HIGH SCHOOL SCIENCE Physical Science 11: Chemical Bonds - Willmar

HIGH SCHOOL SCIENCE

Physical Science 11: Chemical Bonds

WILLMAR PUBLIC SCHOOL

2013-2014 EDITION

CHAPTER 11

Chemical Bonds

In this chapter you will: 1. Recognize stable electron configurations. 2.Predict an element's chemical properties using

the number of valence electrons and electron dot diagrams. 3.Describe how ionic bonds form and relate the properties of ionic compounds to the structure of crystal lattices. 4.Describe how covalent bonds form and the attractions that keep atoms together in molecules. 5. Compare polar and nonpolar bonds. 6.Name and determine chemical formulas for ionic and molecular compounds. 7. Describe the structure and strength of bond in metals.

SECTION 11.1

What are Chemical Bonds?

OBJECTIVES: 1. Recognize stable electron configurations. 2. Predict an element's chemical properties using

the number of valence electrons and electron dot diagrams.

Vocabulary: chemical bond electron dot diagram chemical formula subscript

There is an amazing diversity of matter in the universe, but there are only about 100 elements. How can this relatively small number of pure substances make up all kinds of matter? Elements can combine in many different ways.

Did you ever make cupcakes from scratch, like the boy pictured above? You mix together flour, sugar, eggs, and other ingredients to make the batter, put the batter into cupcake papers, and then put them into the oven to bake. The cupcakes that come out of the oven after baking are different from any of the individual ingredients that went into the batter. Like the ingredients that join together to make cupcakes, atoms of different elements can join together to form entirely different substances called compounds. In cupcakes, the eggs and other wet ingredients cause the dry ingredients to stick together. What causes elements to stick together in compounds?

Elements form compounds when they combine chemically. Their atoms join together to form molecules, crystals, or other structures. The atoms are held together by chemical bonds. A chemical bond is a force of attraction between atoms or ions. It occurs when atoms share or transfer valence electrons. Valence electrons are the electrons in the outer energy level of an atom.

Water (H2O) is an example of a chemical compound. Water molecules always consist of two atoms of hydrogen and one atom of oxygen. Like water, all other chemical compounds consist of a fixed ratio of elements. It doesn't matter how

2

much or how little of a compound there is. It always has the same composition.

The same elements may combine in different ratios. If they do, they form different compounds. Both water (H2O) and hydrogen peroxide (H2O2) consist of hydrogen and oxygen. However, they have different ratios of the two elements. As a result, water and hydrogen peroxide are different compounds with different properties. If you've ever used hydrogen peroxide to disinfect a cut, then you know that it is very different from water! Both carbon dioxide (CO2) and carbon monoxide (CO) consist of carbon and oxygen, but in different ratios.

There are different types of compounds. They differ in the nature of the bonds that hold their atoms together. The type of bonds in a compound determines many of its properties. Three types of bonds are ionic, covalent, and metallic bonds. An ionic bond is the force of attraction that holds together oppositely charged ions. Ionic bonds form crystals. Table salt contains ionic bonds. A covalent bond is the force of attraction that holds together two nonmetal atoms that share a pair of electrons. One electron is provided by each atom, and the pair of electrons is attracted to the positive nuclei of both atoms. The water molecule represented above contains covalent bonds. A metallic bond is the force of attraction between a positive metal ion and the valence electrons that surround it--both its own valence electrons and those of other ions of the same metal. The ions and electrons form a latticelike structure. Only metals form metallic bonds.

Chemical properties depend on an element's electron configuration. When the highest occupied energy level of an atom is filled with electrons, the atom is stable and not likely to react. The noble gases have stable electron configurations since their outer shell is full. The chemical properties of an element depend on the number of valence electrons.

An electron dot diagram is a model of an atom in which each dot represents a valence electron. The symbol in the center represents the nucleus and all other electrons in the atom. Electron dot diagrams for carbon and chlorine are shown below. The show paired electrons and unpaired electrons. These valence electrons are available for bonding. The unpaired electrons would like to be in a pair.

3

Elements that do not have complete sets of valence electrons tend to react. By reacting, they achieve electron configurations similar to those of noble gases.

You can make a simple salad dressing using just the two ingredients: oil and vinegar. Recipes for oil-and-vinegar salad dressing vary, but they typically include about three parts oil to one part vinegar, or a ratio of 3:1. For example, if you wanted to make a cup of salad dressing, you could mix together 3 cup of oil and 1 cup of vinegar. Chemical compounds also have "ingredients" in a certain ratio. However, unlike oil-and-vinegar salad dressing, a chemical compound always has exactly the same ratio of elements. This ratio can be represented by a chemical formula. A chemical formula is a notation that shows what elements a compound contains and the ratio of the atoms or ions of these elements in the compound.

For example, the chemical formula for magnesium chloride is MgCl2. The 2 written to the right and slightly below the symbol of the chlorine is a subscript. Subscripts are used to show the relative numbers of atoms of the element present. If there is only one atom of an element in a formula, no subscript is needed. From the formula, you can tell that there is one magnesium ion for every two chlorine ions in magnesium chlorine. A magnesium atom cannot reach a stable electron configuration by reacting with just one chlorine atom since it has two valence electrons. It must transfer electrons to two chlorine atoms.

4

 ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download