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Atoms and Elements

Atoms and Elements
Chapter 4 Atoms and Elements www.ThesisScientist.comExperiencing Atoms • Atoms are incredibly small • All of our senses, sense atoms www.ThesisScientist.comExperiencing Atoms • There are about 91 elements found in nature. Over 20 have been made in laboratories. • Each kind of atom is unique Carbon is not Hydrogen They have different properties Structure, magnetic meaning they can attract and repel other atoms, melting, boiling, electrical, stability, reactivity (attract and repel), etc… www.ThesisScientist.comThe Divisibility of Matter • Ultimate particle Upon division, eventually a particle is reached which can no longer be divided. 11 Atoms are 10 m 15 We detect particles at 10 m 35 In theory particles 10 m exist, we don’t have instruments that sensitive “Nothing exists except atoms and empty space; everything else is opinion.” Democritus 460–370 B.C. www.ThesisScientist.comModern Evidence for Atoms IBM's Almaden Research Center in San Jose, California, April 1990 www.ThesisScientist.comSizes of Atoms • Using compositions of compounds and assumed formulas, Dalton was able to determine the relative masses of the atoms. Dalton based his scale on H = 1 amu. We now base it on C12 = 12 amu exactly. Unit = atomic mass unit. Amu or dalton. • Absolute sizes of atoms: 24 Mass of H atom= 1.67 x 10 g. 25 3 Volume of H atom = 2.1 x 10 cm . www.ThesisScientist.comThe Atom Is Divisible • Work done by J. J. Thomson and others proved that the atom had pieces called electrons. • Thomson found that electrons are much smaller than atoms and carry a negative charge. th The mass of the electron is 1/1836 the mass of a hydrogen atom. The charge on the electron is the fundamental unit of charge that we call –1 charge unit. www.ThesisScientist.comPlum Pudding Atom • • • • •• • •• •• • • • •• • • • • • • www.ThesisScientist.comRutherford’s Experiment • How can you prove something is empty • Put something through it. Use large target atoms. Use very thin sheets of target so they do not absorb ―bullet‖. Use very small particles as ―bullet‖ with very high energy. But not so small that electrons will effect it. • Bullet = alpha particles; target atoms = gold foil a particles have a mass of 4 amu charge of +2 c.u.  Gold has a mass of 197 amu and is very malleable. www.ThesisScientist.comRutherford’s Experiment Alpha particles striking screen Radioactive sample Lead box Fluorescent screen Gold foil www.ThesisScientist.comPlum Pudding Atom • • If atom was like • • •• • a plum pudding, •• •• • all the a particles • • •• • should go • • • • straight through. • Very few of the a particles Nuclear Atom do not go through. . Most a particles go straight through. . . Some a particles go through, but are deflected. www.ThesisScientist.comRutherford’s Interpretation— The Nuclear Model 1. The atom contains a tiny dense center called the nucleus.  The amount of space taken by the nucleus is only about 1/10 trillionth the volume of the atom. 2. The nucleus has essentially the entire mass of the atom.  The electrons weigh so little they contribute practically no mass to the atom. 3. The nucleus is positively charged.  The amount of positive charge balances the negative charge of the electrons. 4. The electrons are dispersed in the empty space of the atom surrounding the nucleus.  Like water droplets in a cloud. www.ThesisScientist.comSome Problems • How could beryllium have 4 protons stuck together in the nucleus Shouldn’t they repel each other • If a beryllium atom has 4 protons, then it should weigh 4 amu, but it actually weighs 9.01 amu Where is the extra mass coming from Each proton weighs 1 amu. Remember: The electron’s mass is only about 0.00055 amu and Be has only 4 electrons—it can’t account for the extra 5 amu of mass. www.ThesisScientist.comThere Must Be Something Else There • To answer these questions, Rutherford proposed that there was another particle in the nucleus—it is called a neutron. • Neutrons have no charge and a mass of 1 amu. The masses of the proton and neutron are both approximately 1 amu. www.ThesisScientist.comSubatomic Mass Mass Location Charge Symbol particle g amu in atom + + Proton 1.67262 1.0073 nucleus 1+ p, p , H 24 x 10 Electron 0.00091 0.00055 empty space e, e 1 24 x 10 0 Neutron 1.67493 1.0087 nucleus 0 n, n 24 x 10 www.ThesisScientist.comThe Modern Atom • We know atoms are composed of three main pieces—protons, neutrons, and electrons. • The nucleus contains protons and neutrons. 13 • The nucleus is only about 10 cm in diameter. • The electrons move outside the nucleus with an average distance 8 of about 10 cm.  Therefore, the radius of the atom is 5 about 10 times larger than the radius of the nucleus. www.ThesisScientist.comSome Notes on Charges • There are two kinds of charges, called positive and negative. • Opposite charges attract.  + attracted to –. • Like charges repel.  + repels +.  – repels –. • To be neutral, something must have no charge or equal amounts of opposite charges. www.ThesisScientist.comThe Nature of Electrical Charge • Electrical charge is a fundamental property of protons and electrons. • Positively and negatively charged objects attract each other. • Like charged objects repel each other. + to +, or  to . • When a proton and electron are paired, the result is a neutral charge. Because they have equal amounts of charge. www.ThesisScientist.comPractice—An Atom Has 20 Protons. Determine if Each of the Following Statements Is True or False • If it is a neutral atom, it will have True 20 electrons. • If it also has 20 neutrons, its mass True will be approximately 40 amu. • If it has 18 electrons, it will have False a net 2 charge. www.ThesisScientist.comElements • Each element has a unique number of protons in its nucleus.  All carbon atoms have 6 protons in their nuclei. • The number of protons in the nucleus of an atom is called the atomic number.  Z is the shorthand designation for the atomic number.  Because each element’s atoms have a unique number of protons, each element can be identified by its atomic number.  The elements are arranged on the Periodic Table in order of their atomic numbers. • Each element has a unique name and symbol.  The symbol is either one or two letters One capital letter or one capital letter + one lower case letter. www.ThesisScientist.comThe Periodic Table of Elements Atomic number Element symbol Atomic mass www.ThesisScientist.comReview • What is the atomic number of boron, B 5 • What is the atomic mass of silicon, Si 28.09 amu • How many protons does a chlorine atom have 17 • How many electrons does a neutral neon atom have 10 • Will an atom with 6 protons, 6 neutrons and 6 electrons be electrically neutral Yes • Will an atom with 27 protons, 32 neutrons, and 27 electrons be electrically neutral Yes • Will an Na atom with 10 electrons be electrically neutral No www.ThesisScientist.comMendeleev • Ordered elements by atomic mass. • Saw a repeating pattern of properties. • Periodic law—When the elements are arranged in order of increasing relative mass, certain sets of properties recur periodically • Used pattern to predict properties of undiscovered elements. • Where atomic mass order did not fit other properties, he reordered by other properties.  Te I www.ThesisScientist.comPeriodic Pattern nm H O 2 a/b H 1 H 2 m Li O m/nm BeOnm B O nm CO nm N O nm O nm 2 2 3 2 2 5 2 b a/b a a a Li Be B C N O F 7 LiH 9 BeH 11 ( BH ) 12 CH 14 NH 16 H O 19 HF 2 3 n 4 3 2 m Na O m MgOm Al O nm/m SiO nm P O nm SO nm Cl O 2 2 3 2 4 10 3 2 7 b b a/b a a a a Na Mg Al Si P S Cl 23 NaH24 MgH 27 (AlH )28 SiH 31 PH 32 H S 35.5 HCl 2 3 4 3 2 m = metal, nm = nonmetal, m/nm = metalloid a = acidic oxide, b = basic oxide, a/b = amphoteric oxide www.ThesisScientist.comMendeleev's Predictions for Ekasilicon (Germanium) Property Silicon’s Tin’s Predicted Measured props props value value Atomic 28 118 72 72.6 mass Color Gray White Gray Gray metal white Density 2.32 7.28 5.5 5.4 Reaction Resists Reacts Resists Resists with acid acid, acid, both both and base reacts resists base base Oxide SiO SnO Eks O GeO 2 2 1 2 2 www.ThesisScientist.comPeriodicity = Metal = Metalloid = Nonmetal www.ThesisScientist.comMetals • Solids at room temperature, except Hg. • Reflective surface.  Shiny • Conduct heat. • Conduct electricity. • Malleable. Can be shaped. • Ductile.  Drawn or pulled into wires. • Lose electrons and form cations in reactions. • About 75 of the elements are metals. • Lower left on the table. www.ThesisScientist.comNonmetals • Found in all 3 states. • Poor conductors of heat. • Poor conductors of electricity. • Solids are brittle. • Gain electrons in reactions to become anions. • Upper right on the table. Except H. www.ThesisScientist.comMetalloids • Show some properties of metals and some of nonmetals. • Also known as Properties of Silicon: semiconductors. Shiny Conducts electricity Does not conduct heat well Brittle www.ThesisScientist.comPractice—Classify Each Element as Metal, Nonmetal, or Metalloid. • Xenon, Xe Nonmetal Metal • Tungsten, W • Bromine, Br Nonmetal • Arsenic, As Metalloid Metal • Cerium, Ce www.ThesisScientist.comThe Modern Periodic Table • Elements with similar chemical and physical properties are in the same column. • Columns are called Groups or Families. Designated by a number and letter at top. • Rows are called Periods. • Each period shows the pattern of properties repeated in the next period. www.ThesisScientist.comThe Modern Periodic Table, Continued • Main group = representative elements = ―A‖ groups. • Transition elements = ―B‖ groups. All metals. • Bottom rows = inner transition elements = rare earth elements. Metals Really belong in periods 6 and 7. www.ThesisScientist.com= Alkali metals = Halogens = Alkali earth metals = Lanthanides = Noble gases = Actinides = Transition metals www.ThesisScientist.comImportant Groups—Hydrogen • Nonmetal. • Colorless, diatomic gas.  Very low melting point and density. • Reacts with nonmetals to form molecular compounds.  HCl is an acidic gas.  H O is a liquid. 2 • Reacts with metals to form hydrides.  Metal hydrides react with water to form H . 2 • hydrogen halides dissolve in water to form acids. www.ThesisScientist.comImportant Groups— Alkali Metals • Group IA = Alkali metals. • Hydrogen is usually placed here, lithium though it doesn’t belong. • Soft, low melting points, low density. • Flame tests: Li = red, Na = yellow, and sodium K = violet. • Very reactive, never found uncombined in nature. potassium • Tend to form water soluble compounds that are crystallized from seawater then molten salt electrolyzed. rubidium Colorless solutions. • React with water to form basic (alkaline) solutions and H : 2 cesium 2 Na + 2 H O  2 NaOH + H 2 2 Releases a lot of heat. www.ThesisScientist.comImportant Groups—Alkali Earth Metals • Group IIA = Alkali earth metals. • Harder, higher melting, and denser than alkali metals.  Mg alloys used as structural materials. beryllium • Flame tests: Ca = red, Sr = red, and Ba = yellowgreen. magnesium • Reactive, but less than corresponding alkali metal. calcium • Form stable, insoluble oxides from strontium which they are normally extracted. • Oxides are basic = alkaline earth. barium • Reactivity with water to form H : 2 Be = none, Mg = steam, Ca, Sr, Ba = cold water. www.ThesisScientist.comImportant Groups—Halogens • Group VIIA = Halogens. • Nonmetals. • F and Cl gases, Br liquid, and 2 2 2 I solid. 2 fluorine • All diatomic. • Very reactive. chlorine • Cl and Br react slowly with 2, 2 water: bromine Br + H O  HBr + HOBr 2 2 • React with metals to form ionic iodine compounds. • hydrogen halides all acids:  HF weak HCl HBr HI. www.ThesisScientist.comImportant Groups—Noble Gases • Group VIIIA = Noble gases. • All gases at room temperature.  Very low melting and boiling points. • Very unreactive, practically inert. • Very hard to remove electron from or give an electron to. www.ThesisScientist.comImportant Groups—Noble Gases • This unreactive and • The difficulty to remove and electron from Noble Gases has to do with the number of electrons • From a Quantum point of view these elements have all the electrons they need. • Which means, all the other atoms do NOT have all the electrons they need…want. www.ThesisScientist.comAtom Happiness • The number of proton determines the physical and chemical properties of an atom • When the protons=electrons atoms are electrically neutral • But Atom Happiness only comes about when the atom has the same number of Electrons as a noble gas www.ThesisScientist.comAtom Happiness • Atoms will not gain and loose protons because protons are at the center of atoms, very far away from outside, in an electrical shroud of negative charge • Atoms will gain and loose electrons, which are on the outside surface of atoms • When an atom gains or looses an electron the electrical balance is lost • But, atoms are happier with a charge www.ThesisScientist.comIons • Ions with a positive charge are called cations.  More protons than electrons.  Form by losing electrons. • Ions with a negative charge are called anions.  More electrons than protons.  Form by gaining electrons. • Chemically, ions are much different than the neutral atoms.  Because they have a different structure. www.ThesisScientist.comAtomic Structures of Ions • Nonmetals form anions. • For each negative charge, the ion has 1 more electron than the neutral atom. + ─ +  F = 9 p and 9 e ; F = 9 p and 10 e . + 3─ +  P = 15 p and 15 e ; P = 15 p and 18 e . • Anions are named by changing the ending of the name to –ide.  ─ fluorine F + 1e F fluoride ion  2─ oxygen O + 2e O oxide ion • The charge on an anion can often be determined from the group number on the periodic table.  Group 7A  1, Group 6A 2. www.ThesisScientist.comAtomic Structures of Ions, Continued • Metals form cations. • For each positive charge the ion has 1 less electron than the neutral atom. +  + +   Na atom = 11 p and 11 e ; Na ion = 11 p and 10 e . +  2+ +   Ca atom = 20 p and 20 e ; Ca ion = 20 p and 18 e . • Cations are named the same as the metal. +  sodium Na  Na + 1e sodium ion 2+  calcium Ca  Ca + 2e calcium ion • The charge on a cation can often be determined from the group number on the periodic table.  Group 1A  1+, Group 2A 2+, (Al, Ga, In)  3+. www.ThesisScientist.comExample 4.5—Find the Number of Protons 2+ and Electrons in Ca . 2+ Given: Ca + 0 Find: p , e , n atomic + Solution Map: symbol p e number + − Relationships: ion charge = p − e + − Solution: + ion charge = p − e Z = 20 = p − +2 = 20 − e − ─18 = ─ e − 18 = e + − www.ThesisScientist.com Check: For cations, p e , so the answer is reasonable.Practice—Fill in the Table. + Ion p e 1 Cl +1 K 2 S +2 Sr www.ThesisScientist.comPractice—Fill in the Table, Continued. + Ion p e 1 Cl 17 18 +1 K 19 18 2 S 16 18 +2 Sr 38 36 www.ThesisScientist.comValence Electrons and Ion Charge • The highest energy electrons in an atom are called the valence electrons. • Metals form cations by losing their valence electrons to get the same number of electrons as the previous noble gas.  Main group metals. + 3+  Li = 2 e = He; Al = 10 e = Ne. • Nonmetals form anions by gaining electrons to have the same number of electrons as the next noble gas.  2  Cl = 18 e = Ar; Se = 36 e = Kr. www.ThesisScientist.comIon Charge and the Periodic Table • The charge on an ion can often be determined from an elements position on the periodic table. • Metals are always positive ions, nonmetals are negative ions. • For many main group metals, the cation charge = the group number. • For nonmetals, the anion charge = the group number – 8. www.ThesisScientist.com1A 2A 3A 5A 6A 7A + Li 2+ 3 2 Be N O F 3+ 2+ + 3 2 Mg Al Na P S Cl 3 + 2+ 3+ As 2 K Ca Ga Se Br + 2+ 3+ 2 Rb Sr In Te I + 2+ Cs Ba www.ThesisScientist.comStructure of the Nucleus • Soddy discovered that the same element could have atoms with different masses, which he called isotopes. There are two isotopes of chlorine found in nature, one that has a mass of about 35 amu and another that weighs about 37 amu. • The observed mass is a weighted average of the weights of all the naturally occurring atoms. The atomic mass of chlorine is 35.45 amu. www.ThesisScientist.comIsotopes • All isotopes of an element are chemically identical. Undergo the exact same chemical reactions. • All isotopes of an element have the same number of protons. • Isotopes of an element have different masses. • Isotopes of an element have different numbers of neutrons. • Isotopes are identified by their mass numbers. Protons + neutrons. www.ThesisScientist.comIsotopes, Continued • Atomic Number.  Number of protons.  Z • Mass Number  = Protons + Neutrons.  Whole number.  A  Percent natural abundance = Relative amount found in a sample. www.ThesisScientist.comNeon Percent Number of Number of A, mass natural Symbol protons neutrons number abundance 20 Ne20 or 10 10 20 90.48 Ne 10 21 Ne21 or 10 11 21 0.27 Ne 10 22 Ne22 or 10 12 22 9.25 Ne 10 www.ThesisScientist.comIsotopes • Cl35 makes up about 75 of chlorine atoms in nature, and Cl37 makes up the remaining 25. • The average atomic mass of Cl is 35.45 amu. • Cl35 has a mass number = 35, 17 protons and 18 neutrons (35 17). 35 Cl 17 Atomic symbol A X = XA A = Mass number Z Z = Atomic number www.ThesisScientist.comExample 4.8—How Many Protons and 52 Cr Neutrons Are in an Atom of 24 52 Given: therefore A = 52, Z = 24 Cr 24 Find: + 0 p and n Solution Map: atomic mass 0 symbol n numbers + 0 mass number = p + n Relationships: Solution: + 0 Z = 24 = p A = Z + n 0 52 = 24 + n 0 28 = n 0 + www.ThesisScientist.com Check: For most stable isotopes, n p .Practice—Complete the Following Table. Atomic Mass Number Number Number Number Number of of of protons electrons neutrons Calcium40 Carbon13 +3 Aluminum27 www.ThesisScientist.comPractice—Complete the Following Table, Continued. Atomic Mass Number Number Number Number Number of of of protons electrons neutrons Calcium40 20 40 20 20 20 Carbon13 6 13 6 6 7 +3 Aluminum27 13 27 13 10 14 www.ThesisScientist.comMass Number Is Not the Same as Atomic Mass • The atomic mass is an experimental number determined from all naturally occurring isotopes. • The mass number refers to the number of protons + neutrons in one isotope. Natural or manmade. www.ThesisScientist.comExample 4.9─Ga69 with Mass 68.9256 Amu and Abundance of 60.11 and Ga71 with Mass 70.9247 Amu and Abundance of 39.89. Calculate the Atomic Mass of Gallium. Given: Ga69 = 60.11, 68.9256 amu Cu71 = 39.89, 70.9247 amu Find: atomic mass, amu Solution Map: isotope masses, avg. atomic mass isotope fractions Relationships: Atomic Mass  fractional abundance of isotopemass of isotope  n n Solution: Atomic Mass  0.601168.9256 amu 0.398970.9247 amu Atomic Mass  63.723041 69.72 amu Check: The average is between the two masses, closer to the major isotope. www.ThesisScientist.comPractice—If Copper Is 69.17 Cu63 with a Mass of 62.9396 Amu and the Rest Cu65 with a Mass of 64.9278 Amu, Find Copper’s Atomic Mass. www.ThesisScientist.comPractice—If Copper Is 69.17 Cu63 with a Mass of 62.9396 Amu and the Rest Cu65 with a Mass of 64.9278 Amu, Find Copper’s Atomic Mass, Continued. Given: Cu63 = 69.17, 62.9396 amu Cu65 = 10069.17, 64.9278 amu Find: atomic mass, amu Solution Map: isotope masses, avg. atomic mass isotope fractions Relationships: Atomic Mass  fractional abundance of isotopemass of isotope  n n Solution: Atomic Mass  0.691762.9396 amu 0.308364.9278 amu Atomic Mass  63.5525 63.55 amu Check: The average is between the two masses, closer to the major isotope. www.ThesisScientist.com
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