AP TOPIC 2: Atoms, Ions & Nomenclature

Transcription

1 AP TOPIC 2: Atoms, Ions & Nomenclature The History of Atomic Theory Circa BC. Greek philosopher Democritus proposes the idea of matter being made up of small, indivisible particles (atomos). Late 18 th Century. Lavoisier proposes the Law of conservation of mass and Proust proposes the Law of constant composition. Early 19 th Century. Using the previously unconnected ideas above, John Dalton formulates his Atomic Theory. (i) (ii) (iii) (iv) (v) Dalton s Atomic Theory Elements are made from tiny particles called atoms. All atoms of a given element are identical. (N.B. see isotopes below). The atoms of a given element are different to those of any other element. 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). 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 19 th and 20 th 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. Thomson Millikan Rutherford, Marsden and Geiger Cathode Ray Tube Cathode Ray Deflection Oil Drop Experiment Gold Foil Experiment Negative particles of some kind exist Mass/charge ratio of the electron determined Charge on the electron Nucleus present in atom Electron Electron Electron The nucleus of an atom and the proton In the first part of the 20 th 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 CHARGE MASS POSITION in atom PROTON 1 1 amu Nucleus NEUTRON 0 1 amu Nucleus ELECTRON 1 amu denotes atomic mass units amu Outside of the nucleus C:\ Page 1 of 13

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 2 H He Li 7 11 Na K Rb Cs Fr Be 9 12 Mg Ca Sr Ba Ra 226 T R A N S I T I O N M E T A L S Sc Ti V Cr Mn Fe Co Ni Cu Zn Y La* Ac** 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 48 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 Ne Ar Kr Xe Rn 222 *Lanthanides **Actinides 58 Ce Th Pr Pa Nd U Pm Np Sm Pu Eu Am Gd Cm Tb Bk Dy Cf Ho Es Er Fm Tm Md Yb No Lu Lr 257 KEY: Metal Semi Metal Nonmetal 13 Al Si P 31 In this example Al is a metal, Si is a semimetal (metalloid) and P is a nonmetal. C:\ Page 2 of 13

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. Consider the following pairs; does either pair represent a pair of isotopes? (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 integers. 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 integer values mean that there is more than one isotope of chlorine that exists in nature, in this case 35 Cl and 37 Cl. 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 75% and 37 Cl 25%. C:\ Page 3 of 13

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 Average atomic mass ((35) (75)) ((37) (25)) 35.5 (A non integer in this case) 100 Another example is provided by Boron. Boron has two isotopes 10 B and 11 B. They have the abundance 18.7% and 81.3% respectively. Task 2b Average atomic mass ((10) (18.7)) ((11) (81.3)) (Again a non integer) 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? C:\ Page 4 of 13

5 Radioactivity Radioactivity is the spontaneous decay of certain atoms with the evolution of alpha, beta and gamma particles. The radiation comes from the nucleus of the atom i.e. it is a nuclear reaction. ALPHA BETA GAMMA NATURE A helium nucleus Essentially electrons High energy and high frequency electromagnetic radiation CHARGE MASS MOVEMENT IN ELECTRIC FIELD Toward negative plate Toward positive plate None PENETRATING POWER Least Intermediate Greatest Radioactive decay reactions Alpha decay (the loss of a Helium nucleus) m X n m4 Y n2 4 2 Beta decay (a neutron splits to give a proton and an electron) m X n m Z n1 0 1 Gamma decay (a rearrangement of the nuclear particles only) m X n m X n (gamma radiation) Other nuclear reactions Positron emission (a positron has the same mass an electron, but has a positive charge) m X n m Z n1 0 1 Electron capture (a captured electron combines with a proton in the nucleus to form a neutron) m X n 0 e 1 m Z n1 C:\ Page 5 of 13

6 Halflife The halflife of a radioactive nucleus is the time taken for half of the atoms to decay. It is independent of the initial quantity of atoms. There are three methods of determining halflife. (i) (ii) Graphically Use of the expression No log ( ) k t N N o = count rate initially N = count rate at time t K = radioactivity constant t = time N If the half life is known then o 2 and therefore N log 2 k t 1/2 t 1/2 = halflife (iii) Use of the expression Fraction of remaining activity 1 n 2 n = number of halflives Task 2c 1. After 21 days a radioactive isotope has only one eighth of its original activity. What is the halflife of the radioactive isotope? 2. An isotope has a halflife of 3.00 min. What fraction of the original quantity remains after? (i) (ii) 30.0 min min 3. The halflife of C 14 is 5585 years. A sample of carbon gave a count rate of Another sample of carbon gave a count rate of Calculate the age of the second sample. 4. In 6.00 hours, the activity of a radioactive compound falls from 999 counts per hour to 634 counts per hour. What is the halflife of the isotope? C:\ Page 6 of 13

7 Transmutation of elements It is possible, via nuclear reactions, to artificially produce elements Rutherford. Alpha particle bombardment. 14 N 7 4 He 2 1 H 1 17 O Cockcroft and Walton. H used as the bombarding particle, it has less charge than the alpha particles and so feels less repulsion from the target nucleus. 7 Li 3 1 H 1 4 He 2 4 He 2 Neutron induced transmutation. Neutrons being neutral feel no repulsion from the positive target nucleus. 31 P 15 1 n 0 32 P 15 gamma 1970 Use of accelerated heavier nuclei. A machine will accelerate the positive nucleus toward the target. 250 Cf B Lw n 0 Mass Deficit When atoms are formed by the combination of protons, neutrons and electrons the mass of the atom is found to be less than that of the sum of the individual particles. This appears to contradict the law of conservation of mass. The explanation for the mass deficit is that when the particles combine a small amount of the mass is converted to energy (binding energy) and released to the surroundings. Predicting stability As a general guideline only, stable (nonradioactive) nuclei tend to have neutron: proton ratios closer to 1:1. Nuclei that have higher neutron: proton ratios tend to want to lower the ratio by converting a neutron to a proton and an electron. The electrons are released as particles. Nuclear and electron rearrangement Radioactivity is a nuclear process that involves rearrangement of the nuclei of atoms, whereas chemical reactions involve the rearrangement of the electrons. Nuclear Fission The process of heavy nuclei capturing neutrons, splitting to form other, smaller nuclei and releasing more neutrons is known as nuclear fission. In the process large amounts of energy can be released, exacerbated by the production of more neutrons each time leading to a potential chain reaction. Nuclear Fusion The combination of smaller nuclei into larger ones with the release of energy is known as nuclear fusion. These reactions are less easy to perform than nuclear fission since they involve the combination of two nuclei that are both positively charged and therefore repel one another. C:\ Page 7 of 13

8 Uses of Radioactivity Some examples of the uses of radiation are listed below 1. Medicine. E.g. 131 I for thyroid and brain imaging, 58 Co for diagnosis of pernicious anaemia and 67 Ga for lung function. 2. Isotopic dating. See questions above. 3. Thickness control in Engineering. A radioactive source is placed on one side of a sheet of paper or metal as it is made and a detector is placed on the other side. Any change in the thickness of the material will be reflected in a change in the observed radioactivity count. 4. Leak detection. Radioactive sources can be injected into pipework and will be detected emerging where the leak is. 5. Nuclear Fission (power and atomic bomb). Uranium nuclei can be bombarded with neutrons and converted to other nuclei. The process occurs with the loss of massive amounts of energy in a chain reaction. These huge amounts of energy can be used in a constructive or destructive way. C:\ Page 8 of 13

9 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 nonmetal elements. Formulae show the number of each type of atom present written as subscripts. The lack of a subscript means only one of that type of atom is present. Examples of molecules Substance and Formula 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 nonmetals 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 Formula/Symbol Cation or Anion Description Sodium ion (Na ) Cation Monatomic Chloride ion (Cl ) Anion Monatomic Carbonate ion (CO 3 2 ) Anion Polyatomic Ammonium ion (NH 4 ) Cation Polyatomic C:\ Page 9 of 13

10 Nomenclature of inorganic compounds Binary compounds of metals and nonmetals (Ionic compounds) Binary compounds are those formed between two elements. In compounds where one is a metal and one a nonmetal 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 nonmetal, 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 formulae are listed below. For a more complete list see Negative ions (ANIONS) Positive ions (CATIONS) Name Charge Symbol Name Charge Symbol Bromide 1 Br Aluminum 3 Al 3 Chloride 1 Cl Barium 2 Ba 2 Fluoride 1 F Calcium 2 Ca 2 Hydride 1 H Copper (I) 1 Cu Iodide 1 I Copper (II) 2 Cu 2 Nitride 3 N 3 Hydrogen 1 H Oxide 2 O 2 Iron (II) 2 Fe 2 Phosphide 3 P 3 Iron (III) 3 Fe 3 Sulfide 2 S 2 Lead (II) 2 Pb 2 Lead (IV) 4 Pb 4 Lithium 1 Li Magnesium 2 Mg 2 Manganese (II) 2 Mn 2 Nickel (II) 2 Ni 2 Potassium 1 K Silver 1 Ag Sodium 1 Na Strontium 2 Sr 2 Tin (II) 2 Sn 2 Tin (IV) 4 Sn 4 Zinc 2 Zn 2 N.B. Most transition metal ions and a few other metal ions, include a number written as a Roman numeral after the name. These metals form ions with varying charges and this numeral identifies the charge in each case. Task 2d 1. Name the compounds below (i) NaCl (ii) SrO (iii) AlN (iv) BaCl 2 (v) K 2 O (vi) CuO (vii) Cu 2 O C:\ Page 10 of 13

11 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 nonmetal name modified to an ic ending. E.g. Hydrochloric acid. Polyatomic ions and oxoanions 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 formulae are listed below. For a more complete list see Common Polyatomic ions Name Charge Formula NH 4 Ammonium 1 Carbonate 2 Chromate (VI) 2 2 CO 3 2 CrO 4 Dichromate (VI) 2 2 Cr 2 O 7 Ethanedioate 2 Hydrogen carbonate 1 2 C 2 O 4 HCO 3 Hydrogen sulfate 1 HSO 4 Hydroxide 1 OH Manganate (VII) (Permanganate) 1 MnO 4 Nitrate 1 NO 3 Nitrite 1 NO 2 3 Phosphate 3 PO 4 2 Sulfate 2 SO 4 Sulfite 2 2 SO 3 Polyatomic anions where oxygen is combined with another nonmetal are called oxoanions and can be named systematically. In these oxoanions certain nonmetals (Cl, N, P and S) form a series of oxoanions 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 2 ) and sulfate (SO 4 2 ). When there are four members in the series the hypo and perprefixes are used additionally. Some oxoanions contain hydrogen and are named accordingly e.g. HPO 4 2, hydrogen phosphate. The prefix thio means that a sulfur atom has replaced an atom of oxygen in an anion. 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. C:\ Page 11 of 13

12 Oxoacids Oxoacids are formed when hydrogen ions combine with polyatomic oxoanions. This gives a combination of hydrogen, oxygen and another nonmetal. Such acids are called oxoacids. To name an oxoacid use the name of the oxoanion 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 nonmetal. Task 2e Formula and name of oxoacid Formula and name of oxoanion 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 1. What are the formulae for the following ionic compounds? (i) (ii) (iii) (iv) (v) (vi) (vii) Ammonium nitrate Copper (II) bromide Copper (I) bromide Zinc hydrogensulfate Aluminum sulfate Sodium perchlorate Copper (II) iodite C:\ Page 12 of 13

13 Binary compounds of two nonmetals (Molecules) If the two elements in a binary compound are nonmetals, then the compound is molecular, i.e. they form a molecule. To name a molecular compound of two nonmetals, 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 supercede their systematic names, e.g. water not dihydrogen monoxide. Task 2f 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 Hydrates Hydrates are ionic formula units with water associated with them. The water molecules are incorporated into the solid structure of the ions. To name a hydrate use the normal name of the ionic compound followed by the term hydrate with an appropriate prefix to show the number of water molecules per ionic formula unit. E.g. Copper(II) sulfate pentahydrate. Strong heating can generally drive off the water in these salts. Once the water has been removed the salts are said to be anhydrous (without water). C:\ Page 13 of 13