 4 e-
  6. As | gained 3 e-
  1. Fe | lost 2 e-
  2. Al | lost 3 e-
  3. Fe | lost 3 e-
  4. I | gained 1 e-
  5. Cu | lost 2 e-
  6. Cu | lost 3 e-


For the following elements, predict the charge they will have based on what you know about the number of valence electrons in a neutral atom of that element. Remember, atoms gain or lose electrons so that they end up with a full valence shell. This tendency is sometimes called the octet rule because full shells have eight electrons. (Except for Shell 1).

  1. H
  2. He
  3. S
  4. Si
  5. F
  6. B
  7. Mg
  8. Al
  9. C
  10. K
  1. Be
  2. Ar
  3. N
  4. Cl
  5. Li
  6. O
  7. Ne
  8. P
  9. Na
  10. Ca
Each of these elements loses or gains the minimum number of electrons to obtain a full valence shell. Can you explain why this might be so?


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Lewis Dot Structures
Lewis dot diagrams are a simplified way to show how the electrons are arranged in their outer shell. This is where chemical reactions take place. Dots are placed around the element symbol in the following way: only the electrons in the valence shell are shown, they are added in pairs up to a total of four pairs. Here is an example:
lewis.dots.chlorine (1K)
Draw the Lewis Dot Structures for neutral atoms of elements 1-18:
H

Lewis Dot Diagrams

 He
Li Be B C N O F Ne
Na Mg Al Si P S Cl Ar
We will use these Lewis dot diagrams more later to describe how atoms bond to form compounds.
General Questions
Answer the following questions:
  1. Name:
    1. three elements that have only a single electron in their valence shell
    2. two elements that have exactly two electrons in their valence shell
    3. three elements with filled valence shells
    1. Lithium, sodium, and potassium are all metals that react with water to liberate hydrogen gas. What, if anything, do atoms of each of these metals have in common? Here is a link to a page with videos showing the chemical and physical properties of several groups from the periodic table.
    2. Helium is an unreactive gas and neon does not take part in chemical reactions either. What, if anything, do atoms of these elements have in common?
  3. The description of the atom that we have used so far is similar in some ways to our solar system. What part of the solar system is like the nucleus of an atom? What parts correspond to the electron shells surrounding an atomic nucleus? What are some other similarities? What are some obvious differences?


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  1. What are electron shells? What makes one shell different from another shell? Recall what you learned about atoms when we studied light.
  2. Define valence shell.
  3. Name the three ways that an atom can fill its valence shell and define them.
  4. Why do atoms tend to fill their valence shells?
  5. What key principle determines the charge that an atom will be most likely to have? Give an example of how that principle works at the atomic level. Give an example of the same principle at an everyday-things level.
  6. What makes the periodic table such a useful tool? Explain your answer using the terms and concepts learned in this lesson.
  7. Why are elements with similar properties found in columns in the periodic table? Again, explain your answer using what you have learned about electron structure.
For this packet to be complete, print out one copy of the periodic table for each student.
Here is the homework assignment that goes with this group activity.
Last updated: Jan 25, 2013       Home