Atoms in Combination: The Chemical Bond Trefil & Hazen The Sciences Chapter 10

Atoms bind together in chemical reactions by the rearrangement of electrons. Outer electrons = “valence electrons” Valent = combining power

Figure 8-13 The periodic table of the elements. The weights of the elements increase from left to right. Each vertical column groups elements with similar chemical properties.

Figure 8-13 The periodic table of the elements. The weights of the elements increase from left to right. Each vertical column groups elements with similar chemical properties.

Each shell can hold only a limited number of electrons The innermost shell can hold 2; the second and third shells, 8 each; the fourth and fifth shells 18 each; the sixth and seventh shells 32 each. Does this remind you of the periodic table?

Figure 8-13 The periodic table of the elements. The weights of the elements increase from left to right. Each vertical column groups elements with similar chemical properties.

Figure 8-14 A representation of electrons in a number of common atoms.

Figure 8-14 A representation of electrons in a number of common atoms.

Every object in nature tries to reach a state of lowest energy. When atoms come together, the electrons rearrange themselves to minimize the potential energy in the system. This situation requires atoms to either exchange or share electrons.

Most atoms adopt one of three simple strategies to achieve a filled outer shell: they give away electrons, accept electrons, or share electrons.

Chemical Bonds result from redistribution of electrons to form a more stable electron configuration between two or more atoms. Al 2 O 3 H 2 O CaCl 2 CH 4 CO 2 MgO NaCl NH 3

Ionic Bond

Sodium atom Loses one electron Becomes a sodium ion, Na+ Stable [Ne] Chlorine atom Gains one electron Becomes a chloride ion, Cl- Stable [Ar]

Metals lose e- Nonmetals gain e-

Figure 10-1 Sodium, a highly reactive element, readily transfers its single valence electron to chlorine, which is one electron shy of the “magic” number 18. The result is the ionic compound sodium chloride—ordinary table salt. In these diagrams, electrons are represented as dots in shells around a nucleus.

Courtesy Robert Capece Figure 10-9 Individual salt crystals reveal a characteristic cube shape.

Figure 10-2 The atomic structure of a sodium chloride crystal consists of a regular pattern of alternating sodium and chloride ions.

Figure 10-3 Calcium and chlorine neutral-atom electron configurations (left), and their configurations after electrons have been transferred from the calcium to the chlorine atoms (right).

Metallic bond – electrons are redistributed so they are shared by all the atoms as a whole.

Figure 10-4 Metallic bonding, in which a bond is created by the sharing of electrons among several metal atoms.

Naming ionic compounds with variable charge transition metals… Fe 3+ and O 2- Fe 2 O 3 iron (III) oxide Sn 4+ and O 2- SnO 2 tin (IV) oxide

Figure 10-5 Two hydrogen atoms become an H 2 molecule by sharing each of their electrons in a covalent bond. This bonding may be represented schematically in a dot diagram (a), or by the merging of two atoms with their electron clouds (b).

Covalent Bond Hydrogen atom Has one unpaired valence electron Becomes stable by sharing its outer electron with one of carbon’s [He] Carbon atom Has four unpaired valence electrons Becomes stable by sharing its lone electrons with one from hydrogen [Ne]

Covalent Bond

Figure 10-7 The water molecule and its polarity.

Bonding Ionic Electron transfer Metal loses e- Nonmetals gain e- Electrostatic attraction holds the crystal together NaCl crystal Covalent Electron sharing Two nonmetals share a pair of valence electrons which orbit both atoms, holding them together H 2 O molecule

Chemical Reaction --the rearrangement of atoms that occurs when substances undergo chemical change During a chemical reaction, a new material is formed by this shuffling of atoms Bonds break, new bonds form Reactants  Products

Figure In pictorial form, an oxidation reaction involves the transfer of electrons to oxygen atoms. When natural gas (CH 4 ) burns, it combines with two oxygen molecules (O 2 ) to form a molecule of carbon dioxide (CO 2 ) and two molecules of water (H 2 O).

Reactants  Products A chemical equation shows the material about to react to the left of an arrow that points to the newly formed materials. 2Na + Cl 2  2NaCl 2H 2 O 2  2H 2 O + O 2 Zn + 2HCl  ZnCl 2 + H 2 CH 4 + 2O 2  CO 2 + 2H 2 O

Types of Chemical Reactions Synthesis A + B  AB Calcium + Oxygen  Calcium Oxide Ca + O 2  CaO

Types of Chemical Reactions Decomposition AB  A + B CaCO 3  CaO + CO 2 (Calcium carbonate yields calcium oxide and carbon dioxide)

“Polyatomic Ions” Groups of nonmetals bonded together with an overall net charge. ammoniumNH 4 + carbonate(CO 3 ) 2- hydroxideOH - nitrateNO 3 - phosphatePO 4 3- sulfateSO 4 2- CaCO 3 H 2 SO 4 NaOHNH 4 NO 3

Types of Chemical Reactions Single Displacement A + BX  AX + B Zn + HCl  ZnCl 2 + H 2 (Zinc and hydrochloric acid react to form zinc chloride and hydrogen.)

Types of Chemical Reactions Double Displacement AX + BY  AY + BX CaCO 3 + 2HCl  CaCl 2 + H 2 CO 3 Calcium carbonate and hydrochloric acid yield calcium chloride and carbonic acid.

Types of Chemical Reactions Synthesis = Building Decomposition = Breaking Down Single Displacement = Metal Displaced Double Displacement = Metals Swap Places

Types of Chemical Reactions Combustion C 2 H 5 OH + 3O 2  2CO 2 + 3H 2 O CH 4 + 2O 2  CO 2 + 2H 2 O 2H 2 + O 2  2H 2 O Fuel + Oxygen  release energy Energy releasing reaction involving oxygen and forming oxides of reactants.

Types of Chemical Reactions Acid – Base a.k.a. neutralization HCl + Mg(OH) 2  MgCl 2 + H 2 O Acid + Base  Salt + Water Acid = substance that releases H + in solution Base = substance that accepts H + in solution (usually by releasing OH - ) HClNaOHH 2 SO 4 Mg(OH) 2

Types of Chemical Reactions Synthesis = Building Decomposition = Breaking Down Single Displacement = Metal Displaced Double Displacement = Metals Swap Places Combustion = Burn Fuel, Release Energy Acid-Base = Neutralization, Yield salt + water

Law of Conservation of Matter Matter, like energy, is neither created nor destroyed in any process. “One may take it for granted that in every reaction there is an equal quantity of matter before and after.” Lavoisier, 1768

“Balancing Equations” 2Ca + O 2  2CaO 2 Ca2 2 O2