Period (periodic table)

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In the periodic table of the elements, elements are arranged in a series of rows (or periods) so that those with similar properties appear in a column. Elements of the same period have the same number of electron shells; with each group across a period, the elements have one more proton and electron and become less metallic. This arrangement reflects the periodic recurrence of similar properties as the atomic number increases. For example, the alkaline metals lie in one group (group 1) and share similar properties, such as high reactivity and the tendency to lose one electron to arrive at a noble-gas electronic configuration. The periodic table of elements has a total of 118 elements.

The Madelung energy ordering rule describes the order in which orbitals are arranged by increasing energy according to the Madelung rule. Each diagonal corresponds to a different value of n + l.

Modern quantum mechanics explains these periodic trends in properties in terms of electron shells. As atomic number increases, shells fill with electrons in approximately the order shown at right. The filling of each shell corresponds to a row in the table.

In the s-block and p-block of the periodic table, elements within the same period generally do not exhibit trends and similarities in properties (vertical trends down groups are more significant). However in the d-block, trends across periods become significant, and in the f-block elements show a high degree of similarity across periods (particularly the lanthanides).

Periods[edit]

Seven periods of elements occur naturally on Earth. For period 8, which includes elements which may be synthesized after 2013, see the extended periodic table.

A group in chemistry means a family of objects with similarities like different families. There are 7 periods, going horizontally across the periodic table.

Period 1[edit]

Group 1/17 2/18
#
Name
1
H
2
He

The first period contains fewer elements than any other, with only two, hydrogen and helium. They therefore do not follow the octet rule. Chemically, helium behaves as a noble gas, and thus is taken to be part of the group 18 elements. However, in terms of its nuclear structure it belongs to the s block, and is therefore sometimes classified as a group 2 element, or simultaneously both 2 and 18. Hydrogen readily loses and gains an electron, and so behaves chemically as both a group 1 and a group 17 element.

  • Hydrogen (H) is the most abundant of the chemical elements, constituting roughly 75% of the universe's elemental mass.[1] Ionized hydrogen is just a proton. Stars in the main sequence are mainly composed of hydrogen in its plasma state. Elemental hydrogen is relatively rare on Earth, and is industrially produced from hydrocarbons such as methane. Hydrogen can form compounds nt in water and most organic compounds.[2]
  • Helium (He) exists only as a gas except in extreme conditions.[3] It is the second lightest element and is the second most abundant in the universe.[4] Most helium was formed during the Big Bang, but new helium is created through nuclear fusion of hydrogen in stars.[5] On Earth, helium is relatively rare, only occurring as a byproduct of the natural decay of some radioactive elements.[6] Such 'radiogenic' helium is trapped within natural gas in concentrations of up to seven percent by volume.[7]

Period 2[edit]

Group 1 2 13 14 15 16 17 18
#
Name
3
Li
4
Be
5
B
6
C
7
N
8
O
9
F
10
Ne

Period 2 elements involve the 2s and 2p orbitals. They include the biologically most essential elements besides hydrogen: carbon, nitrogen, and oxygen.

  • Lithium (Li) is the lightest metal and the least dense solid element.[8] In its non-ionized state it is one of the most reactive elements, and so is only ever found naturally in compounds. It is the heaviest primordial element forged in large quantities during the Big Bang.
  • Beryllium (Be) has one of the highest melting points of all the light metals. Small amounts of beryllium were synthesised during the Big Bang, although most of it decayed or reacted further within stars to create larger nucleii, like carbon, nitrogen or oxygen. Beryllium is classified by the International Agency for Research on Cancer as a group 1 carcinogen.[9] Between 1% and 15% of people are sensitive to beryllium and may develop an inflammatory reaction in their respiratory system and skin, called chronic beryllium disease.[10]
  • Boron (B) does not occur naturally as a free element, but in compounds such as borates. It is an essential plant micronutrient, required for cell wall strength and development, cell division, seed and fruit development, sugar transport and hormone development,[11][12] though high levels are toxic.
  • Carbon (C) is the fourth most abundant element in the universe by mass after hydrogen, helium and oxygen[13] and is the second most abundant element in the human body by mass after oxygen,[14] the third most abundant by number of atoms.[15] There are an almost infinite number of compounds that contain carbon due to carbon's ability to form long stable chains of C—C bonds.[16][17] All organic compounds, those essential for life, contain at least one atom of carbon;[16][17] combined with hydrogen, oxygen, nitrogen, sulfur, and phosphorus, carbon is the basis of every important biological compound.[17]
  • Nitrogen (N) is found mainly as mostly inert diatomic gas, N2, which makes up 78% of the Earth's atmosphere. It is an essential component of proteins and therefore of life.
  • Oxygen (O) comprising 21% of the atmosphere and is required for respiration by all (or nearly all) animals, as well as being the principal component of water. Oxygen is the third most abundant element in the universe, and oxygen compounds dominate the Earth's crust.
  • Fluorine (F) is the most reactive element in its non-ionized state, and so is never found that way in nature.
  • Neon (Ne) is a noble gas used in neon lighting.

Period 3[edit]

Group 1 2 13 14 15 16 17 18
#
Name
11
Na
12
Mg
13
Al
14
Si
15
P
16
S
17
Cl
18
Ar

All period three elements occur in nature and have at least one stable isotope. All but the noble gas argon are essential to basic geology and biology.

Period 4[edit]

Group 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Atomic number
Name
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
From left to right, aqueous solutions of: Co(NO3)2 (red); K2Cr2O7 (orange); K2CrO4 (yellow); NiCl2 (green); CuSO4 (blue); KMnO4 (purple).

Period 4 includes the biologically essential elements potassium and calcium, and is the first period in the d-block with the lighter transition metals. These include iron, the heaviest element forged in main-sequence stars and a principal component of the earth, as well as other important metals such as cobalt, nickel, copper, and zinc. Almost all have biological roles.

Period 5[edit]

Group 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Atomic number
Name
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

Period 5 contains the heaviest few elements that have biological roles, molybdenum and iodine. (Tungsten, a period 6 element, is the only heavier element that has a biological role.) It includes technetium, the lightest exclusively radioactive element.

Period 6[edit]

Group 1 2 3 (Lanthanides) 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
#
Name
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

Period 6 is the first period to include the f-block, with the lanthanides (also known as the rare earth elements), and includes the heaviest stable elements. Many of these heavy metals are toxic and some are radioactive, but platinum and gold are largely inert.

Period 7[edit]

Group 1 2 3 (Actinides) 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
#
Name
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
Uut
114
Fl
115
Uup
116
Lv
117
Uus
118
Uuo

All elements of period 7 are radioactive. This period contains the heaviest element which occurs naturally on earth, californium. All of the subsequent elements in the period have been synthesized artificially. Whilst one of these (einsteinium) is now available in macroscopic quantities, most are extremely rare, having only been prepared in microgram amounts or less. Some of the later elements have only ever been identified in laboratories in quantities of a few atoms at a time.

Although the rarity of many of these elements means that experimental results are not very extensive, periodic and group trends in behaviour appear to be less well defined for period 7 than for other periods. Whilst francium and radium do show typical properties of Groups 1 and 2 respectively, the actinides display a much greater variety of behaviour and oxidation states than the lanthanides. Initial studies suggest Group 14 element flerovium appears to be a noble gas instead of an other metal, and group 18 element ununoctium probably is not a noble gas.[18] These peculiarities of period 7 may be due to a variety of factors, including a large degree of spin-orbit coupling and relativistic effects, ultimately caused by the very high positive electrical charge from their massive atomic nuclei.

Period 8[edit]

No element of the eighth period has yet been synthesized. A g-block is predicted. It is not clear if all elements predicted for the eighth period are in fact physically possible. There may therefore be no ninth period.

Legend for the background color
Background color shows subcategory in the metal–nonmetal range:
Metal Metalloid Nonmetal Unknown
chemical
properties
Alkali metal Alkaline earth metal Lan­thanide Actinide Transition metal Other metal Polyatomic nonmetal Diatomic nonmetal Noble gas

References[edit]

  1. ^ Palmer, David (November 13, 1997). "Hydrogen in the Universe". NASA. Retrieved 2008-02-05. 
  2. ^ "hydrogen". Encyclopædia Britannica. 2008. 
  3. ^ "Helium: physical properties". WebElements. Retrieved 2008-07-15. 
  4. ^ "Helium: geological information". WebElements. Retrieved 2008-07-15. 
  5. ^ Cox, Tony (1990-02-03). "Origin of the chemical elements". New Scientist. Retrieved 2008-07-15. 
  6. ^ "Helium supply deflated: production shortages mean some industries and partygoers must squeak by.". Houston Chronicle. 2006-11-05. 
  7. ^ Brown, David (2008-02-02). "Helium a New Target in New Mexico". American Association of Petroleum Geologists. Retrieved 2008-07-15. 
  8. ^ Lithium at WebElements.
  9. ^ "IARC Monograph, Volume 58". International Agency for Research on Cancer. 1993. Retrieved 2008-09-18. 
  10. ^ Information about chronic beryllium disease.
  11. ^ "Functions of Boron in Plant Nutrition" (PDF). U.S. Borax Inc. 
  12. ^ Blevins, Dale G.; Lukaszewski, Krystyna M. (1998). "Functions of Boron in Plant Nutrition". Annual Review of Plant Physiology and Plant Molecular Biology 49: 481–500. doi:10.1146/annurev.arplant.49.1.481. PMID 15012243. 
  13. ^ Ten most abundant elements in the universe, taken from The Top 10 of Everything, 2006, Russell Ash, page 10. Retrieved October 15, 2008.
  14. ^ Chang, Raymond (2007). Chemistry, Ninth Edition. McGraw-Hill. p. 52. ISBN 0-07-110595-6. 
  15. ^ Freitas Jr., Robert A. (1999). Nanomedicine. Landes Bioscience. Tables 3-1 & 3-2. ISBN 1-57059-680-8. 
  16. ^ a b "Structure and Nomenclature of Hydrocarbons". Purdue University. Retrieved 2008-03-23. 
  17. ^ a b c Alberts, Bruce; Alexander Johnson; Julian Lewis; Martin Raff; Keith Roberts; Peter Walter. Molecular Biology of the Cell. Garland Science. 
  18. ^ See references in the articles Flerovium, Ununoctium