Chemical Foundations: Elements, Atoms & Ions

Intro  Since ancient times, humans have tried to harness chemical rxns for their benefit Prior to 1000 B.C. ○ Metals for ornaments & tools ○ Embalming fluids 400 B.C. Greeks proposed 4 elements – fire, earth, water & air Alchemy – some tried to make gold; others developed many lab methods & equip, and isolated many elements Robert Boyle – insisted that science should be grounded firmly in experiments (best known for work w/ gases) ○ Defined an element – any substance that couldn’t be broken down into simpler substances

The Elements  All matter can be broken down chemically into ~100 elements Single atoms Macroscopic samples Combined w/ other elements Diatomic molecules (two atoms of same element) Allotropes – diff forms of an element ○ Ex. Carbon: diamond, graphite (pencil lead), & buckminsterfullerene (C 60 ) – p106 fig. 4.18

4.3 Dalton’s Atomic Theory  18 th century truths Most natural mat’ls are mixture or pure substances Pure substances are either elements or cmpds Law of constant composition – a given cmpd always contains the same proportions, by mass, of its elements – in any form - always

 1808 – John Dalton offered an explanation (Dalton’s Atomic Theory) Elements are made of tiny particles called atoms All atoms of a given element are identical Atoms of a given element are different from those of any other element Atoms of different elements can combine to form compounds. Compounds always have the same relative numbers & types of atoms Atoms are indivisible; they cannot be created nor destroyed in chem rxns – they get rearranged or re-grouped No experiments were conducted

4.4 Formulas of Compounds  Compound – composed of atoms of 2+ elements always in the exact same ratio H2OH2O No subscript for only 1 atom The # of each atom is indicated by a subscript to the right of the symbol Each atom is represented by a symbol

4.5 The Structure of the Atom  1 st evidence of parts of an atom – 1890’s  J.J. Thomson Showed that atoms can be made to emit tiny negative particles (they repelled a negative electric field)  the electron! Whole atoms are neutral – concluded that atoms must contain positively charged particles too  Lord Kelvin (William Thomson) Coined the term “plum-pudding model” for JJ Thomson’s new idea of the atom

 1911 – Ernest Rutherford Worked with alpha (  ) particles (7500x’s the weight of electrons) ○ Some particles were deflected by something in the air ○ Gold-foil experiment Hypothesized particles would go straight through Some were surprisingly, deflected at large angles Concluded that the deflections could only occur by a center of positive charge Nuclear atom – an atom w/ a dense, positive charge – the nucleus 1919 – concluded the + charge came from positively charged protons ○ e - = -1 ; p + = +1 ○ 1932 w/ James Chadwick, showed that nuclei have neutrally charged neutron particles also (n 0 = 0)

4.6 introduction to the Modern Concept of Atomic Structure The nuclear atom: Nucleus cm cm Atoms differ w/ diff #’s of e - & p + but the # of e - determines the chemical behavior of an atom; the # of p + identifies the element of an atom

4.7 Isotopes  With Chadwick’s discovery of the neutron, Dalton’s atomic theory had to be modified Atoms of an element have identical #’s of e - & p +, but atoms of an element may have different #’s of n 0  isotopes!!!  Atomic number (Z) - # of p +  Mass number (A)– sum of p + & n 0  Isotopic symbol: A Z X How do we calculate the # n 0 ?

4.10 Ions  Atoms are neutral (zero net charge)  Ions – atoms that have gained/lost e - Ex. Na  Na + + e - Ex. O + 2 e -  O -2  Ion charges & the P.T. Groups 1,2,& 3 correspond to # e - lost Groups 15, 16 & 17 correspond to # e - needed to be like noble gas

4.11 cmpds that contain ions  Cmpds must have overall net zero chg Many cmpds are ionic – made of cations & anions The total + chg must = total – chg ○ Ex. Na + + Cl -  NaCl ○ Ex. Mg +2 + Cl -  MgCl 2 Mg +2 + Cl - Cl -  MgCl 2 Cmpds have diff properties than elements ○ Ionic cmpds are crystalline, dissolve in water, conduct electricity when in water (ions sep)

4.8 Intro to the P.T.  Elements are arranged in order of inc. atomic number  Dmitri Mendeleev (Russian) arranged elements in specific rows (periods) & columns (groups) according to similarities in chem prop’s of various “families”

 Main-Group Elements Groups 1,2,13-18 (or 3-8) S- & p-blocks of P.T.  Alkali Metals Group 1 elements Metals, solid Lose 1 e - React w/ H 2 O to form alkaline solns Highly reactive Always found as compounds  Alkaline-Earth Metals Group 2 elements Metals, solid Lose 2 e - Highly reactive (slightly less than Alkali metals) Usu. found as compounds  Halogens (Greek,“salt maker”) Group 17 (7) elements Nonmetals, gas, liquid or solid Most reactive nonmetals Gain 1 e - React w/ most metals to produce salts (love Alkali metals)

 Noble Gases Group 18 (8) elements Non-metals, gases Full set of e - in highest energy level Low reactivity (not completely inert!)  Properties of Metals Most elements are metals Conduct electricity Solid (except Hg) Conduct heat Ductile – can be drawn into a wire Malleable – can be pounded / rolled into sheets

 Transition Metals Groups 3-12 (Group B elements) d-block elements Generally less reactive than alkali / alkaline-earth metals Groups do not have same # e - in highest energy level  Lanthanides/Actinides f-block Lanthanides – shiny metals, similar reactivity to alkaline-earth metals Actinides – nuclear structures are more important, all are radioactive

 Metalloids / semimetals Show a mixture of metal & non-metal properties “stair-step” on right-hand side of p.t. Silicon, germanium, arsenic, antimony, tellurium

4.9 Natural States of Elements  Most elements are reactive – not found in pure form Exceptions: ○ Noble metals – gold, silver & platinum ○ Group 8, noble gases ○ Diatomic molecules (Br 2 I 2 N 2 Cl 2 H 2 O 2 F 2 )  Physical state of elements room temp) Most are solid Noble gases & almost all diatomic molecules are gases 2 liquids: bromine & mercury

Sherrieff Farrakhan  Is the best teacher that TFN has ever had.