Introduction to the periodic table

The universe is, as far as we know,  composed of approximately 120 elements. These are pure substances with a fixed number of protons: hydrogen has 1, helium 2, carbon 6 etc.It seems reasonable to expect some sort of organization to them. That is what the current periodic table is about: trying to show how the elements in the universe are organized.in a functional manner

Aufbau rule

The electrons generally arrange themselves in a pattern shown below
 

Electron Configuration Pattern: s2 p6 d10 f14 g18 h22 i26…											total running count  of electrons
1s2											2 (He)
	2s2										4 (Be)
2p6		3s2									12 (Mg)
	3p6		4s2								20
3d10		4p6		5s2							38
	4d10		5p6		6s2						56
4f14		5d10		6p6		7s2					88
	5f14		6d10		7p6		8s2				120
5g18		6f14		7d10		8p6		9s2			170
	6g18		7f14		8d10		9p6		10s2		220

The first number is the shell, the letter is the subshell type, and the last number is the number of elements in that orbital group.
 
Note that this pattern is infinite, although to date that largest confirmed element has 116 electrons, and no elements have been discovered beyond the seventh row. For fascinating reading on the extended periodic table have a look at the wikipedia article. Here is an extended periodic table that includes 218 elements.





Exceptions to the aufbau rule

Note also that numerous exceptions to the aufbau rule (or are they?) exist:
 

Chromium	[Ar] 4s1  3d5
Copper	[Ar]  4s1 3d10
Niobium	[Kr]  5s1 4d4
Molybdenum	[Kr]  5s1 4d5
Ruthenium	[Kr]  5s1 4d7
Rhodium	[Kr]  5s1 4d8
Palladium	[Kr]  5s0 4d10
Silver	[Kr]  5s1 4d10
Lanthanum	[Xe] 6s2  5d1
Cerium	[Xe] 6s24f1 5d1
Gadolinium	[Xe] 6s2 4f7 5d1
Platinum	[Xe]  6s14f14 5d9
Gold	[Xe]  6s14f14 5d10
Actinium	[Rn] 7s26d1
Thorium	[Rn] 7s26d2
Protactinium	[Rn]7s2 5f2 6d1
Uranium	[Rn]7s2 5f3 6d1
Neptunium	[Rn] 7s25f4 6d1
Curium	[Rn] 7s25f7 6d1

Ouch! That is a lot of exceptions. I hate it when something symmetrical and perfect gets messed up...it makes me wonder if there is in fact a perfect pattern that is hiding within these exceptions. The two most common trends are when an element is one element shy of halfway or completely filling an orbital group it wil promote an  electron to do so. So 3d4 (Copper) and 3d9 Chromium) are exceptions, as are 4f6 (gadolinium) and 4f13 (platinum). One would predict that both lanthanum and actinium should be in the f block, but since they are exceptions and do not contain any f electrons, they are placed in the d block. Same for lutetium and lawrencium. Many periodic tables give the observed electron configurations for each element:


A few other disappointments to the aufbau principle, and one good thing about it.

1. Ions are formed by removing electrons from the valence shells, not the ending electrons in the aufbau order. A simple look at valence does a better job at predicting ion formation than electron configuration. For example, Scandium will lose a 4s electron, even though the electron configuration ends in 3d1.

2. The electron dot structure correctly predicts carbon will form four bonds, but the electron configuration predicts 2 (and is therefore "hybridized" to give the correct answer, which sounds like a fix to me). Once again valence electrons work better- they predict bonding behavior better than electron configuration.

3. At least the aufbau order shows how shells can have more contain more than eight electrons, but no element can have more than eight valence electrons- they fill up out of order.

Temporary names

Note that until the elements are confirmed they are assigned temporary names using a simple system with short sounds for numbers 0-9 with a "ium suffix. 0 = nil, 1 = un, 2 = bi, 3 = tri, 4 = quad, 5 = pent, g = hex, 7 = sept, 8 = oct, and 9 = en. The symbols, and pronunciations are shown below. Thus for element 118 we have un-un-octi-ium (ununoctium) with a symbol Uuo.  Ubn is Unbibium, element 122, etc.

digit	root	symbol
0	nil	n
1	un	u
2	b(i)	b
3	tr(i)	t
4	quad	q
5	pent	p
6	hex	h
7	sept	s
8	oct	o
9	en(n)	e
Suffix	-ium	none

The periodic table attempts to show this order while at the same time organizing the elements with the same number of valence electrons in increasing columns. The creators chose not to organize by isotopic abundance, or by the state the element is in, or by melting point or boiling point. 
 
As a result of these competing priorities, and perhaps for historical reasons as well, the periodic table lacks symmetry. In the table on the left most rows and columns are  of different length, and it is in two pieces. This is not a simple table. Even the extended periodic table looks like it could be modified to form a symmetrical right triangle, but it isn't. Could it be that we humans just haven’t figured it out yet? Is there a better, more symmetrical way to display the elements in the periodic table? I’m hoping you can do better. Somebody should.