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Earth Chemistry
2 Combinations of Atoms
SECTION
KEY IDEAS As you read this section, keep these questions in mind:
• • • •
What are compounds and molecules? What are chemical formulas? What are two types of chemical bonds? What is the difference between compounds and mixtures?
What Are Compounds? Different elements can combine to form compounds. A compound is a substance that forms when atoms of two or more elements join together. A compound has different properties than the elements that form it.
READING TOOLBOX Compare As you read this section, use a double-door fold to compare the terms compound and mixture.
MOLECULES
The atoms in a compound are joined together in groups called molecules. A molecule is the smallest unit of a compound that has the chemical properties of that compound. Most molecules are made of atoms of more than one kind of element. However, some molecules are made of atoms of the same element. For example, the element oxygen naturally exists as molecules of two oxygen atoms. Molecules that are made up of only two atoms are called diatomic molecules.
READING CHECK 1. Identify What is a molecule?
What Are Chemical Formulas? A chemical formula is a group of letters and numbers that describes a compound. The letters are symbols from the periodic table. They represent the elements in the compound. The numbers show how many atoms of an element are in a molecule of the compound. For example, the chemical formula for water is H2O. The table below shows how to interpret this chemical formula. Formula Symbol Element H2O
LOOKING CLOSER
Interpretation
H
hydrogen There are two hydrogen atoms in each water molecule.
O
oxygen
There is one oxygen atom in each water molecule.
2. Identify Which two elements combine to form water?
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Combinations of Atoms continued
What Are Chemical Equations? A chemical reaction happens when atoms combine in new ways to form new substances. The substances that react are called reactants. The new substances that form are called products. The diagram below shows the chemical reaction between methane and oxygen.
+
+
LOOKING CLOSER
3. Identify Label the reactants and the products in this chemical reaction.
Methane CH4
Oxygen 2O2
Carbon dioxide CO2
Water 2H2O
Methane and oxygen react to form carbon dioxide and water.
Scientists use chemical equations to represent chemical reactions. In a chemical equation, the chemical formulas for the reactants appear on the left side of an arrow. The chemical formulas for the products appear on the right side of the arrow. The chemical reaction below represents the reaction between methane and oxygen. CH4 + 2O2 ➞ CO2 + 2H2O
Talk About It
Discuss Work with a partner to prove that the chemical equation for the reaction of methane and oxygen is balanced. Share your work with the class.
Critical Thinking 4. Apply Concepts Look at the equation below: 2Mg ! O 2 ➞ __MgO What coefficient should be added to the right-hand side to balance this equation?
BALANCED EQUATIONS
Look again at the chemical equation for the reaction of methane and oxygen. The chemical formulas for oxygen (O2) and water (H2O) have the number 2 in front of them. These numbers are called coefficients. They show that two molecules of oxygen react with each molecule of methane. They also show that the reaction forms two molecules of water. Scientists use coefficients to balance chemical equations. A chemical reaction is balanced when the number of each kind of atom is the same on both sides of the arrow. The number of atoms of an element must be the same in the reactants and the products. For example, if there are four oxygen atoms in the reactants, there must be four oxygen atoms in the products. To calculate the number of atoms present, multiply the coefficient by the subscript. You can use only coefficients to balance a chemical equation. You cannot change the chemical formulas of the reactants or products.
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Combinations of Atoms continued
What Are Chemical Bonds? A chemical bond is a force that holds the atoms in a molecule together. There are two main ways atoms can form chemical bonds. Atoms can form bonds by • transferring electrons from one atom to another • sharing electrons Scientists study the properties of elements to predict how atoms will bond. Atoms of some kinds of elements lose or gain electrons easily. Atoms of other elements are more likely to bond by sharing electrons. Elements bond in ways that make their atoms more stable. Most atoms are stable when they have eight valence electrons.
READING CHECK 5. Identify What are the two main ways atoms can bond?
What Happens When Atoms Transfer Electrons? An atom has the same number of protons and electrons. Each proton has a charge of 1!. Each electron has a charge of 1". Therefore, the positive and negative charges in an atom exactly balance out. The atom is neutral. It has no charge. If a neutral atom gains or loses electrons, it is no longer neutral. If the atom loses an electron, it becomes positively charged. If it gains an electron, it becomes negatively charged. IONS AND IONIC BONDS
An ion is a particle that has a positive or negative charge. Ions form when atoms or molecules gain or lose electrons. For example, a neutral chlorine atom has 7 valence electrons. If the atom accepts an electron, it will have 8 valence electrons and become chemically stable. The extra electron will also give the atom a negative charge. The neutral chlorine atom becomes an ion. An ionic bond is the attraction between ions with opposite charges. A compound that forms from ionic bonds is an ionic compound. Sodium chloride, or table salt, is an ionic compound. The compound can form when a sodium atom loses one electron. The sodium atom becomes a sodium ion with a 1! charge. The chlorine atom gains the electron and becomes a chloride ion with a 1" charge. The oppositely charged ions attract one another. This attraction is an ionic bond.
Critical Thinking 6. Apply Concepts Does an ion with a positive charge have more protons or electrons?
READING CHECK 7. Explain Why do the ions in an ionic bond stay together?
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Combinations of Atoms continued
LOOKING CLOSER
-
+
Sodium chloride (NaCl) forms when sodium ions (Na+) and chloride ions (Cl–) form ionic bonds.
8. Identify Which ion in the diagram has a positive charge?
Sodium
Chloride
What Happens When Atoms Share Electrons?
READING CHECK 9. Define What is a covalent bond?
Some atoms do not transfer electrons easily. These atoms may bond by sharing electrons. When atoms share electrons, the electrons move around the nuclei of all of the atoms, instead of just one of them. The attraction between the positive nuclei and the negative electrons holds the atoms together. A bond that forms when atoms share electrons is called a covalent bond. A compound formed from covalent bonds is a covalent compound. Water is a covalent compound. In a water molecule, two hydrogen atoms share valence electrons with an oxygen atom. This sharing of electrons gives each atom a more stable number of electrons. Some covalent bonds form between atoms of the same element. For example, remember that oxygen naturally exists as diatomic molecules. The two oxygen atoms in an oxygen molecule are joined by a covalent bond.
LOOKING CLOSER
10. Describe How does the covalent bond in an oxygen molecule form?
A covalent bond joins two oxygen atoms in a molecule of oxygen (O2).
Oxygen
Oxygen
POLAR COVALENT BONDS
In a polar covalent bond, the atoms in the bond do not share their electrons equally. The electrons spend more time around one of the atoms. The bonds in water molecules are polar covalent bonds. The oxygen atoms attract electrons more than the hydrogen atoms do. The electrons spend more time around the oxygen atom than around the hydrogen atoms. Therefore, the oxygen atom has a slight negative charge. The hydrogen atoms have slight positive charges. Copyright © Holt McDougal. All rights reserved.
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Combinations of Atoms continued
What Is a Mixture? Elements and compounds can combine to form mixtures. A mixture is a combination of two or more substances that are not joined by chemical bonds. A mixture is different from a compound. Remember that the properties of substances change when they react to form compounds. When substances form a mixture, their properties do not change. Unlike the parts of a compound, the parts of a mixture can be separated by physical means. For example, you can separate a mixture of sand and iron shavings using a magnet. Compounds cannot be separated into elements by physical means.
READING CHECK 11. Compare How is a compound different from a mixture?
HETEROGENEOUS MIXTURES
There are two main types of mixtures: heterogeneous mixtures and homogeneous mixtures. The parts of a heterogeneous mixture are not evenly mixed together. Different parts of the mixture have different amounts of each substance. A mixture of mud and water is an example of a heterogeneous mixture.
Critical Thinking
HOMOGENEOUS MIXTURES
Talk About It
In a homogeneous mixture, the parts of the mixture are evenly mixed. Every part of the mixture has the same properties and composition. A homogeneous mixture of two or more substances is a solution. Sea water is a solution. In sea water, ionic compounds such as sodium chloride are mixed evenly with water molecules. All parts of a sample of sea water have the same composition. Gases and solids can also form solutions. Air is an example of a solution of gases. An alloy is a solution of two or more metals. Steel is an alloy made of iron, carbon, and many other elements. The table below shows some other examples of heterogeneous and homogeneous mixtures. Heterogeneous Mixtures
Homogeneous Mixtures
salad
tea
granite and many other rocks
nail polish
soil
paint
vegetable soup
lemonade
12. Apply Concepts You stir salt into a bowl of water. Is this a compound or a mixture?
Discuss Make a list of the foods you ate yesterday. Share your list with a partner. Together, figure out which of the foods were homogeneous mixtures and which were heterogeneous mixtures.
LOOKING CLOSER
13. Identify What is one other example of a heterogeneous mixture?
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Section 2 Review SECTION VOCABULARY compound a substance made up of atoms of two or more different elements joined by chemical bonds covalent bond a bond formed when atoms share one or more pairs of electrons ion an atom or molecule that has gained or lost one or more electrons and has a negative or positive charge ionic bond the attractive force between oppositely charged ions, which form when electrons are transferred from one atom or molecule to another
mixture a combination of two or more substances that are not chemically combined molecule a group of atoms that are held together by chemical forces solution a homogeneous mixture throughout which two or more substances are uniformly dispersed
1. Organize Complete the concept map with the terms compounds, mixtures,
covalent bonds, ionic bonds, heterogeneous, and homogeneous. substances can combine to form
whose atoms are joined by
which can be
2. Analyze This chemical equation represents a chemical reaction:
2H2 ! O2 ➞ 2H2O Identify the reactants and products of this reaction. Explain whether the equation is balanced or not.
3. Identify Give one example of a homogeneous solution and one example of a
heterogeneous solution. Use examples that are not given in this section.
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