There are currently seven periods in the periodic table of chemical elements, culminating with atomic number 118. If further elements with higher atomic numbers than this were to be discovered, they would be placed in additional periods, laid out (as with the existing periods) to illustrate periodically recurring trends in the properties of the elements concerned. Any additional periods are expected to contain a larger number of elements than the seventh period, as they are calculated to have an additional socalled gblock, containing 18 elements with partially filled gorbitals in each period. An eightperiod table containing this block was suggested by Glenn T. Seaborg in 1969.^{[1]}
No elements in this region have been synthesized or discovered in nature. (Element 122 was claimed to exist naturally in April 2008, but this claim was widely believed to be erroneous.)^{[2]} The first element of the gblock may have atomic number 121, and thus would have the systematic name unbiunium. Elements in this region are likely to be highly unstable with respect to radioactive decay, and have extremely short half lives, although element 126 is hypothesized to be within an island of stability that is resistant to fission but not to alpha decay. It is not clear how many elements beyond the expected island of stability are physically possible.
According to the orbital approximation in quantum mechanical descriptions of atomic structure, the gblock would correspond to elements with partiallyfilled gorbitals. However, spinorbit coupling effects reduce the validity of the orbital approximation substantially for elements of high atomic number.^{[3]}
Contents 
s^{1}  s^{2}  p^{1}  p^{2}  p^{3}  p^{4}  p^{5}  p^{6}  
1  1 H 
2 He 

2  3 Li 
4 Be 
5 B 
6 C 
7 N 
8 O 
9 F 
10 Ne 

3  11 Na 
12 Mg 
d^{1}  d^{2}  d^{3}  d^{4}  d^{5}  d^{6}  d^{7}  d^{8}  d^{9}  d^{10}  13 Al 
14 Si 
15 P 
16 S 
17 Cl 
18 Ar 

4  19 K 
20 Ca 
21 Sc 
22 Ti 
23 V 
24 Cr 
25 Mn 
26 Fe 
27 Co 
28 Ni 
29 Cu 
30 Zn 
31 Ga 
32 Ge 
33 As 
34 Se 
35 Br 
36 Kr 

5  37 Rb 
38 Sr 
f^{1}  f^{2}  f^{3}  f^{4}  f^{5}  f^{6}  f^{7}  f^{8}  f^{9}  f^{10}  f^{11}  f^{12}  f^{13}  f^{14}  39 Y 
40 Zr 
41 Nb 
42 Mo 
43 Tc 
44 Ru 
45 Rh 
46 Pd 
47 Ag 
48 Cd 
49 In 
50 Sn 
51 Sb 
52 Te 
53 I 
54 Xe 

6  55 Cs 
56 Ba 
57 La 
58 Ce 
59 Pr 
60 Nd 
61 Pm 
62 Sm 
63 Eu 
64 Gd 
65 Tb 
66 Dy 
67 Ho 
68 Er 
69 Tm 
70 Yb 
71 Lu 
72 Hf 
73 Ta 
74 W 
75 Re 
76 Os 
77 Ir 
78 Pt 
79 Au 
80 Hg 
81 Tl 
82 Pb 
83 Bi 
84 Po 
85 At 
86 Rn 

7  87 Fr 
88 Ra 
g^{1}  g^{2}  g^{3}  g^{4}  g^{5}  g^{6}  g^{7}  g^{8}  g^{9}  g^{10}  g^{11}  g^{12}  g^{13}  g^{14}  g^{15}  g^{16}  g^{17}  g^{18}  89 Ac 
90 Th 
91 Pa 
92 U 
93 Np 
94 Pu 
95 Am 
96 Cm 
97 Bk 
98 Cf 
99 Es 
100 Fm 
101 Md 
102 No 
103 Lr 
104 Rf 
105 Db 
106 Sg 
107 Bh 
108 Hs 
109 Mt 
110 Ds 
111 Rg 
112 Cn 
113 Uut 
114 Uuq 
115 Uup 
116 Uuh 
117 Uus 
118 Uuo 
8  119 Uue 
120 Ubn 
121 Ubu 
122 Ubb 
123 Ubt 
124 Ubq 
125 Ubp 
126 Ubh 
127 Ubs 
128 Ubo 
129 Ube 
130 Utn 
131 Utu 
132 Utb 
133 Utt 
134 Utq 
135 Utp 
136 Uth 
137 Uts 
138 Uto 
139 Ute 
140 Uqn 
141 Uqu 
142 Uqb 
143 Uqt 
144 Uqq 
145 Uqp 
146 Uqh 
147 Uqs 
148 Uqo 
149 Uqe 
150 Upn 
151 Upu 
152 Upb 
153 Upt 
154 Upq 
155 Upp 
156 Uph 
157 Ups 
158 Upo 
159 Upe 
160 Uhn 
161 Uhu 
162 Uhb 
163 Uht 
164 Uhq 
165 Uhp 
166 Uhh 
167 Uhs 
168 Uho 
9  169 Uhe 
170 Usn 
171 Usu 
172 Usb 
173 Ust 
Blocks of the periodic table
sblock  pblock  dblock  fblock  gblock 
(Undiscovered elements are coloured in a lighter shade)
All of these hypothetical undiscovered elements are named by the International Union of Pure and Applied Chemistry (IUPAC) systematic element name standard which creates a generic name for use until the element has been discovered, confirmed, and an official name approved.
The positioning of the gblock in the table (to the left of the fblock, to the right, or in between) is speculative. The positions shown in the table above corresponds to the assumption that the Madelung rule will continue to hold at higher atomic number; this assumption may or may not be true. At element 118, the orbitals 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f, 5s, 5p, 5d, 5f, 6s, 6p, 6d, 7s and 7p are assumed to be filled, with the remaining orbitals unfilled. The orbitals of the eighth period are predicted to be filled in the order 8s, 5g, 6f, 7d, 8p. However, after approximately element 120, the proximity of the electron shells makes placement in a simple table problematic; for example, calculations suggest that it may be elements 165 and 166 which occupy the 9s block (leaving the 8p orbital incomplete) assuming they are physically possible.^{[5]}
The number of physically possible elements is unknown. The lightspeed limit on electrons orbiting in everbigger electron shells theoretically limits neutral atoms to a Z of approximately 173,^{[6]} after which it would be nonsensical to assign the elements to blocks on the basis of electron configuration. However, it is likely that the periodic table actually ends much earlier, possibly soon after the island of stability,^{[7]} which is expected to center around Z = 126.^{[8]}
Additionally the extension of the periodic and nuclides tables is restricted by the proton drip line and the neutron drip line.
The Bohr model exhibits difficulty for atoms with atomic number greater than 137, for the speed of an electron in a 1s electron orbital, v, is given by
where Z is the atomic number, and α is the fine structure constant, a measure of the strength of electromagnetic interactions.^{[9]} Under this approximation, any element with an atomic number of greater than 137 would require 1s electrons to be traveling swifter than c, the speed of light. Hence a nonrelativistic model such as the Bohr model is inadequate for such calculations.
The semirelativistic Dirac equation also has problems for Z > 137, for the ground state energy is
where m_{0} is the rest mass of the electron. For Z > 137, the wave function of the Dirac ground state is oscillatory, rather than bound, and there is no gap between the positive and negative energy spectra, as in the Klein paradox.^{[10]} Richard Feynman pointed out this effect, so the last element expected under this model, 137 (untriseptium), is sometimes called feynmanium.
However, a realistic calculation has to take into account the finite extension of the nuclearcharge distribution. This results in a critical Z of ≈ 173 (unseptrium), such that nonionized atoms may be limited to elements equal to or lower than this.^{[6]}

