Chapter 2: Atoms and Elements Chem 103: Chapter 2 Chapter 2: Atoms and Elements Copyright 2006, David R. Anderson

The Atomic Theory of Matter: Dalton's postulates Atomic Structure Subatomic particles: protons, neutrons, and electrons Atomic number and mass number Isotopes Atomic weight and isotopic abundances The mole and molar masses The Periodic Table: Classification of the elements rows (periods) and columns (groups) main group, transition, and inner transition elements metals, nonmetals, and metalloids alkali metals, alkaline earth metals, chalcogens, halogens, and noble gases

~400 BC Democritus: “atomos” (indivisible) I. The Atomic Theory of Matter Dalton’s postulates ~400 BC Democritus: “atomos” (indivisible) -but Plato & Aristotle argued against it -ignored for > 2000 years 1803 John Dalton proposed: (> 100 years before the discovery of the nucleus, protons, and electrons) 1. All matter is composed of tiny, indestructible (indivisible) particles called atoms. 2. All atoms of a given element are the same; atoms of different elements are different. 3. A compound is composed of atoms in a fixed number ratio. 4. Chemical reactions involve only exchanges of atoms; atoms are neither created, destroyed, nor transmuted.

Dalton’s theory explained: the Law of Conservation of Mass: I. The Atomic Theory of Matter Dalton’s theory explained: the Law of Conservation of Mass: 2HgO  2Hg + O2 the Law of Definite Proportions: CO2 and predicted: the Law of Multiple Proportions: e.g., CO2 O/C = 2.67 CO O/C = 1.33 no loss or change in atoms,  no loss or gain of mass always same number (same proportion) of same atoms (same masses) (simple whole number ratio)

nucleus: protons and neutrons > 99.95% of mass of atom II. Atomic Structure nucleus: protons and neutrons > 99.95% of mass of atom electron cloud: < 0.05% of mass Analogies: size: 73 million Cu atoms = diameter of a penny 1 million C atoms = thickness of a sheet of paper emptiness: nucleus/electron cloud = pinhead/baseball stadium mass: if matter were composed of only nuclear material, a matchbox full would weigh 25 billion tons

i.e., protons, neutrons ~ 2000 times heavier than electrons Atomic Structure Subatomic particles: protons, neutrons, and electrons where charge = 1.602  1019 C 1 u = 1.661  1024 g i.e., protons, neutrons ~ 2000 times heavier than electrons

Z atomic number = number of protons in the atom (identity) Atomic Structure Atomic number and mass number atomic number = number of protons in the atom (identity) mass number = number of protons + neutrons notation: AX X = symbol for the element Z = atomic number = number of protons A = mass number = number of protons + neutrons  number of neutrons = A  Z e.g., How many neutrons are in 12C, 206Pb? e.g., What is the mass number of phosphorus with 16 neutrons? Z redundant

= atoms with the same atomic number (same number of protons) Atomic Structure C. Isotopes = atoms with the same atomic number (same number of protons) but different mass (different number of neutrons) 12C 13C 1H 2H 3H 79Br 81Br deuterium tritium D. Atomic weight and isotopic abundance atomic “weight” = weighted average of all naturally occurring isotopes e.g., Naturally occurring carbon is 98.892% 12C (12.00000 amu) 1.108% 13C (13.00335 amu)  average atomic mass = (0.98892  12.00000) + (0.01108  13.00335) = 12.011 amu “atomic weight”

Boron is 19.91% 10B (10.0129 u) and 80.09% 11B (11.0093 u). Atomic Structure Atomic weight and isotopic abundances Boron is 19.91% 10B (10.0129 u) and 80.09% 11B (11.0093 u). What is the average atomic mass of boron? Chlorine occurs as two isotopes, 35Cl (34.96885 u) and 37Cl (36.96590 u). Its atomic weight is 35.4527 u. What are the abundances of 35Cl and 37Cl?

mole = number of atoms of 12C in exactly 12 g of 12C Atomic Structure The mole and molar masses mole = number of atoms of 12C in exactly 12 g of 12C i.e., 1 atom of 12C has a mass of 12 amu 1 mole of 12C has a mass of 12 g find: 1 mol = 6.022  1023 (N, Avogadro’s number) huge number: 1 mol of marbles would cover the earth 3 miles deep! 1 mol of dollar bills would be enough to pay each of the 6 billion people on earth $3 million per second for 100 years! 1 mol of Cu = 6.022  1023 atoms of Cu 1 mol of NH3 = 6.022  1023 molecules of NH3 1 mol of M&M’s = 6.022  1023 M&M’s

molar mass = mass, in g, of one mole of a substance Atomic Structure The mole and molar masses molar mass = mass, in g, of one mole of a substance molar mass of C = 12.011 g/mol (atomic weight) molar mass of Ag = 107.8682 g/mol Using molar masses to convert between mass, moles, and number of particles: e.g., A 1-carat diamond has a mass of 200.0 mg. How many moles of carbon are in a 1-carat diamond? How many atoms? e.g., A reaction requires 0.350 mol of Na. How many grams is this?

The Periodic Table: Classification of the Elements

chemical and physical properties vary in a periodic fashion The Periodic Table: Classification of the Elements A. periods (rows) chemical and physical properties vary in a periodic fashion groups (columns): families of elements elements in a group have similar properties B. main group (representative) elements groups 1A - 8A transition elements groups 1B - 8B inner transition elements lanthanides (rare earth elements) actinides

lustrous, malleable, ductile, electrically conductive nonmetals The Periodic Table: Classification of the Elements C. metals lustrous, malleable, ductile, electrically conductive nonmetals not lustrous, malleable, ductile, nor electrically conductive metalloids (semimetals, semiconductors) in between D. named groups group 1A: alkali metals (form alkaline solutions in water) group 2A: alkaline earth metals (") group 6A: chalcogens (copper-forming ) [not often used] group 7A: halogens (salt-forming) group 8A: noble gases (inert gases; unreactive)