Atoms and Ions PDF

5.5 Atoms and Ions You have surely heard that we are all supposed to drink a lot of water: at least 2 L a day, and more if it is very hot or if we are exercisin...

5.5 Atoms and Ions You have surely heard that we are all supposed to drink a lot of water: at least 2 L a day, and more if it is very hot or if we are exercising (Figure 1). While this is generally good advice, too much water can actually be bad for your health. This was the conclusion reached during a scientific study of runners in the 2002 Boston Marathon. During the race, 13 % of the runners sampled in the study developed a condition known as hyponatremia. Symptoms of hyponatremia include disorientation and a loss of balance. The cause? They drank too much water during the race. Excess water can dilute the concentration of sodium in the blood to dangerously low levels—so low that three runners were at risk of dying if left untreated. People have actually died from this condition. When scientists refer to sodium in blood, they are not talking about the shiny metal that reacts vigorously with water that you saw in Figure 2 in Section 5.4. Rather, they are referring to sodium ions. Many ions are necessary for our health. Calcium and phosphorus ions are essential components of bone; iron ions help carry oxygen around the body. Ions get Figure 1 Is water harming this athlete? into our bodies in our food, and our bodies regulate their concentration. How Do Atoms and Ions Differ? As you learned in Section 5.4, an atom is an electrically neutral particle ion a charged particle that results when with an equal number of electrons and protons. An ion is an atom that has an atom gains or loses one or more become charged by gaining or losing electrons. For example, sodium atoms electrons lose one electron when they react with other atoms. Each resulting sodium ion contains 11 positive charges (on protons) and only 10 negative charges (on electrons). Since it has one more positive charge than negative, the sodium ion has an ionic charge of +1. As a result, chemists gave sodium ions the chemical symbol Na1+ or Na+. (Note that the number 1 is usually omitted in chemical symbols.) The other alkali metals also form ions with a single positive charge. Fluorine is one of the most reactive elements. When fluorine reacts, it tends to gain an electron from another atom to form a stable ion called fluoride. Because the fluoride ion has one extra negative charge, it has an ionic charge of −1. The chemical symbol of this ion is therefore F−. In fact, all the halogens form ions with a single negative charge. So, sodium and the other alkali metals (Group 1) lose an electron to form +1 ions, and the halogens (Group 17) gain an electron to form –1 ions. LEARNING TIP sodium, Na+ fluoride, F− How to Represent a Charge on an Ion positive charge (protons) +11 +9 Chemists say that a magnesium ion has negative charge (electrons) −10 −10 a charge of +2, which is read as “plus ionic charge +1 −1 two,” but they write the ion as Mg2+ (with the number first and the plus sign Why do we not find Na2+ or F2−: sodium ions with a +2 charge or fluoride last). You will get used to this convention as you see it used. ions with a −2 charge? To understand why Na+ and F− are the only stable ions that these elements form, we need to consider their Bohr−Rutherford diagrams in relation to those of the noble gases. 188 Chapter 5 Chemicals and Their Properties NEL  The noble gases (Group 18) are stable due to their full outer orbits. Sodium ions and fluoride ions are also stable. Why is this so? To explain the non-reactivity of sodium and fluoride ions, we can compare their Bohr−Rutherford diagrams with that of neon. Neon is the noble gas that is closest to sodium and fluorine on the periodic table. Again, we can use the Bohr−Rutherford model and the periodic table to help us predict the formation of ions. While we are considering the formation of ions from atoms, we do not need to be concerned about the number of neutrons in the nucleus. We can therefore omit them from our Bohr–Rutherford diagrams for now. Sodium In the process of forming a sodium ion (which has a positive charge), a sodium atom must react with another atom and lose one electron. The most likely electron to be lost is the one farthest from the nucleus: the single electron in the third orbit. This farthest electron is the least tightly held to the nucleus. As a result, the sodium ion has the same stable electron arrangement as a neon atom: an outer orbit filled with eight electrons (Figure 2). 11p+ 11p+ 10p+ Na Na+ Ne Figure 2 The sodium atom loses its outermost electron to form an ion. The sodium ion is stable because its outer orbit is full, like that of neon. Fluorine DID YOU KNOW? Fluorine has one less electron than neon. Fluorine tends to react with Fluoride and Dental Cavities other atoms to gain one electron. This reaction gives it the same stable Fluoride ions help make tooth enamel strong. For decades, Canadians have arrangement of electrons as neon. With this extra electron, the fluorine atom been getting fewer dental cavities now has 10 electrons and only 9 protons. It therefore becomes a fluoride ion thanks to fluoride in their drinking with a single negative ionic charge: F– (Figure 3). water. Some dentists have recently noticed an increase in cavities among young children, possibly because these children are drinking bottled water, without fluoride, rather than tap water. 9p+ 9p+ 10p+ GO TO NELSON SCIENCE – F F Ne Figure 3 The fluorine atom gains one electron to become a fluoride ion, F. Fluoride is stable – because its outer orbit is full, like that of neon. NEL 5.5 Atoms and Ions 189  Aluminum The Bohr−Rutherford diagram of aluminum shows that aluminum has three outer electrons (Figure 4(a)). To have a stable outer orbit (like a noble gas) aluminum could—in theory—either gain five electrons or lose three. Experimental evidence shows that metals tend to lose electrons, while non-metals tend to gain them. The result is an aluminum ion with ionic charge +3: Al3+ (Figure 4(b)). 13p+ 13p+ (a) Al (b) Al3+ Figure 4 (a) An aluminum atom has three electrons in its outer orbit. (b) Losing these electrons leaves the aluminum ion positively charged. Sulfur Sulfur has six electrons in its third orbit (Figure 5(a)). To achieve a stable electron arrangement, a sulfur atom reacts with other atoms and gains two electrons. When it does, sulfur forms an ion with the chemical symbol S2− (Figure 5(b)). This is called a sulfide ion. Sulfur can also form compounds without forming ions. You will learn more about these compounds in Section 5.11. DID YOU KNOW? 16p+ 16p+ Hydrogen Hydrogen can form both positive and negative ions. It can gain one electron to fill its only orbit, forming an ion with a charge of −1. More often, though, (a) S (b) S2– hydrogen loses its only electron to form an ion with a charge of +1. Figure 5 (a) A sulfur atom has six electrons in its outer orbit. (b) Filling the outer orbit with electrons makes the sulfide ion negatively charged. Naming Ions cation a positively charged ion We can classify ions as cations—those that have positive charges, and anion a negatively charged ion anions—those that have negative charges. The name of a positive ion is the same as the name of the element: LEARNING TIP sodium forms sodium ions, for example. The name of a negative ion is Cations and Anions determined by adding “ide” to the stem of the name. For example, oxygen Remember “cation” contains the letter forms oxide ions and phosphorus forms phosphide ions. “t,” which looks like a + sign; anions are negatively charged. 190 Chapter 5 Chemicals and Their Properties NEL  T RY THIS IONS AND THE PERIODIC TABLE SKILLS HANDBOOK SKILLS: Analyzing, Communicating 3.B., 10. Some elements gain or lose electrons to form stable ions. Is there 1 18 1 2 a pattern to how elements form ions? In this activity you will 1 H+ explore how some of the first 20 elements in the periodic table 2 13 14 15 16 17 3 4 5 6 7 8 9 10 form ions. In the process, you will learn how to predict the ionic 2 F– charge of elements based on their location on the periodic table. 11 12 13 14 15 16 17 18 1. Figure 6 represents part of the first four rows of the periodic 3 Na+ Al3+ S2– table. The atomic numbers of the first 20 elements are 19 20 indicated. Symbols are also provided for the ions of five 4 elements. Draw a Bohr–Rutherford diagram for the ion formed by each of the remaining elements in Figure 6. Figure 6 (Omit the elements that are greyed out.) K/U C C. How can the ionic charge be predicted from the location of 2. Copy Figure 6 into your notebook. From your Bohr–Rutherford the element on the periodic table? T/I diagrams, determine the chemical symbol of each ion and record in your periodic table. C A D. Use your answer in C to predict the chemical symbol of the ion of each of the following elements: T/I A. Describe the patterns or similarities that exist within a period and within a group for the Bohr–Rutherford diagrams. T/I A (a) barium, BaC05-F51-UBOS10SB.ai (b) iodine, I (c) rubidium, Rb (d) arsenic, As B. Describe the patterns or similarities that exist within a period E. You were not required to determine the ions for the and within a group for the ionic charges. T/I A greyed-out cells in Figure 6. Why? T/I IN SUMMARY Ions are atoms that have gained or lost electrons. Cations have fewer electrons than protons and Many ions have complete outer orbits, so they are therefore have a positive charge. stable. Atoms and ions can be represented by Anions have more electrons than protons and Bohr−Rutherford diagrams. therefore have Ontario Science a negative charge.10Anions SB often Some ions—in the appropriate concentrations— have “ide” at the end of their names. 0-17-635519-7 are necessary for good health. FN C05-F51-UCOS10SB CO Crowle Art Group CHECK YOUR LEARNING Deborah Crowle 1. Compare a sodiumPassion to 1st pass 5. List three atoms or ions that have the same number of (a) a sodium atom Approved(b) a neon atom K/U electrons as each of the following: Not Approved 2. (a) Draw the Bohr−Rutherford diagram (without neutrons) (a) S2− (b) Al3+ (c) P3− (d) Kr (e) Cs+ T/I for an atom of each of the following elements: lithium, 6. Suppose that a new element has been made. Chemical tests oxygen, calcium, and phosphorus. show that it is an alkaline earth metal. T/I (b) Draw the Bohr−Rutherford diagram (without neutrons) for (a) Predict how many electrons there will be in the outer orbit. the ion formed by each of the elements in (a). (b) Predict the ionic charge of the ion that this element forms. (c) Write the chemical symbol for each ion. 7. Justify why these ions do not exist under normal conditions. A (d) Name the noble gas with the same electron arrangement (a) K2+ (b) O2− as each ion. T/I C 8. (a) What is the trend in the ionic charges of the elements in 3. Distinguish between a cation and an anion. K/U Groups 1, 2, and 13 of the periodic table? 4. Name these ions. K/U (b) What is the trend in ionic charges of the elements in (a) Mg2+ (b) S2− (c) Fe3+ (d) Br− (e) N3− Groups 15 to 17? K/U 9. What type of drink would you recommend for endurance runners who suffer from hyponatremia? Why? K/U NEL 5.5 Atoms and Ions 191