1. What is a dot diagram? 2. Drawing dot diagrams. Name:

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1 Name: Skill Sheet 29.2 Dot Diagrams You have learned that atoms are composed of protons, neutrons, electrons. The electrons occupy energy levels that surround the nucleus in the form of an electron cloud. The electrons that are involved in forming chemical bonds are called valence electrons. Atoms can have up to eight valence electrons. These electrons exist in the outermost region of the electron cloud often called the valence shell. The most stable atoms have eight valence electrons. When an atom has eight valence electrons, it is said to have a complete octet. Atoms will gain or lose electrons in order to complete their octet. In the process of gaining or losing electrons, atoms will form chemical bonds with other atoms. The method we use to visually represent an atom's valence state is called a dot diagram, you will practice drawing these in the following exercise. 1. What is a dot diagram? Dot diagrams are composed of two parts the chemical symbol for the element dots surrounding the chemical symbol. Each dot represents one valence electron. If an element, such as oxygen, has six valence electrons, then six dots will surround the chemical symbol as shown to the right. Boron has three valence electrons, so three dots surround the chemical symbol for boron as shown to the right. There can be up to eight dots around a symbol, depending on the number of valence electrons the atom has. The first four dots are single, then as more dots are added, they fill in as pairs. 2. Drawing dot diagrams Using a periodic table, complete the following chart. With this information, draw a dot diagram for each element in the chart. Remember, only the valence electrons are represented in the diagram, not the total number of electrons. Element Chemical symbol Total number of electrons Number of valence electrons Dot diagram Potassium K Nitrogen N Carbon C Beryllium Be Neon Ne Sulfur S 1

2 Skill Sheet 29.2 Dot Diagrams 3. Using dot diagrams to represent chemical reactivity Once you have a dot diagram for an element, you can predict how an atom will achieve a full valence shell. For instance, it is easy to see that chlorine has one empty space in its valence shell. It is likely that chlorine will try to gain one electron to fill this empty space rather than lose the remaining seven. However, potassium has a single dot or electron in its dot diagram. This diagram shows how much easier it is to lose this lone electron than to find seven to fill the seven empty spaces. When the potassium loses its electron, it becomes positively charged. When chlorine gains the electron, it becomes negatively charged. Opposite charges attract, this attraction draws the atoms together to form what is termed an ionic bond, a bond between two charged atoms or ions. Because chlorine needs one electron, potassium needs to lose one electron, these two elements can achieve a complete set of eight valence electrons by forming a chemical bond. We can use dot diagrams to represent the chemical bond between chlorine potassium as shown above. For magnesium chlorine, however, the situation is a bit different. By examining the electron or Lewis dot diagrams for these atoms, we see why magnesium requires two atoms of chlorine to produce the compound, magnesium chloride, when these two elements chemically combine. Magnesium can easily donate one of its valence electrons to the chlorine to fill chlorine s valence shell, but this still leaves magnesium unstable; it still has one lone electron in its valence shell. However, if it donates that electron to another chlorine atom, the second chlorine atom has a full shell, now so does the magnesium. The chemical formula for potassium chloride is KCl. This means that one unit of the compound is made of one potassium atom one chlorine atom. The formula for magnesium chloride is MgCl 2. This means that a one unit of the compound is made of one magnesium atom two chlorine atoms. Now try using dot diagrams to predict chemical formulas. Fill in the table below: Elements Dot diagram for each element Dot diagram for compound formed Chemical formula Na F Br Br Mg O 2

3 Name: Skill Sheet 29.3 Chemical Equations Chemical symbols provide us with a shorth method of writing the name of an element. Chemical formulas do the same for compounds. But what about chemical reactions? To write out, in words, the process of a chemical change would be long tedious. Is there a shorth method of writing a chemical reaction so that the all the information is presented correctly is understood by all scientists? Yes! This is the function of chemical equations. You will practice writing balancing chemical equations in this skill sheet. 1. What are chemical equations? Chemical equations show what is happening in a chemical reaction. They provide you with the identities of the reactants (substances entering the reaction) the products (substances formed by the reaction). They also tell you how much of each substance is involved in the reaction. Chemical equations use symbols for elements formulas for compounds. The reactants are written to the left of the arrow. Products go on the right side of the arrow. The arrow should be read as yields or produces. This equation, therefore, says that hydrogen gas (H 2 ) plus oxygen gas (O 2 ) yields or produces the compound water (H 2 O). 2. Practice writing chemical equations Write chemical equations for the following reactions: H 2 + O 2 H 2 O Reactants Products Chemical Equation Hydrochloric acid HCl Sodium hydroxide NaOH Calcium carbonate CaCO 3 Potassium iodide KI Aluminum fluoride AlF 3 Magnesium nitrate Mg(NO 3 ) 2 Water H 2 O Sodium chloride NaCl Potassium carbonate K 2 CO 3 Calcium iodide CaI 2 Aluminum nitrate Al(NO 3 ) 3 Magnesium fluoride MgF 2 1

4 Skill Sheet 29.3 Chemical Equations 3. Conservation of atoms Take another look at the chemical equation for making water: Did you notice that something has been added? The large number in front of H 2 tells how many atoms of H 2 are required for the reaction to proceed. The large number in front of H 2 0 tells how many molecules are formed by the reaction. These numbers are called coefficients. Using coefficients, we can balance chemical equations so that the equation demonstrates conservation of atoms. The law of conservation of atoms says that no atoms are lost or gained in a chemical reaction. The same types numbers of atoms must be found in the reactants the products of a chemical reaction. Coefficients are placed before the chemical symbol for single elements before the chemical formula of compounds to show how many atoms or molecules of each substance are participating in the chemical reaction. When counting atoms to balance an equation, remember that the coefficient applies to all atoms within the chemical formula for a compound. For example, 5CH 4 means that 5 atoms of carbon 20 atoms of hydrogen are contributed to the chemical reaction by the compound methane. 4. Balancing chemical equations 2H 2 + O 2 2H 2 O To write a chemical equation correctly, first write the equation using the correct chemical symbols or formulas for the reactants products. If a reaction is to occur between sodium chloride iodine to form sodium iodide chlorine gas, we would write: NaCl + I 2 NaI + Cl 2 Remember that certain elements, when found alone, are diatomic; iodine chlorine are two examples. Next, count the number of atoms of each element present on the reactant product side of the chemical equation: Reactant Side of Equation Element Product Side of Equation 1 Na 1 1 Cl 2 2 I 1 For the chemical equation to be balanced, the numbers of atoms of each element must be the same on either side of the reaction. This is clearly not the case with the equation above. We need coefficients to balance the equation. 2

5 Skill Sheet 29.3 Chemical Equations First, choose one element to balance. Let s start by balancing chlorine. Since there are two atoms of chlorine on the product side only one on the reactant side, we need to place a 2 in front of the substance containing the chlorine, the NaCl. 2NaCl + I 2 NaI + Cl 2 This now gives us two atoms of chlorine on both the reactant product sides of the equation. However, it also give us two atoms of sodium on the reactant side! This is fine often balancing one element will temporarily unbalance another. By the end of the process, however, all elements will be balanced. We now have the choice of balancing either the iodine or the sodium. Let's balance the iodine. (It doesn t matter which element we choose.) There are two atoms of iodine on the reactant side of the equation only one on the product side. Placing a coefficient of 2 in front of the substance containing iodine on the product side: 2NaCl + I 2 2NaI + Cl 2 There are now two atoms of iodine on either side of the equation, at the same time we balanced the number of sodium atoms! In this chemical reaction, two molecules of sodium chloride react with one diatomic molecule of iodine to produce two molecules of sodium iodide one diatomic molecule of chlorine. Our equation is balanced! 5. Polyatomic ions balancing equations Some compounds contain multiple numbers of polyatomic ions. For example, Mg(OH) 2 is formed when two complete hydroxide (-OH) groups attach to the element magnesium. Therefore, in a chemical equation: represents 3 atoms of magnesium, 6 atoms of oxygen, 6 atoms of hydrogen. Each hydroxide group contributes 2 oxygen atoms 2 hydrogen atoms to the compound. There are 3 molecules of the compound in the reaction, so there are 6 total atoms of oxygen hydrogen in the reaction. Be aware of this as you balance equations. If your equation contains a polyatomic ion with a subscript, like SO 4, you really have to watch your math! For example, if you have 3 molecules of aluminum sulfate, 3Al 2 (SO 4 ) 3, you actually have nine atoms of sulfur thirty-six atoms of oxygen! Can you figure out why? 6. Practice balancing equations 3Mg( OH) 2 Balance the equations on the next page using the appropriate coefficients. Remember that balancing one element may temporarily unbalance another. You will have to correct the imbalance in the final equation. Check your work by counting the total number of atoms of each element the numbers should be equal on the reactant product sides of the equation. 1. Al + O 2 Al 2 O 3 3

6 Skill Sheet 29.3 Chemical Equations 2. CO + H 2 H 2 O + CH 4 3. HgO Hg + O 2 4. CaCO 3 CaO + CO 2 5. C + Fe 2 O 3 Fe + CO 2 6. N 2 + H 2 NH 3 7. K + H 2 O KOH + H 2 8. P + O 2 P 2 O 5 9. Ba(OH) 2 + H 2 SO 4 H 2 O + BaSO CaF 2 + H 2 SO 4 CaSO 4 + HF 11. KClO 3 KClO 4 + KCl 4