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Group 18 is an element group that consists of elements whose outermost (valence) shell is completely filled with electrons. Since adding an electron to a Group 18 element means putting it into a higher, weakly bound shell, Group 18 elements don't readily acquire a negative charge. Removing an electron from a Group 18 element takes a comparatively large amount of energy, Since the electron transfers which cause chemical bonding are difficult for Group 18 elements, they are chemically inert. For this reason, they are called noble gases.

Their inertness means that Group 18 elements don't bond to each other. The first six noble gases; helium, neon, argon, krypton, xenon and radon; are gases under ambient conditions. Og is predicted to be a solid under ambient conditions. They are also monatomic gases, rather than diatomic (like O2 or N2).

Reactivity of Group 18 increases from period to period, as demonstrated by decreasing ionization energy. Helium, neon, and argon are all but completely inert, Krypton has a limited chemistry, and xenon has an extensive chemistry. Radon probably does too, but has not been extensively studied for the obvious reason. Oganesson is not only expected to be more reactive than the lighter Group 18 elements, but is expected to be more reactive than fellow Period 7 elements Cn or Fl.

It is likely that periodicity will largely break down in Period 8, but that period is predicted to end in a p-block, with ionization energy increasing to a maximum at Group 18, then falling abruptly as Period 9 begins. This same pattern is likely to occur between Periods 9 and 10, and may occur between Periods 10 and 11.

Elements in group 18[]

(Elements that are pale blue are undiscovered/new)

2
He
10
Ne
18
Ar
36
Kr
54
Xe
86
Rn
118
Og
172
Usb
226
Bbh

At Z = 172, the theory of atomic structure may still apply. Usb may be a noble fluid, but it's unlikely to be a "gas" and is probably quite reactive. By Z = 226, atomic theory has broken down completely. If Period 9 has the same structure as Period 8, Bbh will represent a local maximum in ionization energy, but is unlikely to be either a gas or noble.

9-Period Periodic Table of Elements
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
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
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
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
Nh
114
Fl
115
Mc
116
Lv
117
Ts
118
Og
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
169
Uhe
170
Usn
171
Usu
172
Usb
9 173
Ust
174
Usq
Alkali metal Alkaline earth metal Lanthanide Actinide Superactinide Transition metal Post-transition metal Metalloid Other nonmetal Halogen Noble gas
predicted predicted predicted predicted predicted predicted predicted predicted predicted
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