Atoms, Molecules and Ions PDF

Atoms, Molecules and Ions Chapter 2 Prof.Dr. Wael Abou-Elmagd  Dalton’s Atomic Theory (1808) 1. Elements are composed of extremely small particles called atoms. All atoms of a given element are identical, having the same size, mass and chemical properties. The atoms of one e...

Atoms, Molecules and Ions Chapter 2 Prof.Dr. Wael Abou-Elmagd  Dalton’s Atomic Theory (1808) 1. Elements are composed of extremely small particles called atoms. All atoms of a given element are identical, having the same size, mass and chemical properties. The atoms of one element are different from the atoms of all other elements. 2. Compounds are composed of atoms of more than one element. The relative number of atoms of each element in a given compound is always the same. 3. Chemical reactions only involve the rearrangement of atoms. Atoms are not created or destroyed in chemical reactions.  The Structure of the Atom  Experiments showed that an atom consists of two kinds of particles: a nucleus, the atom’s central core, which is positively charged and contains most of the atom’s mass, and one or more electrons. An electron is a very light, negatively charged particle that exists in the region around the atom’s positively charged nucleus. Nuclear Structure; Isotopes  A proton is a nuclear particle having a positive charge equal to that of the electron and a mass more than 1800 times that of the electron.  The atomic number (Z) is therefore the number of protons or electrons in the nucleus of an atom.  Neutron is a nuclear particle having a mass almost identical to that of the proton but no electric charge. Note that: An element is a substance whose atoms all have the same atomic number. The mass number (A) is the total number of protons and neutrons in a nucleus. Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutrons A nuclide is an atom characterized by a definite atomic number and mass number. Element Symbol   Isotopes are atoms whose nuclei have the same atomic number but different mass numbers; that is, the nuclei have the same number of protons but different numbers of neutrons. Example 1 A representation of two isotopes of carbon  1 2 3 Example 2 1H 1H (D) 1H (T)  Do You Understand Isotopes? How many protons, neutrons, and electrons are in 146 C? 6 protons, 8 (14 - 6) neutrons, 6 electrons How many protons, neutrons, and electrons are in 116 C? 6 protons, 5 (11 - 6) neutrons, 6 electrons  Noble Gas Halogen Group A modern form of the periodic table Period Alkali Earth Metal Alkali Metal Metals, Nonmetals, and Metalloids  A metal is a substance or mixture that has a characteristic luster, or shine, and is generally a good conductor of heat and electricity.  A nonmetal is an element that does not exhibit the characteristics of a metal. The solid nonmetals are usually hard, brittle substances.  A metalloid, or semimetal, is an element having both metallic and nonmetallic properties. These elements, such as silicon (Si) and germanium (Ge), are usually good semiconductors A molecule is an aggregate of two or more atoms in a definite arrangement held together by chemical bonds H2 H2O NH3 CH4 A diatomic molecule contains only two atoms H2, N2, O2, Br2, HCl, CO A polyatomic molecule contains more than two atoms O3, H2O, NH3, CH4 An ion is an atom, or group of atoms, that has a net positive or negative charge. cation – ion with a positive charge If a neutral atom loses one or more electrons it becomes a cation. 11 protons 11 protons Na 11 electrons Na+ 10 electrons anion – ion with a negative charge If a neutral atom gains one or more electrons it becomes an anion. 17 protons 17 protons Cl 17 electrons Cl- 18 electrons A monatomic ion contains only one atom Na+, Cl-, Ca2+, O2-, Al3+, N3- A polyatomic ion contains more than one atom OH-, CN-, NH4+, NO3-  Do You Understand Ions? 27 3 + How many protons and electrons are in 13 Al ? 13 protons, 10 (13 – 3) electrons How many protons and electrons are in 78 34 Se 2- ? 34 protons, 36 (34 + 2) electrons ionic compounds consist of a combination of cations and an anions The sum of the charges on the cation(s) and anion(s) in each formula unit must equal zero The ionic compound NaCl Formula of Ionic Compounds 2 x +3 = +6 3 x -2 = -6 Al2O3 Al3+ O2- 1 x +2 = +2 2 x -1 = -2 CaBr2 Ca2+ Br- 1 x +2 = +2 1 x -2 = -2 Na2CO3 Na+ CO32- Some Polyatomic Ions (Table 2.3) NH4+ ammonium SO42- sulfate 2- 2- CO3 carbonate SO3 sulfite - - HCO3 bicarbonate NO3 nitrate ClO3- chlorate NO2- nitrite Cr2O72- dichromate SCN- thiocyanate 2- - CrO4 chromate OH hydroxide Chemical Nomenclature  Binary Compounds: Compounds composed of two elements  Binary Ionic Compounds (Type 1): contains a positive ion (cation) always written first in the formula and a negative ion (anion)  Rules: 1. The cation is always named first and the anion second 2. A monatomic (meaning one atom) cation takes its name from the name of the element 3. A monatomic anion is named by taking the root of the element name and adding –ide  Binary Ionic Compounds Compound Ions Present Name NaCl Na+, Cl- Sodium chloride KI K+, I- Potassium iodide CaS Ca2+, S2- Calcium sulfide Li3N Li+, N3- Lithium nitride CsBr Cs+, Br- Cesium bromide MgO Mg2+, O2- Magnesium oxide  Naming compounds Binary Ionic Compounds (Type II): Metals that form more than one type of positive ion. Fe2+ and Fe3+ Transition metals form several positive oxidation states Charge on the metal ion must be specified Roman numeral indicates the charge of the cation. Iron (II) chloride and iron (III) chloride The ion with the higher charge has a name ending in –ic and the one with the lower charge has a name ending in –ous; ferrous chloride and ferric chloride  Naming Compounds Ionic Compounds with Polyatomic Ions: Need to know the names of the polyatomic ions (Table 2.5). NH4+  ammonium, SO42-  sulfate Na2SO4 Sodium sulfate KH2PO4 Potassium dihydrogen phosphate Fe(NO3)3 Iron(III) nitrate CsClO4 Cesium perchlorate NaOCl sodium hypochlorite Al2(Cr2O7)3 Aluminum dichromate Sr(CN)2 Strontium cyanide  Naming compounds Binary Covalent Compounds (Type III): Formed between two nonmetals Rules:  The first element in the formula is named first, using the full element’s name  Second element is name as if it were an anion  Use prefixes to denote the number of atoms present  Never use mono – prefix for naming the first element CO ==> carbon monoxide, not monocarbon monoxide P2O5 ==> diphosphorus pentoxide S2Cl4 ==> disulfur tetrachloride NO2 ==> nitrogen dioxide N2O5 ==> dinitrogen pentoxide  Formulas from Names Name Chemical Formula Diphosphorus pentasulfide P2S5 (two non metals) Cesium peroxide Cs2O2 (Cs1+, O22-) Aluminum fluoride AlF3 (Al3+, F-1) Vanadium (v) fluoride VF5 (V5+, F-1) Dioxygen difluoride O2F2 (two non metals) Gallium oxide Ga2O3 (Ga3+, O2-) Ammonium dichromate (NH4)2Cr2O7 (NH4+, Cr2O72-) Cupric phosphate Cu3(PO4)2 (Cu2+, PO43-) Flowchart for Naming Binary Compounds  Acids When dissolved in water produce a solution containing free H+ ions (protons) An acid is a molecule with one or more H+ ions attached to an anion If the anion does not contain oxygen, the acid is named with the prefix hydro – and the suffix –ic HCl Hydrochloric acid HCN Hydrocyanic acid  Acids If the anion contains oxygen, the acidic name is formed from the root name of the anion with the suffix of –ic or –ous depending on the anion HNO3 Nitric acid (Nitrate anion) H2SO4 Sulfuric acid (Sulfate anion) H3PO4 Phosphoric acid (Phosphate anion) HC2H3O2 Acetic acid (Acetate anion) H2SO3 Sulfurous acid (sulfite anion) HNO2 Nitrous acid (nitrite anion) Flowchart for Naming Acids A base can be defined as a substance that yields hydroxide ions (OH-) when dissolved in water. NaOH sodium hydroxide KOH potassium hydroxide Ba(OH)2 barium hydroxide  Chemical equations  Chemical reactions involve a rearrangement of the ways atoms are grouped together. A chemical equation represents a chemical reaction.  Reactants are shown to the left of the arrow.  Products are shown to the right of the arrow.  In a chemical reaction atoms are not created or destroyed. All atoms present in the reactants must be accounted for in the products.  Same number of each type of atom on both sides of the arrow. 34 Balancing a Chemical Equation  Unbalanced Equation:  Balancing the Equation:  The balanced equation: CH4 + 2O2 CO2 + 2H2O  Physical States  Physical states of compounds are often given in a chemical equation. Example  Exercise When blue light shines on a mixture of hydrogen and chlorine gas, the elements react explosively to form gaseous hydrochloric acid. What is the unbalanced equation for this process? a) H2(g) + CH4 (g) HCl(g) b) HCl(g) H(g) + Cl(g) c) H(g) + Cl(g) HCl(g) d) H2(g) + Cl2(g) 2HCl(g) How to Write and Balance Equations 1. Read the description of the chemical reaction. What are the reactants, the products, and their states?Write the appropriate formulas. Hydrogen gas (H2) and oxygen gas (O2) combine to form liquid water (H2O). 2. Write the unbalanced equation that summarizes the information from step 1. H2(g) + O2(g) H2O(l) How to Write and Balance Equations 3. Balance the equation by inspection, starting with the most complicated molecule. Equation is unbalanced by counting the atoms on both sides of the arrow. How to Write and Balance Equations 3. Balance the equation by inspection, starting with the most complicated molecule. We must balance the equation by adding more molecules of reactants and/or products.  How to Write and Balance Equations 4. Check to see that the coefficients used give the same number of each type of atom on both sides of the arrow. Also check to see that the coefficients used are the smallest integers that give the balanced equation. The balanced equation is: 2H2(g) + O2(g) 2H2O(l) preferred or could be: 4H2(g) + 2O2(g) 4H2O(l) Exercise Balance the following equation in standard form (lowest multiple integers) and determine the sum of the coefficients? FeO(s) + O2(g) Fe2O3(s) a) 3 b) 4 c) 7 d) 14  Exercise Which of the following correctly balances the chemical equation given below? There may be more than one correct balanced equation. If a balanced equation is incorrect, explain what is incorrect about it. CaO + C CaC2 + CO2 I. CaO2 + 3C CaC2 + CO2 II. 2CaO + 5C 2CaC2 + CO2 III. CaO + (2.5)C CaC2 + (0.5)CO2 IV. 4CaO + 10C 4CaC2 + 2CO2  Exercise Of the three that are correct, which one is preferred most (the most accepted convention)? Why? CaO + C CaC2 + CO2 I. CaO2 + 3C CaC2 + CO2 II. 2CaO + 5C 2CaC2 + CO2 III. CaO + (2.5)C CaC2 + (0.5)CO2 IV. 4CaO + 10C 4CaC2 + 2CO2  Concept Check When balancing a chemical equation, which of the following statements is false? a) Subscripts in the reactants must be conserved in the products. b) Coefficients are used to balance the atoms on both sides. c) When one coefficient is doubled, the rest of the coefficients in the balanced equation must also be doubled. d) Phases are often shown for each compound but are not critical to balancing an equation. Notice  The number of atoms of each type of element must be the same on both sides of a balanced equation.  Subscripts must not be changed to balance an equation.  A balanced equation tells us the ratio of the number of molecules which react and are produced in a chemical reaction.  Coefficients can be fractions, although they are usually given as lowest integer multiples.