Transcript Chapter 8
Chapter 22 Chemical Bonds Section 1 Stability in Bonding Compounds Compounds are elements that are combined Compounds usually have new or different properties than the elements that make up the compound Formulas A chemical formula tells what elements are in a compound, and how many atoms of each element in a unit of that compound – H2O In each unit, there are 2 hydrogen atoms and 1 oxygen atom Chemical Bonds The force that holds two atoms together is called a chemical bond. They may form by the attraction of a positive nucleus and a negative electron They can also form between a positive an negative ion Remember, opposites attract! Electron Dot Structure It is a diagram showing valence electrons. It is useful when showing chemical bonds. Valance Electrons All atoms have valence electrons. Atoms want to be stable. To be stable, the valence shell needs to be full. – Most elements need 8 electrons in their outer shell to be full. This is called the Octet Rule. – Exceptions: hydrogen & helium which need 2 This is why elements bond. Formation of a positive ion A positive ion forms when an atom loses one or more electrons. When the atom loses an electron, it does not become a different element because it still has the same number of protons. A positively charged ion is called a cation. Elements on the left side of the periodic table tend to lose some of their electrons. Groups 1, 2, 3 When an element from group 1 loses all its valence electrons, it picks up a +1 charge. When an element from group 2 loses all its valence electrons, it picks up a +2 charge. When an element from group 3 loses all its valence electrons, it picks up a +3 charge. Formation of a negative ion Elements on right side of the periodic table have great attraction for electrons so they gain electrons to fill their valence shell. Elements become anions when they gain an electron, and then become negatively charged. Anion Rule of Thumb Group 5, 6, and 7 elements could lose all their valence electrons, but as a rule of thumb they usually gain them. Remember each element wants 8 valence electrons. So, 8 – Group # = electrons needed – Group 5 gains 3 and has a 3- charge – Group 6 gains 2 and has a 2- charge – Group 7 gains 1 and has a 1- charge 22.1 Bond Analysis: paragraphs 1. Compare / contrast Na and Cl with NaCl. 2. What does the formula tell about BaF2? 3. Explain why some elements are stable but others are more stable as compounds 4. In CH3COOH, list the elements and how many there are of each. Chapter 22 Section 2 Formation and Nature of Ionic Bonding Electrostatic Force NaCl forms from attraction between oppositely charged ions. They are held together by an electrostatic force. – This force is called an ionic bond Ionic Bonds In ionic bonds, electrons are transferred from one atom to another Ionic bonds form between metals and nonmetals – This is to fulfill the octet rule Atoms want to have a full outer shell of electrons http://www.middleschoolchemistry.com/mu ltimedia/chapter4/lesson5 Ionic Bond Electron Exchange Formation of an ionic bond Ionic bonds form between metals and nonmetals Na atom transfers valence electron to Cl – Na becomes a positive ion, Na + – Cl becomes a negative ion, Cl - Different Ionic Bonds Ionic bonds between metals and the nonmetal oxygen are called oxides. Most other ionic bonds are salts. Ionic Compounds Many ionic compounds are binary – Ionic compounds contain only 2 elements – Ionic compounds contain a metallic cation and a nonmetallic anion Why do atoms bond? Atoms want to have a full outer shell – Noble gas configuration With ionic bonding , electrons are transferred Electrons can also be shared What is a Covalent Bond? The chemical bond that results from the sharing of valence electrons – Shared electrons are part of outer energy levels of both atoms involved Generally occurs when elements are relatively close on the periodic table What is a Molecule? Formed when two or more atoms bond covalently Covalent Bond Electron Sharing Formation of a Covalent Bond H, N, O, F, Cl, Br and I occur in nature as diatomic molecules The molecules formed are more stable this way – Hydrogen is H2 – Oxygen is O2 – Etc. Sigma Bonds Sigma bonds are single covalent bonds. They occur when an electron pair is shared in the center between 2 atoms. The valence atomic orbital of one atom overlaps or merges with another. Multiple Covalent Bonds In many molecules, atoms attain a noblegas configuration by sharing more then one pair of electrons between 2 atoms. C, N, O and S most often form multiple bonds. Double or Triple Bonds Bonds can be double or triple – Double bonds occur when 2 pairs of electrons are shared – Triple bonds occur when 3 pairs of electrons are shared Bond Length This is the distance from the center of one nucleus to the center of the other nucleus of 2 bonded atoms. Single bonds are the longest. Multiple bonds are shorter. – Double bonds are longer then triple bonds The more bonds there are means there is more attraction, so the distance is shorter. Covalent Bond Strength Covalent bonds are not as strong as ionic bonds. – Melting temperature of salt is 801 degrees C, sugar is 185 degrees C. Covalent bonds can be broken. The strength depends on how much distance separates the nuclei. – The shorter the bond length means the bond is stronger due to higher attraction forces. 22.2 Bond Analysis: paragraphs 1. Explain why at atom may make an ionic bond with only certain other atoms. 2. Compare/contrast the possession of electrons in ionic and covalent bonds. 3. Name the types of particles formed by covalent bonds. Section 3 Formulas for Ionic Compounds How do we determine charge? Binary ionic compounds are composed of positively charged monatomic ions of a metal and negatively charged monatomic ions of a nonmetal. – A monatomic ion is a one-atom ion. Charge depends on its place on the periodic table. Look at periodic table for Oxidation Numbers or for Group Numbers. What are the charges on the following monatomic ions? Beryllium Be2+ Iodine I- Nitride N3- Oxidation Number The charge of a monatomic ion can also be known as the oxidation state. – Transition metals have more then 1 oxidation number Oxidation State Oxidation state equals the number of electrons transferred from an atom to form an ion. – When sodium and chlorine react, sodium atom transfers 1 electron to chlorine atom. Oxidation state of sodium is 1+ Oxidation state of chlorine is 1– Negative sign shows the electron was transferred to chlorine. Writing Chemical Formulas In chemical formulas, the symbol of the cation is always written first. The anion symbol follows the cation. Subscripts are used to represent the number of ions of each element in the ionic compound. – If there are no subscripts, the number is one. What is going on with this chemical reaction? Na+ Cl- Answer Ratio is 1:1 Sum of the oxidation numbers if they react is zero. One sodium ion transfers 1 electron to one chloride ion. The compound formula is NaCl. 8-3 Part 2 Naming ions and ionic compounds Naming Ionic Compounds Name cation first and the anion second. Monatomic cations use the element name. – Cs+ is cesium Monatomic anions take their name from the root of the element name plus the suffix –ide. – CsBr is cesium bromide – Examples: oxide, phosphide, nitride, sulfide Naming Ionic Compounds cont. Group 1A and 2A metals have only 1 oxidation number. Transition metals and metals on the right side of the periodic table have more then 1 oxidation number – Fe2+ and O2- is FeO, named iron (II) oxide – Fe3+ and O2- is Fe2O3, named iron (III) oxide Special Ions Name Cu(I) Cu(II) Fe(II) Fe(III) Cr(II) Cr(III) Pb(II) Pb(IV) Ox # 1+ 2+ 2+ 3+ 2+ 3+ 2+ 4+ Polyatomic Ions Some ionic compounds are not binary – Compounds may have more than 2 elements These are called polyatomic ions – Poly means many NaHCO3 Polyatomic Ion Examples Charge Name Formula 1+ Ammonium NH4+ 1- Acetate Chlorate Hydroxide Nitrate C2H3O2ClO3OHNO3- 2- Carbonate Sulfate CO32SO42- 3- Phosphate PO43- Naming Ionic Compounds cont. If a compound contains a polyatomic ion, simply name the ion. – Polyatomic ion OH- is called hydroxide, so – NaOH is sodium hydroxide 22.3 Bond Analysis: paragraphs 1. Write formulas for potassium iodide, magnesium hydroxide, aluminum sulfate, and chlorine heptoxide. 2. Write the names for KCl, Cr2O3, NH4Cl, Ba(ClO3)2, and PCl3. 3. Explain why sodium and potassium will or will not react to bond with each other.