Periodic Properties of Elements Chemistry 100 Chapter 7

The Modern Periodic Table

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 A 2  the distance between one nucleus and the other is d, then the radius of an A atom is ½d

An “Atomic Size” Calculation

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 Cl 2 is 1.99Å long, so we give the Cl atom a radius of 0.99Å. We predict that the C-Cl bond should be = 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 I 1, is the energy required to remove one electron from the neutral atom.  Example Na (g)  Na + (g) + e - The second ionization energy I 2, is the energy required to remove the second electron.  Example Na + (g)  Na 2+ (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. I 1 < I 2 < I 3 < I 4

Na [Ne]3s 1 Si [Ne]3s 2 3p 2 Cl [Ne]3s 2 3p 5 Mg [Ne]3s 2 P [Ne]3s 2 3p 3 Ar [Ne]3s 2 3p 6 = [Ar] Al [Ne]3s 2 3p 1 S [Ne]3s 2 3p 4 1) More difficult to remove electron from smaller atom 2) I 1 < I 2 < I 3 < I 4 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]3s 2 3p 5 Cl - configuration [Ne]3s 2 3p 6 The ion has the same electron configuration as Ar - a closed shell The Cl - ion is readily formed

Electron Affinity Values

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) + H 2 O(l)  Ca(OH) 2 (aq) Metal oxide + acid  salt + water MgO(s) + 2HCl(aq)  MgCl 2 (aq) + H 2 O(l)

Non-metals Vary greatly in appearance. Seven exist as diatomic atoms.  H 2 (colourless gas)  F 2 (yellowish gas)  Cl 2 (green gas)  Br 2 (red liquid)  I 2 (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 CO 2 + H 2 O  H 2 CO 3 (aq) non-metal oxide + acid  salt + water SO 3 + 2KOH  K 2 SO 4 (aq) + H 2 O(l)

Aluminum Al 2 O 3  amphoteric oxide (can act as either an acid or a base). Al 2 O 3 (s) + 6 HCl (aq)  2 AlCl 3 (aq) + 3 H 2 O (l) (basic) Al 2 O 3 (s) + 2 NaOH (aq) + 3 H 2 O (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.