Chemistry Lecture 3 and 4 - Atomic Mass, Ions, and Ionic Compounds PDF

ATOMIC MASSES, IONS AND IONIC COMPOUNDS CHE 101_LECTURE 3 AND 4 1 2 ATOMIC MASS UNIT (u) ▪ The atomic mass unit (u) is a unit that describes the masses of individual atoms and molecules ▪ For macroscopic objects, we use units such as grams and kilograms...

ATOMIC MASSES, IONS AND IONIC COMPOUNDS CHE 101_LECTURE 3 AND 4 1 2 ATOMIC MASS UNIT (u) ▪ The atomic mass unit (u) is a unit that describes the masses of individual atoms and molecules ▪ For macroscopic objects, we use units such as grams and kilograms to state their masses, but these units are much too big to comfortably describe the masses of individual atoms and molecules ▪ defined as one-twelfth of the mass of a carbon-12 atom (6 protons and 6 neutrons) Proton 1.00728 u Neutron 1.00866 u Electron 0.000549 u 3 ATOMIC MASS for atoms ▪ Equal to mass number of the atom ▪ Carbon – 12 is 12 u ▪ Oxygen – 16 is 16 u ATOMIC MASS for Elements (Periodic Table) ▪ Most elements exist as a mixture of isotopes, each of which has its own mass ▪ When talking about the mass of an element, we must take the isotopic mixture into account 4 ATOMIC MASS of an ELEMENT ▪ Weighted average of the masses of the isotopes that compose an element Example ▪ Isotope m ▪ 50% with mass 10 u ▪ 50% with mass 11 u 0.50 x 10 u = 5.0 u 0.50 x 11 u = 5.5 u Sum = 10.5 u Atomic mass of the element ▪ No atom in our hypothetical element has a mass of 10.5 u; rather, that is the average mass of the atoms, weighted by their percent occurrence 5 ATOMIC MASS of an ELEMENT Boron ▪ 20% exists as Boron – 10 ▪ 80% exists as Boron – 11 0.20 x 10 u = 2.0 u 0.80 x 11 u = 8.8 u Sum = 10.8 u Atomic mass of Boron ▪ Check periodic table ▪ Virtually all elements exist as mixtures of isotopes, so atomic masses may vary significantly from whole numbers 6 ATOMIC MASS for MOLECULES Molecular Mass ▪ Sum of the masses of the atoms in a molecule N2O3 ▪ Check atomic masses (mass numbers) of each element from the periodic table ▪ N = 14.007 u O = 15.999 u 2(N) = 2 x 14.007 u = 28.014 u 3(O) = 3 x 15.999 u = 47.997 u Sum = 76.001 u Molecular mass of N2O3 7 Questions ▪ What is the molecular mass of each substance? Use periodic table for atomic masses 1. NBr3 1. H2O 2. C2H6 2. O2 3. SO2 3. Cl2 4. CO2 4. H2S 5. SO3 6. CO 7. PF3 8. CO 9. Se2Br2 10. SF4 11. SF6 12. P2S5 8 Questions ▪ What is the molecular mass of each substance? Use periodic table for atomic masses 1. carbon tetrachloride 2. silicon dioxide 3. trisilicon tetranitride 4. disulfur difluoride 5. iodine pentabromide 6. Nitrogen trichloride 7. Diphosphorus pentoxide 9 Questions ▪ Estimate the mass, in whole numbers, of each isotope. 1. hydrogen-1 2. hydrogen-3 3. iron-56 4. phosphorus-31 5. b. carbon-14 6. c. americium-241 10 Questions 1. Determine the atomic mass of each element, given the isotopic composition. a) lithium, which is 92.4% lithium-7 (mass 7.016 u) and 7.60% lithium-6 (mass 6.015 u) a) oxygen, which is 99.76% oxygen-16 (mass 15.995 u), 0.038% oxygen-17 (mass 16.999 u), and 0.205% oxygen-18 (mass 17.999 u) 11 Questions 1. Determine the atomic mass of each element, given the isotopic composition. a) neon, which is 90.48% neon-20 (mass 19.992 u), 0.27% neon-21 (mass 20.994 u), and 9.25% neon-22 (mass 21.991 u) a) uranium, which is 99.27% uranium-238 (mass 238.051 u) and 0.720% uranium-235 (mass 235.044 u) 12  RECAP ▪ Atomic number (No of Protons) ▪ Mass number (No of Protons + No of Neutrons) ▪ Isotopes of elements ▪ Atomic symbols and periodic table ▪ Molecules (combination of two or more atoms) ▪ Molecular formula and rules for naming molecules ▪ Atomic mass of atoms (mass number of the atom) ▪ Atomic mass of element (Average weight of the atomic masses of each isotope) ▪ Molecular mass (sum of masses of atoms in molecule) 13  IONS AND IONIC COMPOUNDS Ions ▪ So far, we have discussed elements and compounds that are electrically neutral. They have the same number of electrons as protons, so the negative charges of the electrons is balanced by the positive charges of the protons ▪ Ions are electrically charges particles (atoms) ▪ Cations are positively charged ▪ Anions are negatively charged ▪ Ionic compounds are compounds formed from positive and negative ions 14  IONS AND IONIC COMPOUNDS Electronic configuration of atoms ▪ ​Electrons are arranged around the nucleus in shells of well defined energy. ▪ Lower energy configurations are more stable ▪ For atoms, these lower energy states are represented by the noble gas elements. ▪ These elements have electron configurations characterized by full valence electron configurations. This makes them stable and unreactive. ▪ Each shell can only contain a given number of electrons 15 Electronic configuration (EC) ▪ ​Draw the EC for He, Ne and Ar ▪ Groups and Periods 16  IONS AND IONIC COMPOUNDS Electronic configuration of atoms ▪ ​The elements in the other groups have valence electron configurations that are not full, so they are unstable ▪ This instability drives them toward the lower energy states represented by the noble gases ▪ In these lower energy states, the outermost energy level has eight electrons (an “octet”) ▪ The tendency of an atom to move toward a configuration in which it possesses eight valence electrons is referred to as the “Octet Rule’’ 17  IONS AND IONIC COMPOUNDS ▪ ​There are two ways for an atom that does not have an octet of valence electrons to obtain an octet in its outer shell Ionic bonding ▪ Transfer of electrons between two atoms until both atoms have octets ▪ some atoms will lose electrons and some atoms will gain electrons ▪ Those that lose electrons become positively charged (cations) ▪ Those that gain electrons become negatively charged (anions) Covalent bonding ▪ Sharing electrons with another atom ▪ These shared electrons simultaneously occupy the outermost shell of both atoms 18  IONS AND IONIC COMPOUNDS ▪ ​There are two ways for an atom that does not have an octet of valence electrons to obtain an octet in its outer shell Ionic bonding ▪ Transfer of electrons between two atoms until both atoms have octets ▪ some atoms will lose electrons and some atoms will gain electrons ▪ Those that lose electrons become positively charged (cations) ▪ Those that gain electrons become negatively charged (anions) Covalent bonding ▪ Sharing electrons with another atom ▪ These shared electrons simultaneously occupy the outermost shell of both atoms 19 ▪ ​Draw the EC for all group elements (first 20) ▪ Determine their possible charges ▪ Note those that loose or gain electrons ▪ Note how to write the ion (number before superscript charge) 20 ▪ While hydrogen is in the first column, it is not considered to be an alkali metal ▪ Hydrogen can form both covalent bonds and ionic bonds, depending on the element that it is interacting with 21  IONS AND IONIC COMPOUNDS Naming ions ▪ ​Cations are named very simply by following the element name with the word ‘ion’ ▪ Na+ = Sodium ion ▪ Fe2+ = Iron two ion = Iron(II) ion ▪ Fe3+ = Iron three ion = Iron(III) ion ▪ For anions, use the stem of the element name and append the suffix -ide to it, ▪ Cl- = Chloride ion ▪ N3- = Nitride ion 22 Questions 1. Name each species. 1. O2− 2. Co 3. Co2+ 4. P3− 5. Sr2+ 23