Transcript Periodic Properties of Elements

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 A2

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 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.
Li
Be B
C
N
O
1.52 1.13 0.88 0.77 0.75 0.73
Na
1.86
K
2.27
Rb
2.47
F
Ne
0.71 0.69
Cl
0.99
Br
1.14
I
1.33
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
Mg [Ne]3s2
Al [Ne]3s23p1
Si [Ne]3s23p2
P [Ne]3s23p3
S [Ne]3s23p4
Cl [Ne]3s23p5
Ar [Ne]3s23p6 = [Ar]
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
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) + 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 semiconductors

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.