Periodic Properties of Elements Chemistry 100 Chapter 7
Atomic size • Atoms do not have a well defined size. • As the distance from the nucleus increases, it becomes less probable that an electron will be found there. • Examine a molecule of A2 • the distance between one nucleus and the other is d, then the radius of an A atom is ½d
Atomic radii • The C-C bond in diamond is 1.54Å, so we assign 0.77Å as the radius of the carbon atom. • The bond in Cl2 is 1.99Å long, so we give the Cl atom a radius of 0.99Å. • We predict that the C-Cl bond should be 0.77 + 0.99 = 1.76Å long. Experimental result is 1.77Å.
Atomic Radii and Periodic Table • As you descend a group, the atoms get larger. • This seems to be intuitive - the atoms lower in a group have more electrons and these fill higher shells. • As you cross a row, radius decreases. • The electrons are in the same shell but the nuclear charge increases as you cross a group - electrons attracted to centre.
Ionization energy • The first ionization energy I1, is the energy required to remove one electron from the neutral atom. • Example Na (g) Na+ (g) + e- • The second ionization energy I2, is the energy required to remove the second electron. • Example Na+(g) Na2+ (g) + e-
IE (Cont’d) • The greater the value of I, the more difficult it is to remove an electron • The first electron is more readily removed than the second, etc. I1 <I2 < I 3 < I4
Na [Ne]3s1 Si [Ne]3s23p2 Cl [Ne]3s23p5 Mg [Ne]3s2 P [Ne]3s23p3 Ar [Ne]3s23p6 = [Ar] Al [Ne]3s23p1 S [Ne]3s23p4 1) More difficult to remove electron from smaller atom 2) I1 <I2 < I 3 < I4 First electron easiest to remove 3) Inner-shell electrons “impossible” to remove
Electron Affinity • Ionization energy measures the energy change associated with the removal of an electron. Cl (g) Cl+(g) + e- E = 1251 kJ/mol • Positive value means energy must be added to atom to remove electron • Electron Affinity measures the energy change related to the addition of an electron Cl (g) + e- Cl-(g) E = -349 kJ/mol
Electron Affinity (cont) • The Cl- ion is more stable than the Cl atom • Cl configuration [Ne]3s23p5 • Cl- configuration [Ne]3s23p6 • The ion has the same electron configuration as Ar - a closed shell • The Cl- ion is readily formed
Metals, Non-metals & Metalloids • Elements which ionize (lose electrons) readily are metals: Sodium, Iron, Lead • Elements which readily gain electrons are non-metals: Chlorine, Sulphur, Argon • Separating them are the metalloids: Boron, Silicon, Arsenic
Metals v Non-metals • Shiny luster, often silvery No luster, many colours • Solids are malleable (can be shaped with hammer) and ductile (can be drawn into wires) Solids often brittle; some are hard, some soft
Metals vs. Nonmetals (Round 2) • Good conductors of heat and electricity Poor conductors (graphite is an exception) • Most metal oxides are basic Most non-metallic oxides are acidic • Tend to form cations (+ve charge) in solution Tend to form anions or oxyanions in solutions
Metals • All but Hg are solids are 25ºC. (What is the other liquid element?) • Low ionization energies; form positive ions • Oxides are basic CaO(s) + H2O(l) Ca(OH)2 (aq) Metal oxide + acid salt + water MgO(s) + 2HCl(aq) MgCl2(aq) + H2O(l)
Non-metals • Vary greatly in appearance. • Seven exist as diatomic atoms. • H2 (colourless gas) • F2 (yellowish gas) • Cl2 (green gas) • Br2 (red liquid) • I2 (purple volatile solid) • Diamond (C) is hard, sulphur is soft.
Nonmetals (Round 2) • Tend to gain electrons to form anions • Oxides are acidic non-metal oxide + water acid CO2 + H2O H2CO3 (aq) non-metal oxide + acid salt + water SO3 + 2KOH K2SO4 (aq) + H2O(l)
Aluminum Al2O3 amphoteric oxide (can act as either an acid or a base). Al2O3(s) + 6 HCl (aq) 2 AlCl3 (aq) + 3 H2O (l) (basic) Al2O3 (s) + 2 NaOH (aq) + 3 H2O (l) 2 NaAl(OH)4 (acidic oxide)
Metalloids • Generally hard, non-malleable solids • In pure state they are non-conductors but with controlled impurities they form semi-conductors • Computer chips are made of Si
Allotropy • Carbon can exist as carbon black (soot), graphite, buckyballs, or diamond. • These are called allotropes - same element, different physical appearances. • Carbon is said to exhibit allotropy
Allotropy (Cont’d) • Tin is a metal at 25ºC. Below 13ºC it can turn into a white, non-metallic powder. • At extremely high pressures, there is a metallic form of hydrogen.