AP Chemistry Chapter 2 Notes: Atoms, Molecules, & Ions

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1 AP Chemistry Chapter 2 Notes: Atoms, Molecules, & Ions The History of Atomic Theory Circa BC. Greek philosopher Democritus proposes the idea of matter being made up of small, indivisible particles (atomos). Late 18th Century. Lavoisier proposes the Law of conservation of mass and Proust proposes the Law of constant composition. Early 19th Century. Using the previously unconnected ideas above, John Dalton formulates his Atomic Theory. Dalton s Atomic Theory (i) Elements are made from tiny particles called atoms. (ii) All atoms of a given element are identical. (N.B. See Isotopes below). (iii) The atoms of a given element are different to those of any other element. (iv) Atoms of different elements combine to form compounds. A given compound always has the same relative numbers and types of atoms. (Law of constant composition). (v) Atoms cannot be created or destroyed in a chemical reaction they are simply rearranged to form new compounds. (Law of conservation of mass). Structure of the Atom and The Periodic Table Several experiments were being carried out in the 19th and 20th centuries that began to identify the subatomic particles that make up the atom. A summary of those experiments is given below. Scientist Experiment Knowledge gained Relating to Crookes J. J. Thompson Millikan Cathode Ray Tube Cathode Ray Deflection Oil Drop Experiment Negative particles of some kind exist Mass/charge ratio of the electron determined Charge on the electron Electron Electron Electron Rutherford, Marsden and Geiger Gold Foil Experiment Nucleus present in atom The nucleus of an atom and the proton In the first part of the 20th Century, following Chadwick s discovery of the neutron, Bohr proposed the idea that the atom was made up of the nucleus containing protons and neutrons that was being orbited by electrons in specific, allowed orbits. This particle model of the electron and atom was expanded a few years after Bohr s original ideas to incorporate the wave nature of the electrons. PARTICLE PROTON CHARGE +1 MASS 1 amu POSITION in atom Nucleus NEUTRON 0 1 amu Nucleus ELECTRON 1 1/1836 amu Outside of the nucleus amu denotes atomic mass units 1

2 The atomic numbers (in the table below printed above the symbol and sometimes referred to as Z) and mass numbers (in the table below printed below the symbol and sometimes referred to as A) that appear on the periodic table have specific meanings. Atomic number = the number of protons in the nucleus of one atom of the element Since all atoms are neutral it also tells us the number of electrons surrounding the nucleus. (N.B. When atoms lose or gain electrons they become charged and form ions). Mass number = the number of protons + the number of neutrons in one atom of the element Period Group T H E P E R I O D I C T A B L E Group 1A 2A 3A 4A 5A 6A 7A 8A 1 H 1 3 Li 7 11 Na K Rb Cs Fr Be 9 12 Mg Ca Sr Ba Ra Sc Y La* Ac** 226 T R A N S I T I O N M E T A L S Ti V Cr Mn Fe Co Ni Cu Zr Hf Rf 41 Nb Ta Db 42 Mo W Sg 43 Tc Re Bh 44 Ru Os Hs 45 Rh Ir Mt 46 Pd Pt Ds 47 Ag Au Rg 30 Zn Cd Hg B Al Ga In Tl C Si Ge Sn Pb N P As Sb Bi O S Se Te Po F Cl Br I At He 4 10 Ne Ar Kr Xe Rn 222 *Lanthanides **Actinided 58 Ce Th Pr Pa Nd U Pm Np Sm Pu Gd Am Tb Cm Dy Bk Ho Cf Er Es Tm Fm Yb Md Lu Lr 257 KEY: Metal Semi Metal Non-metal 13 Al Si P 31 In this example Al is a metal, Si is a semi-metal and P is a non-metal. 2

3 Isotopes Atoms with the same number of protons and electrons, but different numbers of neutrons are called isotopes. This leads to the modification of Dalton s Atomic Theory point (ii) above, to read. All Atoms of the same element contain the same number of protons and electrons but may have different numbers of neutrons. Since it is the electrons in atoms that affect the chemical properties of a substance, isotopes of the same element have the same chemical properties. Task 2a 1. Do either of the following pairs represent isotopes of one another? (i) (ii) 40 K 19 and 40 Ar Sr 38 and 94 Sr Determine the number of protons, electrons and neutrons in, (i) (ii) 210 Pb S 16 On some periodic tables you will find atomic mass numbers that are not whole numbers. What does this mean? A good starting point is to analyze what it does not mean. For example, the atomic mass of Cl is often quoted on a periodic table as 35.5 and can be represented by the following symbol; 35.5 Cl 17 This does not mean that there are 17 protons, 17 electrons and 18.5 neutrons in an atom of chlorine. It is not possible to have a fraction of a neutron, there can only be a whole number of neutrons in an atom. So what does it mean, and where does the 0.5 come from? Here is the explanation. The non-whole number values that are quoted mean there is more than one isotope of chlorine that exists in nature, in this case 35 Cl 17 and 37 Cl 17. A quick calculation will tell you that these two species have the same number of protons and electrons, but different (whole) numbers of neutrons (18 and 20 respectively). That is, they are isotopes of one another. These isotopes happen to exist naturally in the following abundance; 35 Cl 17 75% and 37 Cl 17 25%. 3

4 A simple calculation can be applied to work out the average atomic mass when considering all the isotopes present in a natural sample. Average atomic mass = (% of each isotope)(atomic mass of 100 each isotope) In this case ((75)(35) + (25)(37)) Average atomic mass = = 35.5 (A non -integer in this case) 100 Another example is provided by Boron. Boron has two isotopes 10 B 5 and 11 B 5. They have the abundance 18.7% and 81.3% respectively. ((18.7)(10) + (81.3)(11)) Average atomic mass = = 18.8 (Again a non - integer) 100 Task 2b 1. The noble gas, Neon has three isotopes of masses 22, 21 and 20. If the isotopes have the abundance 8.01%, 1.99% and 90.00% respectively, what is the average atomic mass of neon atoms? 2. A naturally occurring sample of an element consists of two isotopes, one of mass 85 and one of mass 87. The abundance of these isotopes is 71% and 29%. Calculate the average atomic mass of an atom of this element. 3. If 69 Ga and 71 Ga occur in the % s 62.1 and 37.9, calculate the average atomic mass of gallium atoms. 4. Naturally occurring Chlorine molecules, Cl 2 have masses of 70, 72 and 74. They occur in the percentages 56.25%, 37.50% and 6.250%. What is the average atomic mass of chlorine atoms? What is the relative abundance of 35 Cl and 37 Cl isotopes? 4

5 Molecules and Ions Molecules Molecules are formed when a definite number of atoms are joined together by chemical bonds. A molecule can consist of the atoms of only one element, or the atoms of many different elements but always in a fixed proportion. This means that molecules can be elements or compounds. Molecules are usually formed between non-metal elements. Examples of molecules Substance Element or Compound Description Hydrogen (H 2 ), Oxygen (O 2 ), Nitrogen (N 2 ), Fluorine (F 2 ), Chlorine (Cl 2 ), Iodine (I 2 ), Bromine (Br 2 ) Elements Diatomic Water (H 2 O) Compound Polyatomic Ammonia (NH 3 ) Compound Polyatomic Ions Atoms have equal numbers of protons and electrons and consequently have no overall charge. When atoms lose or gain electrons, the proton:electron numbers are unbalanced causing the particles to become charged. These charged particles are called ions. Positive ions (where the number of protons is greater than the number of electrons) are called cations, and negative ions (where the number of electrons is greater than the number of protons) are called anions. Metals tend to form cations and non-metals tend to form anions. These oppositely charged ions form ionic compounds by attracting one another. An ion made up of only one type of atom is called a monatomic ion; one made up from more than one type of atom is called a polyatomic ion. Examples of ions Substance Cation or Anion Description Sodium ion (Na + ) Cation Monatomic Chloride ion (Cl ) Anion Monatomic Carbonate ion (CO 3 ) Anion Polyatomic Ammonium ion (NH 4 + ) Cation Polyatomic 5

6 Nomenclature of inorganic compounds Binary compounds of metals and non-metals (Ionic compounds) Binary compounds are those formed between two elements. In compounds where one is a metal and one a non-metal an ionic compound is formed. Ionic formulae and names can be determined by considering the charge on the ions. To find the formula of an ionic compound the positive and negative charges must be balanced, i.e. there must be no net charge. To name a binary compound of a metal and a non-metal, the unmodified name of the positive ion is written first followed by the root of the negative ion with the ending modified to -ide. E.g. Sodium Chloride. A few common ions, their charges and formula are listed below. For a longer list see AP Common Ions. Negative ions (ANIONS) Positive ions (CATIONS) Name Charge Formula Name Charge Formula Bromide Chloride Br Cl Aluminum Barium 3+ Al 3+ Ba Fluoride Hydride Iodide F H I Calcium Copper (I) Copper (II) Ca Cu + Cu Nitride Oxide Phosphide Sulfide 3 3 N 3 O P 3 S Hydrogen Iron (II) Iron (III) Lead (II) Lead (IV) Lithium Magnesium Manganese (II) Nickel (II) Potassium Silver Sodium Strontium Tin (II) Tin (IV) Zinc H + Fe Fe 3+ Pb Pb 4+ Li + Mg Mn Ni K + Ag + Na + Sr Sn Sn Zn N.B. Most transition metal ions include a number written as a Roman numeral after the name. Most transition metals have varying charges and this number identifies the particular charge on the ion. Task 2c 1. Name the compounds below (i) NaCl (ii) SrO (iii) AlN (iv) BaCl 2 (v) K 2 O (vi) CuO (vii) Cu 2 O 6

7 Binary acids For the purposes of nomenclature an acid can be defined as a compound that produces hydrogen ions when it is dissolved in water. Binary acids are formed when hydrogen ions combine with monatomic anions. To name a binary acid use the prefix hydro followed by the other non-metal name modified to an ic ending. E.g. HCl, hydrochloric acid. Polyatomic ions and oxyanions Polyatomic ions are those where more than one element are combined together to create a species with a charge. Some of these ions can be named systematically, others names must be learned. Some common polyatomic ions, their charges and formula are listed below. For a complete list see AP Common Ions. Common Polyatomic Ions Name Charge Formula Ammonium + NH 4 Carbonate Chromate (IV) CO 3 CrO 4 Dichromate (VI) Ethanedioate Cr 2 O 7 C 2 O 4 Acetate Hydrogen carbonate Hydrogen sulfate Hydroxide Manganate (VII) (Permanganate) Nitrate Nitrite C 2 H 3 O 2 HCO 3 HSO 4 OH MnO 4 NO 3 NO 2 Phosphate 3 PO 4 3 Sulfate Sulfite SO 4 SO 3 Polyatomic anions where oxygen is combined with another non-metal are called oxyanions and can be named systematically. In these oxyanions certain non-metals (Cl, N, P and S) form a series of oxyanions containing different numbers of oxygen atoms. Their names are related to the number of oxygen atoms present, and are based upon the system below. Name Hypo(element)ite (element)ite (element)ate Per(element)ate Number of oxygen atoms Increase in number of oxygen atoms Where there are only two members in such a series the endings are ite and ate. E.g. Sulfite (SO 3 ) and sulfate (SO 4 ). When there are four members in the series the hypo- and per- prefixes are used additionally. Some oxyanions contain hydrogen and are named accordingly e.g. HPO 4, hydrogen phosphate. The prefix thio- means that a sulfur atom has replaced an atom of oxygen in an anion. 7

8 To name an ionic compound that contains a polyatomic ion the unmodified name of the positive ion is written first followed by unmodified name of the negative ion. E.g. Potassium Carbonate. Oxyacids Oxyacids are formed when hydrogen ions combine with polyatomic oxyanions. This gives a combination of hydrogen, oxygen and another non-metal. Such acids are called oxyacids. To name an oxyacid use the name of the oxyanion and replace the -ite ending with ous or the -ate ending with -ic. Then add the word acid. E.g. Sulfuric Acid. To illustrate the names of these oxoanions and oxoacids consider the following example using chlorine as the non-metal. Formula and name of oxyacid Formula and name of oxyanion HClO Hypochlorous acid ClO Hypochlorite HClO 2 Chlorous acid ClO 2 Chlorite HClO 3 Chloric acid ClO 3 Chlorate HClO 4 Perchloric acid ClO 4 Perchlorate Task 2d 1. What are the formulae for the following ionic compounds? (i) Ammonium nitrate (ii) Copper (II) bromide (iii) Copper (I) bromide (iv) Zinc hydrogensulfate (v) Aluminum sulfate (vi) Sodium perchlorate (vii) Copper (II) iodite 8

9 Binary compounds of two non-metals (Molecules) If the two elements in a binary compound are non-metals then the compound is molecular, i.e. they form a molecule. To name a molecular compound of two non-metals, the unmodified name of the first element is followed by the root of the second element with ending modified to -ide. In order to distinguish between several different compounds with the same elements present use the prefixes mono, di, tri, tetra, penta and hexa to represent one, two, three, four, five and six atoms of the element respectively. E.g. Sulfur Dioxide. Some other examples are given below; Formula BCl 3 CCl 4 CO CO 2 NO NO 2 Name Boron trichloride Carbon tetrachloride Carbon monoxide Carbon dioxide Nitrogen monoxide Nitrogen dioxide Note that the prefix mono is only applied to the second element present in such compounds, if the prefix ends with a or o and the element name begins with a or o then the final vowel of the prefix is omitted, and that some compounds have trivial names that have come to supersede their systematic names, e.g. water not dihydrogen monoxide. Task 2e 1. Write formula or names for the following molecular compounds. (i) Dinitrogen tetroxide (ii) N 2 O 5 (iii) PCl 3 (iv) Phosphorous pentachloride (v) SF 6 9

10 Task 2a 1. (i) No different elements (ii) Yes Same element with different numbers of protons and therefore different masses 2. (i) p = 82, e = 82, n = 128 (ii) p = 16, e = 16, n =18 Task 2b ; 37 Cl = 25%, 35 Cl = 75% Task 2c (i) Sodium chloride (ii) Strontium oxide (iii) Aluminum nitride (iv) Barium chloride (v) Potassium oxide (vi) Copper (II) oxide (vii) Copper (I) oxide Task 2d (i) NH 4 NO 3 (ii) CuBr 2 (iii) CuBr (iv) Zn(HSO 4 ) 2 (v) Al 2 (SO 4 ) 3 (vi) NaClO 4 (vii) Cu(IO 2 ) 2 Task 2e (i) N 2 O 4 (ii) Dinitrogen pentoxide (iii) Phosphorous trichloride (iv) PCl 5 (v) Sulfur hexafluoride 10