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Chemical Bonds Chapter 18 17.1 Stability in Bonding Main Idea: When atoms form compounds, each atom is more stable in the compound than it was by itself. • Examples: (discuss with your table examples of things that become more stable when combined) Combined Elements Compounds New Properties • Pure elements (i.e. copper, silver, oxygen) combine chemically to form compounds in the right conditions. • Ex: Copper sulfate is one of the compounds responsible for the color of the Statue of Liberty • Physical and chemical properties of a compound are often different than the properties of the individual elements that combine to create the compound. Formulas • Every element has a chemical symbol. • A chemical formula shows what elements a compound contains and the exact number of the atoms of each element in a unit of that compound. • Subscript – written below • A symbol with no subscript means there is only one atom of the element in the compound. Chemical Bond Formation Why do atoms form compounds? Electron Dot Diagrams (Lewis Dot Structures) refer to WS in NB • The unique noble gases – an atom is chemically stable when its outer energy level is complete. H and He are stable with 2 electrons. All other elements are stable with 8. Helium rarely forms compounds. • Unfilled and filled energy levels – dot diagrams show if the outer energy level is filled. A filled outer energy level creates stability. • Outer levels – getting their fill – atoms with unstable outer energy levels can lose, gain, or share electrons to obtain a stable outer energy level. This is achieved through joining with other atoms. Chemical Bond Formation (cont.) Why do atoms form compounds? • When an element loses electrons, the next-to-the last energy level becomes the new outermost energy level. • Gaining and losing electrons – NaCl Chemical Bond Formation (cont.) Why do atoms form compounds? Sharing electrons– H2O • Draw dot structures for hydrogen and oxygen to show how a water molecule is created. Chemical bond formation – a chemical bond is the force that holds atoms together in a compound. Predict whether these atoms would more likely gain or lose electrons to form an octet. Section 2 - Types of Bonds Main Idea: Atoms form ionic bonds by transferring electrons and form covalent bonds by sharing electrons. Ions • Atoms lose or gain electrons to meet a standard – stability. • An ion is a charged particle that has either more or fewer electrons than protons. • Losing electrons creates a positively charged ion. (cation) • Gaining electrons creates a negatively charged ion. (anion) Transfer of electrons • Superscript- written above • A superscript is used to indicate the charge of an ion. • The charge is determined by the number of electrons gained or lost. • Ex: KI p. 559 The Ionic Bond • An ionic bond is the force of attraction between the opposite charges of the ions in an ionic compound. The number of positive charges must equal the number of negative charges in order to form a compound with a neutral charge. Zero net charge • The compound as a whole is neutral because the sum of the charges on the ions is zero. • Ionic bonds are usually formed between metals and nonmetals. • Ionic compounds are often formed by elements across the periodic table from each other. • They are typically crystalline solids with high melting points. Ex: MgCl2 Molecules • Some atoms of nonmetals are unlikely to lose or gain electrons. • Group 14 elements would have to either gain or lose 4 electrons to achieve stability. This would take a great deal of energy. • Group 14 elements become more chemically stable by sharing electrons. • The neutral particle that forms as a result of electron sharing is called a molecule. The Covalent Bond • The attraction that forms between atoms when they share electrons is a covalent bond. • A single covalent bond is composed of two shared electrons. • A water molecule contains two single bonds. Multiple covalent bonds • A covalent bond can contain more than one pair of electrons. • In a diatomic oxygen molecule two electrons are shared between the two oxygen atoms. • • • • A double bond occurs when atoms share two pairs of electrons. A triple bond contains three shared pairs of electrons. Covalent bonds form between nonmetal elements. Many covalent compounds are solids or gases at room temperature. Equal Sharing • A nonpolar bond is a covalent bond in which electrons are shared equally by both atoms. • Ex: diatomic molecules Unequal Sharing • In some molecules, electrons are not shared equally and the electron charge is concentrated more on one end of the molecule than the other. • The strength of the attraction of each atom to its electrons is related to the size of the atom, the charge of the nucleus, and the total number of electrons the atom contains. • More positive charge in a nucleus attracts electrons more strongly. Partial Charges • Ex. HCl page 562 Tug-of-War • Polar – having opposite ends • A polar bond is a bond in which electrons are shared unequally. Polar and nonpolar molecules • A polar molecule is one in which the unequal sharing of electrons results in a slightly positive end and a slightly negative end, although the molecule is neutral. • A nonpolar molecule does not have oppositely charged ends. • Polarity and Geometry – The polarity of the bonds and the shape of the molecule determine whether the molecule is polar or nonpolar. Writing Formulas and Naming Compounds – Section 3 Main Idea: For both ionic and covalent compounds, you can write a name from the chemical formula or a chemical formula from the name. Writing Chemical Formulas Oxidation Numbers • Oxidation number – a positive or negative number that indicates how many electrons an atom has gained, lost, or shared to become stable. • For ionic compounds, the oxidation number is the same as the charge on the ion. Oxidation Numbers and the Periodic Table • Many elements have 1 oxidation number. • Groups 1, 2, 13-18 fit with the period table groupings. • Many of the transition elements can have more than one oxidation number. Binary Ionic Compounds • Binary compounds are the easiest for which to write formulas. • Binary compounds – composed of two elements • Even though individual ions carry a charge, the compound itself is neutral. • Ex: aluminum oxide (p. 566) Writing Formulas 1. Write the symbol of the element that has the positive oxidation number or charge. (all metals and often Hydrogen) 2. Write the symbol of the element with the negative oxidation number. (nonmetals and Hydrogen when bonded with a metal) 3. Find the least common multiple of the charges of each ion. (The charge, without the sign, of one ion becomes the subscript of the other ion.) Reduce the subscripts to the smallest whole numbers that retain the ratio of the ions. Examples – Binary Ionic Compounds Naming 1. Write the name of the positive ion. 2. Check to see whether the positive ion forms more than one oxidation number. If so, determine the oxidation number of the ion from the formula of the compound. Write the charge of the positive ion using roman numerals in parentheses after the ion’s name. 3. Write the root name of the anion. The root is the first part of the elements name. 4. Add the ending –ide to the root. Examples – Naming Binary Ionic Compounds Compounds with Complex Ions • Many common compounds contain more than two atoms. • A polyatomic ion is a positively or negatively charged, covalently bonded group of atoms. • Polyatomic ions as a whole contain two or more elements. They act like individual ions in forming compounds. Writing Formulas • To write formulas for compounds containing polyatomic ions, follow the rules for binary compounds, with one addition. • When more than one polyatomic ion is needed to balance the charges of the ions, place parentheses around the polyatomic ion before adding the subscript. • Ex: barium chlorate (p. 569) Naming 1. Write the name of the positive ion. 2. Write the name of the negative ion. Naming Binary Covalent Compounds • Some pairs of nonmetals can form more than one compound with each other. • Ex: N2O, NO, NO2, and N2O5 • A different system of naming must be used for covalent compounds. Using Prefixes • Scientists use Greek prefixes to indicate how many atoms of each element are in a binary covalent compound. • Ex: N2O, NO, NO2, and N2O5 Compounds with Added Water • A hydrate is a compound that has water chemically attached to its atoms and written into its chemical formula. • Anhydrous – without water Common hydrates • Hydrate comes from a word that means water. • When naming hydrates you use the same Greek prefixes • Ex: p. 570 cobalt chloride