Period (periodic table)

A period on the

, a total of 118 elements have been discovered and confirmed.

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 in the ordering rule diagram. The filling of each shell corresponds to a row in the table.

In the

elements show a high degree of similarity across periods.

Periods

There are currently seven complete periods in the periodic table, comprising the 118 known elements. Any new elements will be placed into an eighth period; see extended periodic table. The elements are colour-coded below by their block: red for the s-block, yellow for the p-block, blue for the d-block, and green for the f-block.

Period 1

Group	1	18
Atomic # Name	1 H	2 He

The first period contains fewer elements than any other, with only two,

.

• 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 with most elements and is present 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

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

Period 2 elements involve the

. They include the biologically most essential elements besides hydrogen: carbon, nitrogen, and oxygen.

• primordial element forged in large quantities during the Big Bang
  .
• 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]
• diatomic gas, N₂, which makes up 78% of the Earth's atmosphere by volume. It is an essential component of proteins
  and therefore of life.
• Oxygen (O) comprising 21% of the atmosphere by volume 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

Group	1	2	13	14	15	16	17	18
Atomic # 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.

• Sodium (Na) is an alkali metal. It is present in Earth's oceans in large quantities in the form of sodium chloride (table salt).
• Magnesium (Mg) is an alkaline earth metal. Magnesium ions are found in chlorophyll.
• Aluminium (Al) is a post-transition metal. It is the most abundant metal in the Earth's crust.
• integrated circuits. Silicon dioxide is the principal constituent of sand. As Carbon is to Biology, Silicon is to Geology
  .
• nonmetal essential to DNA. It is highly reactive
  , and as such is never found in nature as a free element.
• nonmetal. It is found in two amino acids: cysteine and methionine
  .
• Chlorine (Cl) is a halogen. Since it is one of the most reactive elements, it is often found on the Earth's surface as sodium chloride. Its compounds used as a disinfectant, especially in swimming pools.
• Incandescent lamps
  are often filled with noble gases such as argon in order to preserve the filaments at high temperatures.

Period 4

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

Period 4 includes the biologically essential elements

. Almost all have biological roles.

Completing the fourth period are six p-block elements: gallium, germanium, arsenic, selenium, bromine, and krypton.

Period 5

Group	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
Atomic # 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 has the same number of elements as period 4 and follows the same general structure but with one more post transition metal and one fewer nonmetal. Of the three heaviest elements with biological roles, two (molybdenum and iodine) are in this period; tungsten, in period 6, is heavier, along with several of the early lanthanides. Period 5 also includes technetium, the lightest exclusively radioactive element.

Period 6

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

are largely inert.

Period 7

Group	1	2															3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
Atomic # 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 Nh	114 Fl	115 Mc	116 Lv	117 Ts	118 Og

All elements of period 7 are

) are 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

atomic nuclei

.

Period 8

No element of the eighth period has yet been synthesized. A

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.

See also

• Group (periodic table)

References