Most of these primary headaches are tension headaches. Most people with tension headaches have "episodic" tension headaches that come and go. Only 3.3% of adults have chronic tension headaches, with headaches for more than 15 days in a month.Approximately 12–18% of people in the world have migraines. More women than men experience migraines. In Europe and North America, 5–9% of men experience migraines, while 12–25% of women experience migraines.Cluster headaches are relatively uncommon. They affect only 1–3 per thousand people in the world. Cluster headaches affect approximately three times as many men as women. History
The first recorded classification system was published by Aretaeus of Cappadocia, a medical scholar of Greco-Roman antiquity. He made a distinction between three different types of headache: i) cephalalgia, by which he indicates a shortlisting, mild headache; ii) cephalea, referring to a chronic type of headache; and iii) heterochromia, a paroxysmal headache on one side of the head. Another classification system that resembles the modern ones was published by Thomas Willis, in De Cephalalgia in 1672. In 1787 Christian Baur generally divided headaches into idiopathic (primary headaches) and symptomatic (secondary ones), and defined 84 categories. Children
In general, children experience the same types of headaches as adults do, but their symptoms may be slightly different. The diagnostic approach to headaches in children is similar to that of adults. However, young children may not be able to verbalize pain well. If a young child is fussy, they may have a headache.Approximately 1% of emergency department visits for children are for headache.
In oxidation state +3, bismuth forms trihalides with all of the halogens: BiF3, BiCl3, BiBr3, and BiI3. All of these except BiF3 are hydrolyzed by water.Bismuth(III) chloride reacts with hydrogen chloride in ether solution to produce the acid HBiCl4.The oxidation state +5 is less frequently encountered. One such compound is BiF5, a powerful oxidizing and fluorinating agent. It is also a strong fluoride acceptor, reacting with xenon tetrafluoride to form the XeF+3 cation:
BiF5 + XeF4 → XeF+3BiF−6

Aqueous species
In aqueous solution, the Bi3+ ion is solvated to form the aqua ion Bi(H2O)3+8 in strongly acidic conditions. At pH > 0 polynuclear species exist, the most important of which is believed to be the octahedral complex [Bi6O4(OH)4]6+. Occurrence and production
In the Earths crust, bismuth is about twice as abundant as gold. The most important ores of bismuth are bismuthinite and bismite. Native bismuth is known from Australia, Bolivia, and China. The difference between mining and refining production reflects bismuths status as a byproduct of extraction of other metals such as lead, copper, tin, molybdenum and tungsten. World bismuth production from refineries is a more complete and reliable statistic.Bismuth travels in crude lead bullion (which can contain up to 10% bismuth) through several stages of refining, until it is removed by the Kroll-Betterton process which separates the impurities as slag, or the electrolytic Betts process. Bismuth will behave similarly with another of its major metals, copper. The raw bismuth metal from both processes contains still considerable amounts of other metals, foremost lead.