Chemistry_Grade_10-12 (1).pdf

Fluorine a fluorine atom gains one electron to form a

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Fluorine A fluorine atom gains one electron to form a negative ion (figure 3.12). F F 1 electron gained Figure 3.12: The arrangement of electrons in a fluorine ion. Activity :: Investigation : The formation of ions 54
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CHAPTER 3. THE ATOM - GRADE 10 3.7 1. Use the diagram for lithium as a guide and draw similar diagrams to show how each of the following ions is formed: (a) Mg 2+ (b) Na + (c) Cl (d) O 2 2. Do you notice anything interesting about the charge on each of these ions? Hint: Look at the number of valence electrons in the neutral atom and the charge on the final ion. Observations: Once you have completed the activity, you should notice that: In each case the number of electrons that is either gained or lost, is the same as the number of electrons that are needed for the atoms to achieve a full or an empty valence energy level. If you look at an energy level diagram for sodium (Na), you will see that in a neutral atom, there is only one valence electron. In order to achieve an empty valence level, and therefore a more stable state for the atom, this electron will be lost . In the case of oxygen (O), there are six valence electrons. To fill the valence energy level, it makes more sense for this atom to gain two electrons. A negative ion is formed. 3.7.2 Ionisation Energy Ionisation energy is the energy that is needed to remove one electron from an atom. The ioni- sation energy will be different for different atoms. The second ionisation energy is the energy that is needed to remove a second electron from an atom, and so on. As an energy level becomes more full, it becomes more and more difficult to remove an electron and the ionisation energy increases . On the Periodic Table of the Elements, a group is a vertical column of the elements, and a period is a horizontal row. In the periodic table, ionisation energy increases across a period, but decreases as you move down a group. The lower the ionisation energy, the more reactive the element will be because there is a greater chance of electrons being involved in chemical reactions. We will look at this in more detail in the next section. Exercise: The formation of ions Match the information in column A with the information in column B by writing only the letter (A to I) next to the question number (1 to 7) 55
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3.8 CHAPTER 3. THE ATOM - GRADE 10 1. A positive ion that has 3 less electrons than its neutral atom A. Mg 2+ 2. An ion that has 1 more electron than its neutral atom B. Cl 3. The anion that is formed when bromine gains an electron C. CO 2 3 4. The cation that is formed from a mag- nesium atom D. Al 3+ 5. An example of a compound ion E. Br 2 6. A positive ion with the electron con- figuration of argon F. K + 7. A negative ion with the electron con- figuration of neon G. Mg + H. O 2 I. Br 3.8 The Arrangement of Atoms in the Periodic Table The periodic table of the elements is a tabular method of showing the chemical elements.
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