As children move towards middle and late childhood, the relationship between physical activity and motor skill competence evolves and strengthens (Stodden et al. 2008). During middle childhood, a childâs perceived motor competence more closely estimates their actual motor skill competence.
In 1842 Gustav Mosander separated yttria, found in the mineral gadolinite, into three fractions which he called yttria, erbia, and terbia. The names erbia and terbia became confused in this early period. After 1860, Mosander's terbia was known as erbia, and after 1877, the earlier known erbia became terbia. The erbia of this period was later shown to consist of five oxides, now known as erbia, scandia, holmia, thulia and ytterbia.
Discovery of Iridium. Iridium was discovered in 1803, by English chemist Smithson Tennant in London. He found it in the residue left when crude platinum had been dissolved in aqua regia (a mixture of hydrochloric acid and nitric acid). Smithson Tennant also discovered osmium at the same time.ource: Iridium is found in natural alloys with platinum and osmium in alluvial deposits. Commercially, iridium is recovered as a by-product from the nickel mining industry. Isotopes: Iridium has 34 isotopes whose half-lives are known, with mass numbers from 165 to 198.