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Atoms, Molecules and Ions Chapter 2 - A.P. Chemistry Laws Governing Atoms • law of definite proportions • law of multiple proportions • law of conservation of mass Laws Governing Atoms • law of definite proportions • in a compound the proportion of mass of each element is consistent • law states that the elemental composition of a pure substance is always the same Laws Governing Atoms • law of multiple proportions • when elements combine to form different compounds the ratio of elements is always a small whole number • law states that when two elements form a series of compounds the ratios of the masses to second element to 1g of the first element can always be reduced to small whole numbers Laws Governing Atoms • law of conservation of mass • matter cannot be created or destroyed • since all matter has mass, this must be conserved as matter is conserved • mass can be neither created or destroyed by a chemical reaction Atomic Theory • Dalton’s Atomic Theory • each element is composed of extremely small particles called atoms • all atoms of a given element are identical, the atoms of one element are different than atoms of another element • compounds are formed when atoms of more than one element combine, a given compound always has the same number of elements and atoms • atoms of one element cannot be changed into atoms of another element by chemical means, a chemical reaction involves only separation, combination or rearrangement of atoms, they cannot be created or destroyed Atomic Theory • Democritus • Bohr • Dalton • Thomson • Proust • Schrodinger • Rutherford • Lavoisier • Boyle • History’s Key Players Atoms • the smallest particle of an element that retains the characteristic chemical properties of that element Atoms • electrons - negatively charged particles of insignificant mass orbiting around the nucleus • nucleus - small dense center of an atom • protons - positively charged particles with a mass of 1 amu • neutrons - neutral particles with a mass slightly higher than that of a Atoms • Radioactivity • spontaneous breakdown of an atom by emission of particles and/or radiation • Alpha, Beta or gamma rays • end result of the breakdown is a stable element Atoms • Radioactivity • Alpha rays - positively charged particles called alpha particles • Beta rays - electrons (negative) particles called beta particles • Gamma rays - no charge, very high energy waves Atoms • Numbers • atomic number • atomic mass • mass number Atoms • atomic number - number of protons in the nucleus of each atom of an element Atoms • mass number - the total number of neutrons and protons present in the nucleus of an element • all nuclei have protons and neutrons • exception is hydrogen Atoms • mass number = # protons + # neutrons • mass number = atomic number + number of neutrons Atoms • Isotopes - atoms that have the same atomic number but different mass numbers • same number of protons, different number of neutrons Atoms 11 6C 12 6C 13 6C 14 6C • How many protons, neutrons, and electrons are in (a) an atom of 197Au; (b) an atom of stron:um-‐90? Atomic Mass • Average mass is calculated from the isotopes of an element weighted by their relative abundances. • Because in the real world we use large amounts of atoms and molecules, we use average masses in calculations. Atomic Mass • Naturally occurring chlorine is 75.78% 35Cl, which has an atomic mass of 34.969 amu, and 24.22% 37Cl, which has an atomic mass of 36.966 amu. Calculate the average atomic mass (that is, the atomic weight) of chlorine. Atoms • Give the chemical symbol, including mass number, for each of the following ions: (a) The ion with 22 protons, 26 neutrons, and 19 electrons; (b) the ion of sulfur that has 16 neutrons and 18 electrons. Atoms • The diameter of a US penny is 19 mm. The diameter of a silver atom, by comparison, is only 2.88 Å. How many silver atoms could be arranged side by side in a straight line across the diameter of a penny? Periodic Table • It is a systematic catalog of elements • Elements are arranged in order of atomic number Periodic Table • patterns of chemical and physical properties can be seen in the order of the elements. Periodic Table • the rows on the periodic chart are periods • columns are groups or families • elements in the same group have similar chemical properties Periodic Table • these five groups are known by their names Periodic Table • nonmetals are on the right side of the table (exception is H) Periodic Table • metalloids border the stair step line (except Al, Po, At) Periodic Table • metals are on the left side of the table Molecules and Ions • only the noble gases (group 8A) of the periodic table exist in nature as single atoms • monatomic (single or one atom) Molecules and Ions • molecule - a grouping of at least two atoms in a definite arrangement held together by chemical forces (chemical bonds) Chemical Formulas • molecular compounds are compounds composed of molecules and almost always contain only nonmetals Diatomic Molecules • these seven elements occur naturally as molecules with two atoms Types of Formulas • Empirical formulas give the lowest whole-number ratio of atoms of each element in a compound. • Molecular formulas give the exact number of atoms of each element in a compound. • Example: B2H6 vs. BH3 Types of Formulas • Structural formulas show the order in which atoms are bonded. • Perspective drawings also show the three-dimensional array of atoms in a compound. Formulas • various structures can be used to model the same formula • When atoms lose or gain electrons, they become ions. • Cations are positive and are formed by elements on the left side of the periodic chart. • Anions are negative and are formed by elements on the right side of the periodic chart. Ionic Compound • Ionic compounds (such as NaCl) are generally formed between metals and nonmetals, cations and anions • Because compounds are electrically neutral, one can determine the formula of a compound this way: • The charge on the cation becomes the subscript on the anion. • The charge on the anion becomes the subscript on the cation. • If these subscripts are not in the lowest wholenumber ratio, divide them by the greatest common factor. Common Cations Common Anions Practice Problem • What are the empirical formulas of the compounds formed by (a) Al3+ and Cl– ions, (b) Al3+ and O2– ions, (c) Mg2+ and NO3– ions?