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Aperiod 4 element is one of thechemical elements in the fourth row (orperiod) of theperiodic table of the chemical elements. The periodic table is laid out in rows to illustrate recurring (periodic) trends in the chemical behaviour of the elements as their atomic number increases: a new row is begun when chemical behaviour begins to repeat, meaning that elements with similar behaviour fall into the same vertical columns. The fourth period contains 18 elementsbeginning with potassium and ending withkrypton – one element for each of theeighteen groups. It sees the first appearance ofd-block (which includestransition metals) in the table.
All 4th-period elements arestable,[1] and many are extremely common in theEarth's crust and/orcore; it is the last period with no unstable elements. Many transition metals in the period are verystrong, and therefore common inindustry, especiallyiron.[2] Some aretoxic, with all knownvanadium compounds toxic,[3]arsenic one of the most well-knownpoisons, andbromine a toxic liquid. Conversely, many elements are essential to human survival, such ascalcium, the main component inbones.[2]
Progressing towards increase ofatomic number, theAufbau principle causes elements of the period to putelectrons onto 4s, 3d, and 4p subshells, in that order. However, there are exceptions, such aschromium. The first twelve elements—K,Ca,and transition metals—have from 1 to 12valence electrons respectively, which are placed on 4s and 3d.
Twelve electrons over theelectron configuration ofargon reach the configuration ofzinc, namely 3d10 4s2. After this element, the filled 3d subshell effectively withdraws from chemistry and the subsequenttrend looks much like trends in the periods2 and3.The p-block elements of period 4 have theirvalence shell composed of 4s and 4p subshells of thefourth (n = 4) shell and obey theoctet rule.
Forquantum chemistry namely this period sees transition from the simplifiedelectron shell paradigm to research of manydifferently-shaped subshells. The relative disposition of theirenergy levels is governed by the interplay of various physical effects. The period'ss-block metals put their differentiating electrons onto 4s despite having vacancies among nominally lowern = 3 states – a phenomenon unseen in lighter elements. Contrariwise, the six elements fromgallium tokrypton are the heaviest where all electron shells below the valence shell are filledcompletely. This is no longer possible in further periods due to the existence of f-subshells starting fromn = 4.
| Chemical element | Block | Electron configuration | ||
|---|---|---|---|---|
| 19 | K | Potassium | s-block | [Ar] 4s1 |
| 20 | Ca | Calcium | s-block | [Ar] 4s2 |
| 21 | Sc | Scandium | d-block | [Ar] 3d1 4s2 |
| 22 | Ti | Titanium | d-block | [Ar] 3d2 4s2 |
| 23 | V | Vanadium | d-block | [Ar] 3d3 4s2 |
| 24 | Cr | Chromium | d-block | [Ar] 3d5 4s1 (*) |
| 25 | Mn | Manganese | d-block | [Ar] 3d5 4s2 |
| 26 | Fe | Iron | d-block | [Ar] 3d6 4s2 |
| 27 | Co | Cobalt | d-block | [Ar] 3d7 4s2 |
| 28 | Ni | Nickel | d-block | [Ar] 3d8 4s2 |
| 29 | Cu | Copper | d-block | [Ar] 3d10 4s1 (*) |
| 30 | Zn | Zinc | d-block | [Ar] 3d10 4s2 |
| 31 | Ga | Gallium | p-block | [Ar] 3d10 4s2 4p1 |
| 32 | Ge | Germanium | p-block | [Ar] 3d10 4s2 4p2 |
| 33 | As | Arsenic | p-block | [Ar] 3d10 4s2 4p3 |
| 34 | Se | Selenium | p-block | [Ar] 3d10 4s2 4p4 |
| 35 | Br | Bromine | p-block | [Ar] 3d10 4s2 4p5 |
| 36 | Kr | Krypton | p-block | [Ar] 3d10 4s2 4p6 |
(*) Exception to theMadelung rule
Potassium (K) is analkali metal, underneathsodium and aboverubidium,[4] and the first element of period 4. One of the mostreactive chemical elements, it is usually found only incompounds. It is a silvery metal[5] that tarnishes rapidly when exposed to theoxygen in air, whichoxidizes it. It issoft enough to be cut with a knife[6] and the second least-dense element.[citation needed]Potassium has a relatively lowmelting point; it will melt under a small open flame.[5] It also is less dense than water, and can, in principle, float[7] (although it will react with any water it is exposed to).[5]
Calcium (Ca) is the second element in the period. Analkali earth metal,native calcium is almost never found in nature,[citation needed] because it reacts with water.[8] It has one of the most widely-known biological roles in all animals and some plants, making up structural elements such as bones and teeth.[9] It also has applications incells, such as signals forcellular processeses. It is regarded as the most abundantmineral in the human body.[citation needed]
Scandium (Sc) is the third element in the period, and is the firsttransition metal in the periodic table. Scandium is quite common in nature, but difficult to isolate because its chemistry mirrors that of the otherrare earth compounds quite closely. Scandium has very few commercial applications, the major exception beingaluminiumalloys.
Titanium (Ti) is an element ingroup 4. Titanium is both one of the least dense metals and one of the strongest and most corrosion-resistant. As such, it has many applications, especially in alloys with other elements, such as iron. It is commonly used inairplanes,golf clubs, and other objects that must be strong, but lightweight.
Vanadium (V) is an element ingroup 5. Vanadium is never found in pure form in nature, but is commonly found in compounds. Vanadium is similar to titanium in many ways, such as being very corrosion-resistant, however, unlike titanium, it oxidizes in air even at room temperature. All vanadium compounds have at least some level of toxicity, with some of them being extremely toxic.
Chromium (Cr) is an element ingroup 6. Chromium is, like titanium and vanadium before it, extremely resistant to corrosion, and is indeed one of the main components ofstainless steel. Chromium also has many colorful compounds, and as such is very commonly used in pigments, such aschrome green.
Manganese (Mn) is an element ingroup 7. Manganese is often found in combination with iron. Manganese, like chromium before it, is an important component instainless steel, preventing the iron from rusting. Manganese is also often used in pigments, again like chromium. Manganese is also poisonous; if enough is inhaled, it can cause irreversible neurological damage.
Iron (Fe) is an element ingroup 8. Iron isthe most common on Earth among elements of the period, and probably the most well-known of them. It is the principal component ofsteel.Iron-56 has the lowest energy density of any isotope of any element, meaning that it is the most massive element that can be produced insupergiantstars. Iron also has some applications in the human body;hemoglobin is partly iron.
Cobalt (Co) is an element ingroup 9. Cobalt is commonly used in pigments, as many compounds of cobalt are blue in color. Cobalt is also a core component of many magnetic and high-strength alloys. The only stable isotope,cobalt-59, is an important component ofvitamin B-12, whilecobalt-60 is a component of nuclear fallout and can be dangerous in large enough quantities due to its radioactivity.
Nickel (Ni) is an element ingroup 10. Nickel is rare in the Earth's crust, mainly due to the fact that it reacts with oxygen in the air, with most of the nickel on Earth coming fromnickel iron meteorites. However, nickel is very abundant in theEarth's core; along with iron it is one of the two main components. Nickel is an important component of stainless steel, and in manysuperalloys.
Copper (Cu) is an element ingroup 11. Copper is one of the few metals that is not white or gray in color, the only[citation needed] others beinggold,osmium andcaesium. Copper has been used by humans for thousands of years to provide a reddish tint[clarification needed] to many objects, and is even an essential nutrient to humans, although too much is poisonous. Copper is also commonly used as a wood preservative orfungicides.
Zinc (Zn) is an element ingroup 12. Zinc is one of the main components ofbrass, being used since the 10th century BCE. Zinc is also incredibly important to humans; almost 2 billion people in the world suffer from zinc deficiency. However, too much zinc can cause copper deficiency. Zinc is often used in batteries, aptly namedcarbon-zinc batteries, and is important in many platings, as zinc is very corrosion resistant.
Gallium (Ga) is an element ingroup 13, underaluminium. Gallium is noteworthy because it has a melting point at about 303kelvins, right around room temperature. For example, it will be solid on a typical spring day, but will be liquid on a hot summer day. Gallium is an important component in the alloygalinstan, along with tin. Gallium can also be found in semiconductors.
Germanium (Ge) is an element ingroup 14. Germanium, likesilicon above it, is an importantsemiconductor and is commonly used in diodes and transistors, often in combination with arsenic. Germanium is fairly rare on Earth, leading to its comparatively late discovery. Germanium, in compounds, can sometimes irritate the eyes, skin, or lungs.
Arsenic (As) is an element ingroup 15, the pnictogens. Arsenic, as mentioned above, is often used in semiconductors in alloys with germanium. Arsenic, in pure form and some alloys, is incredibly poisonous to all multicellular life, and as such is a common component in pesticides. Arsenic was also used in some pigments before its toxicity was discovered.
Selenium (Se) is an element ingroup 16, the chalcogens. Selenium is the first nonmetal in period 4, with properties similar tosulfur. Selenium is quite rare in pure form in nature, mostly being found in minerals such aspyrite, and even then it is quite rare. Selenium is necessary for humans in trace amounts, but is toxic in larger quantities. Selenium is red in monomolar structure but metallic gray in its crystalline structure.
Bromine (Br) is an element ingroup 17 (halogen). It does not exist in elemental form in nature. Bromine is barely liquid at room temperature, boiling at about 330 kelvins. Bromine is also quite toxic and corrosive, but bromide ions, which are relatively inert, can be found inhalite, or table salt. Bromine is often used as afire retardant because many compounds can be made to release free bromine atoms.
Krypton (Kr) is anoble gas, placed underargon and overxenon. Being a noble gas, krypton rarely interacts with itself or other elements; although compounds have been detected, they are all unstable and decay rapidly, and as such, krypton is often used in fluorescent lights. Krypton, like most noble gases, is also used in lighting because of its many spectral lines and the aforementioned reasons.
Many period 4 elements find roles in controlling protein function assecondary messengers, structural components, or enzymecofactors. A gradient of potassium is used by cells to maintain amembrane potential which enablesneurotransmitter firing andfacilitated diffusion among other processes. Calcium is a common signaling molecule for proteins such ascalmodulin and plays a critical role in triggeringskeletal muscle contraction in vertebrates. Selenium is a component of thenoncanonicalamino acid,selenocysteine; proteins which contain selenocysteine are known asselenoproteins.Manganese enzymes are utilized by botheukaryotes andprokaryotes, and may play a role in the virulence of some pathogenic bacteria.[10]Vanabins, also known as vanadium-associated proteins, are found in the blood cells of some species ofsea squirts. The role of these proteins is disputed, although there is some speculation that they function as oxygen carriers. Zinc ions are used to stabilize thezinc fingermilieu of manyDNA-binding proteins.
Period 4 elements can also be foundcomplexed with organicsmall molecules to form cofactors. The most famous example of this isheme: an iron-containingporphyrin compound responsible for the oxygen-carrying function ofmyoglobin andhemoglobin as well as the catalytic activity ofcytochrome enzymes.[11]Hemocyanin replaces hemoglobin as the oxygen carrier of choice in the blood of certain invertebrates, includinghorseshoe crabs,tarantulas, andoctopuses.Vitamin B12 represents one of the few biochemical applications for cobalt.
