The atomic number of the chemical element calcium is 20 and its symbol is the letter Ca. Calcium is an alkaline earth metal that reacts with air to form a black oxide-nitride coating. Its heavier homologues, barium and strontium, are the most similar in physical and chemical properties. It is the third most abundant metal after iron and aluminum and the fifth most abundant element in the earth's crust. The most common calcium component on Earth is calcium carbonate, which can be found in limestone and fossilized remains of ancient marine life. Other calcium-rich materials include gypsum, anhydrite, fluorite, and apatite. The word “lime” comes from the Latin word calx, which is obtained by heating limestone.
Several calcium compounds were known to the ancients, although their chemistry was not fully understood until the seventeenth century. Humphry Davy, after whom the element is named, electrolyzed calcium oxide in 1808 to isolate pure calcium. Numerous industries use calcium compounds extensively, including the food and pharmaceutical industries for calcium supplements, the bleaching paper industry, the electrical insulator and cement industries, and the soap industry. On the other hand, the metal has limited use in pure form due to its high reactivity. However, it is often used in small quantities as an alloying agent in steel production and sometimes as a calcium-lead alloy in the manufacture of automobile batteries.
Calcium is the fifth most common element and the most common metal in the human body. Calcium ions (Ca2+) as electrolytes are essential for the physiological and biochemical functions of organisms and cells; It acts as a second messenger in signal transmission pathways, releases neurotransmitters from neurons, contracts all types of muscle cells, is a cofactor in many enzymes and ensures fertilization. Calcium ions outside the cells are very important for maintaining the potential difference between excitable cell membranes, protein synthesis and bone formation.
The properties of the lighter elements in its group, strontium, barium, and radium, are extremely similar to those of calcium, a very ductile silvery metal (usually characterized as pale yellow). Twenty electrons grouped in the [Ar]4s2 electron configuration make up a calcium atom. The two valence electrons in the outermost s-orbital of calcium, like other elements in group 2 of the periodic table, are extremely easily lost in chemical reactions to form a dipopositive ion with the stable electron configuration of a noble gas, in this case argon.
As a result, calcium is almost always divalent in its typical ionic compounds. Given that MX2's enthalpy of formation is significantly higher than that of hypothetical MX, hypothetical monovalent calcium salts would be stable in their element, but not disproportionately stable to divalent salts and calcium metal. This happens as a result of the significantly higher lattice energy provided by the more strongly charged Ca2+ cation than the fictitious Ca+ cation.
The alkaline earth metals are typically defined as calcium, strontium, barium, and radium, while the lighter elements belonging to group 2 of the periodic table may also refer to beryllium and magnesium. Although they behave more like aluminum and zinc, respectively, and have some weak metallic properties of post-transition metals, beryllium and magnesium differ significantly from other members of the group in their physical and chemical behavior. Therefore, the traditional definition of the term "alkali earth metal" excludes them.
Physical Properties of Calcium Element
The melting and boiling points of calcium metal are higher than those of the two close group 842 metals, strontium and magnesium, at 1494 and 2 degrees Celsius, respectively. It forms crystals with a face-centered cubic configuration similar to strontium and transitions to an anisotropic hexagonal close-packed arrangement similar to magnesium above 450 °C. The lowest density of its group is 1,55 g/cm3.
Lead is harder than calcium, but with some effort, calcium can be sliced with a knife. Calcium has an extremely low density, making it a better conductor by mass than copper or aluminum, but a worse conductor of electricity by volume. Because it interacts rapidly with air oxygen, calcium cannot be used as a conductor in most terrestrial applications, but it has been studied in space applications.
Calcium complexes typically have a high coordination number, which is indicated by the structure of the polymeric [Ca(H2O)6]2+ core in hydrated calcium chloride.
Calcium has the characteristic chemistry of a heavy alkaline earth metal. For example, calcium spontaneously interacts with water faster than magnesium and less rapidly than strontium to form calcium hydroxide and hydrogen gas. Also, when combined with oxygen and nitrogen in the air, it forms a mixture of calcium oxide and calcium nitride. When finely separated, it spontaneously burns in air to form nitride.
Calcium is less reactive in bulk; It forms a hydration layer in a short time in humid air, but can be kept indefinitely at room temperature below 30% RH.
In addition to the simple oxide CaO, calcium metal can be directly oxidized to produce the peroxide CaO2 and a yellow superoxide Ca(O2)2, both of which have been observed. Although not as strong as hydroxides of strontium, barium, or alkali metals, calcium hydroxide or Ca(OH)2 is a strong base. There are four known calcium dihalides. Particularly abundant minerals are calcium carbonate (CaCO3) and calcium sulfate (CaSO4).
Calcium metal dissolves instantly in liquid ammonia to produce a dark blue solution, just as strontium and barium, as well as the alkali metals and divalent lanthanides europium and ytterbium.
Large calcium ions (Ca2+) often have high coordination numbers of up to 13 in some intermetallic complexes such as CaZn24. Oxygen chelates such as EDTA and polyphosphates, which are helpful in analytical chemistry to remove calcium ions from hard water, easily complex calcium. Smaller group 2 cations typically form stronger complexes in the absence of steric hindrance, but this trend can be reversed when large polydentate macrocyclics are present.
Although organocalcium compounds are in the same chemical family as magnesium, they are not used as often as organomagnesium compounds in chemistry. This is because they are more difficult and more reactive to synthesize, despite recent research as potential catalysts. Due to the similar ionic radii of Yb2+ (102 pm) and Ca2+, organocalcium compounds resemble organoytterbium compounds more frequently (100 pm).
Bulky ligands tend to favor stability and moderate temperatures are typically required to form most of these compounds. For example, the production of calcium dicyclopentadienyl, Ca(C5H5)2 requires the direct reaction of calcium metal with mercurocene or cyclopentadiene, while the solubility, volatility and kinetic stability of the compound are improved by replacing the C5H5 ligand with the more voluminous C5(CH3)5 ligand.
Günceleme: 02/02/2023 22:55
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