Prescriptive Activities: Facet Cluster 1.5: PF#30-­33 Teacher Page Prescriptive #30, 31, 32, 33: 30 The student has an inaccurate view of the charge involving ions. 31 The student thinks that atoms may be neutral if they don't have an equal number of positive and negative charges. 32 The student thinks that '+' means a gain of electrons. 33 The student thinks that '-­‐' means a loss of electrons. Background The charge on an atom or ion is determined by the numbers of protons and electrons within the atom or ion. Because protons are positively charged and electrons are negatively charged, if an ion has more protons than electrons, it will have an overall positive charge. If there are more electrons than protons it will have an overall negative charge. An atom is considered to be neutral (meaning no charge) when there is an equal number of protons and electrons. When an atom becomes charged (meaning protons and electrons are not equal), it is considered an ion. Remember, that under normal conditions here on earth atoms can gain or lose electrons during chemical reactions. However, unless an atom is radioactive, it will not change the number of protons it has in the nucleus. Therefore, an atom becomes a charged ion by either gaining or losing electrons. Purpose of Activity: In this activity you will build atoms and ions with marbles and peas, in order to master an understanding of charges of atoms and ions. The marbles represent the positively charged protons in the nucleus and the peas represent the negatively charged electrons outside of the nucleus. Materials: Student handout Bag of 16 marbles (“protons”) and 20 peas (“electrons”) Instructions: Create the atoms or ions indicted in the table below by placing the correct number of “protons” in the nucleus and the correct number of “electrons” outside the nucleus in the “atom” diagrammed on page 3. Remember that this is a simplified representation, or model, of an atom. In reality protons weigh about 2000 times more than electrons, and the electrons in this model would be about one third of a mile away from the nucleus. Neutrons have been omitted from this activity for simplicity. 1 Prescriptive Activities: Facet Cluster 1.5: PF#30-­33 Teacher Page Fill in the missing pieces in the table as you create these atoms and ions. Atom/ ion Number of protons Positive charge Number of Negative electrons charge Overall charge of atom or ion Hydrogen atom (H) 1 +1 1 -­‐1 0 Oxide ion (O2-­‐) 8 +8 10 -­‐10 2-­‐ Hydrogen ion (H+) Nitrogen atom (N) Nitride ion (N3-­‐) 7 +7 9 10 4 2 16 2-­‐ Sodium atom (Na) Sodium ion (Na+) Discussion Questions 1. Explain how you went about filling in the missing information for the Sodium ion (Na+). 2. Why does the addition of an extra electron to a neutral atom cause it to have a negative charge? 3. Why can neutrons be omitted from this activity? 4. In this activity the marbles and peas are just a model, or representation, of an actual physical system. List and explain three ways in which this model could be improved to more accurately represent reality. Notes to the teacher This model has a number of constraints that students should be aware of. Firstly the relative masses of the protons and electrons are inaccurate. A proton is approximately 1850 times more massive than an electron. Thus, if a pea weighs 20 grams, then each marble (proton) in this model should weigh about 85 pounds to accurately show the relative masses! The radius of an atom is approximately 10,000 times the size of its nucleus. Thus in this activity the electrons would extend about 1/3 of a mile in all directions rather than remaining within the circle on the page. 2 Prescriptive Activities: Facet Cluster 1.5: PF#30-­33 Teacher Page Neutrons have been omitted for simplicity. Students may suggest this as another constraint of the model. Alternative Activity Another activity students could do on-­‐line to examine the concept of charges on ions is the Build an Atom activity on the PhET website: http://phet.colorado.edu/en/simulation/build-­‐an-­‐atom This PhET activity is slightly more complicated as it includes neutrons in atom building. However, students who prefer to interact with the computer may find the website more motivating and engaging. 3 Prescriptive Activities: Facet Cluster 1.5: PF#30-­33 Teacher Page “Atom” or “Ion” Nucleus 4 Atom/ ion Prescriptive Activities: Facet Cluster 1.5: PF#30-­33 Teacher Page Answer Key Number of protons Positive charge Number of Negative electrons charge Overall charge of atom or ion Hydrogen atom (H) 1 +1 1 -­‐1 0 Oxide ion (O2-­‐) 8 +8 10 -­‐10 2-­‐ Hydrogen ion (H+) 1 +1 0 0 1+ Nitrogen atom (N) 7 +7 7 -­‐7 0 Nitride ion (N3-­‐) 7 +7 10 -­‐10 -­‐3 Fluoride (F-­‐) 9 +9 10 -­‐10 -­‐1 Helium ion (He2+) 4 +4 2 -­‐2 +2 Sulfide (S2-­‐) ion 16 +16 18 -­‐18 2-­‐ Sodium atom (Na) 11 +11 11 -­‐11 0 Sodium ion (Na+) 11 +11 10 -­‐10 +1 5