Lab Tomorrow. LabQuest Emulator.

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1 Lab Tomorrow LabQuest Emulator 1

2 Chapter 2 Elements and Compounds

3 Elements and compounds Mystical signs from other worlds Lunar Eclipse in 1997 with Comet Hale-Bopp 3

4 Questions for this chapter Where did the idea of the atom originate? What are the fundamental units of elements and compounds? What makes a compound unique? Are all atoms of an element exactly the same? What determines the mass of an atom? 4

5 OB JEC TIV ES Section You Should be able to Examples Exercises 2-1 Distinguish between elements and compounds. 1a, 1b, 1e, 1g Match the names and symbols of common elements. Chapter Problems 31, 32, 34 1c, 1d, 1f 1, 5, 6, List the postulates of the atomic theory. 2a, 2b, 2c List the components of an atom, and their relative masses, charges, and location in the atom. 3a, 3b, 3c, 3d 9, Define the terms atomic number, mass number, and isotope. Using the table of elements, determine the number of protons, neutrons, and electrons in any isotope of an element. Demonstrate how the atomic mass of an element is determined from isotopes and their percent abundance. 4a, 4f 14, 18, b, 4c 11, 13, d, 4e, 4g 26, 27, and 2-6 Describe the difference between a molecular and an ionic compound. 44, 45, 56, Write the formulas of simple ionic compounds given the charges on the ions. 2-3, 2-4 6a, 6d, 6e 39, 41, 43, 46, 50 5

6 Elemental Distribution (some facts from text) There are 114 known elements (26 spontaneously decay into other elements) (or maybe more?) The Earth has 88 elements present in measurable amounts, but 10 constitute >99% of the crust (oxygen, silicon, aluminum, iron, calcium, sodium, potassium, magnesium, hydrogen and titanium (carbon ~ 0.08%!)) img next slide The human body is 93% carbon, hydrogen and oxygen 6

7 Names and Symbols of the Elements Symbol - usually the first or second letters of the name for two letter symbols, the first letter is capitalized but the second is not two letters are used for most elements When the name (e.g. gold) and symbol (e.g. Au) use different letters, the element was known for ages (think alchemy.) The Latin name was used for the basis of the symbol. One exception: tungsten, which uses the German name (Wolfram) 7

8 Modern Chemistry starts with Dalton s atomic theory Matter is composed of small indivisible particles called atoms All atoms of the same element are identical and have the same properties Chemical compounds are composed of atoms of different elements combined in small wholenumber ratios Chemical reactions are merely the rearrangement of atoms into different combinations 8

9 Proof of Dalton s Theory? Overwhelming amounts of individual experimental results Scanning Tunnelling Microscopy m.html Probe tip Experiment Results Xenon on Silicon (IBM) Language independent Corrals and cages Now single molecules! 9

10 Evolution of Dalton s Atom 10

11 Molecules and compounds Atoms are the smallest fundamental particle of an element that has the properties of that element Molecules - formed by the chemical combination of two or more atoms Molecular compounds - molecules of atoms of two different elements Covalent bonds - the force holding the atoms together in a molecular compound (C-Cl, vs NaCl) 11

12 Formulas A compound is represented by using the symbols for the elements of which it is composed (CO, NO, NN, etc.) Subscripts are used to indicate how many atoms of a particular element exist in the compound (CO 2, NO 2, N 2 O, N 2, etc.) If there is only one atom of a particular element, the one is assumed 12

13 Formulas, con t Note that changing the subscripts changes the compound consider H 2 O and H 2 O 2 ; NO 2 and N 2 O Oh,oh - Two different compounds can share the same chemical formula dimethyl ether and ethyl alcohol both have the formula C 2 H 6 O (next slide) 13

14 Ethyl alcohol and dimethyl ether (geometric isomers) H H H H H C C O H H C O C H H H H H Ethyl alcohol on the left; dimethyl ether on the right These species are termed geometric isomers Formulas that show the order and arrangement of specific atoms are known as structural formulas 14

15 Elements as we find them (on earth and in periodic table) Default elements exist as individual atoms - Know exceptions Hydrogen, fluorine, chlorine, bromine, iodine and oxygen exist as diatomic molecules (H 2, F 2, Cl 2, Br 2, I 2, O 2 ) Phosphorus exists as a tetratomic molecule, P 4 Some elements exist in a variety of forms (these are called allotropes) Carbon: graphite; diamond; buckminsterfullerine Phosphorus - red and white Sulfur - S 6, S 4 and S 2 15

16 Elementary particles = billiard balls Neutron, no character or properties besides mass. Proton, same mass and size, but with character! Carries positive charge. Electron is even smaller than a marble but also colorful and has character! Carries negative charge. 16

17 Composition of the atom Atoms are composed of three subatomic particles proton - particle with a +1 charge; p neutron - particle with no charge; n electron - particle with a -1 charge; e - The proton and the neutron have roughly the same mass (ca 1 amu or g) The mass of the electron is substantially smaller than the masses of the proton or the neutron 17

18 Size of atom If an atom were the size of a football field, the nucleus would be a fly at the 50 yard line. Or if we are nucleus electron is in Pullman 18

19 Isotopes Most elements occur as a mixture of isotopes The specific isotope is written using isotopic notation This isotope will be referred to by the mass number e.g. copper-63 (although it is not the exact mass!) mass number atomic number 19

20 Atomic number and mass number The protons and the neutrons of the atom are found in the nucleus (and are therefore termed nucleons) The electrons are found outside of the nucleus Atomic number, Z, is the number of protons in the nucleus (and also the number of electrons for a neutral) The mass number is the total number of nucleons = protons + neutrons (amu not g!) 20

21 Isotopes Atoms may have the same atomic number but different mass numbers (meaning the same number of protons and electrons but different numbers of neutrons) The atomic number determines the identity of the element Atoms of the same element with differing mass numbers are termed isotopes Problem 2-11,

22 Counting particles How many protons? Atomic number = protons and electrons 29 = protons; 29 = electrons How many neutrons? Mass number (total nucleons) atomic number (number of protons) = neutrons = 34 = number of neutrons 22

23 Let s talk isotopic mass mass of an isotope compared to a standard the accepted standard is 12 C, which is defined as having a mass unit of exactly 12 amu an amu is defined as one-twelfth the mass of an atom of 12 C 23

24 atomic mass of real elements the atomic mass of an element listed in the periodic table is the weighted average of the atomic masses of all isotopes present in nature Natural abundance 24

25 Calculating atomic masses for boron 2 isotopes, 19.9% occurs as 10 B and 80.1% occurs as 11 B. The isotopic mass of 10 B is and 11 B is Weighted average [(%/100) x mass 1] + [(%/100) x mass 2] = 10 B amu = 1.99 amu 11 B amu = 8.82 amu Atomic mass of boron = amu Problem

26 Electrical nature of matter Charged atoms or molecules are known as ions cations - positively charged anions - negatively charged + + e - + e

27 How charged species arise Neutral atoms and molecules have the same number of protons and electrons Cations LOSE electrons so have more protons than electrons + + e - Anions GAIN electrons so have more electrons than protons + e

28 Counting protons and electrons 1. Consider K +. How many protons? How many electrons? Atomic number for K = 19, therefore 19 protons. NEUTRAL K must have 19 electrons, but K + has positive charge. Must have 1 more proton than electrons, so K + has 18 electrons. 2. Consider P 3-. How many protons? How many electrons? Atomic number of P =, so 15 protons. Charge is 3- so 3 more electrons than protons. P 3- has 18 electrons. 28

29 Chemical Ionic Formula Formula unit - simplest whole-number ratio of ions in an ionic compound to balance charges. For example: Ca 2+ + Br - resulting chemical formula must be electrically neutral so two Br - are needed for each Ca 2+ the resulting formula is CaBr 2 (other examples) 29

30 Ionic compounds and ionic lattice structure Consist of giant lattice of repeating structure Cl - Na + Cl - Na + Another graphic of NaCl Table Salt 2 dimensional representation 30

31 Polyatomic ions Molecular cations or anions consisting of groups of atoms that are covalently bonded to each other examples are NO 3-, SO 4 2- ClO 3 - when more than one appears in a formula unit, the polyatomic ion is put in between parentheses, and a subscript is used to indication the number of the ions that appear in the formula unit - example: Ba(ClO 3 ) 2 Heads up Table 4.2 will need to know, not yet 31