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A chemical element is a chemical substance whose atoms all have the same number of protons.The number of protons is called the atomic number of that element. For example, oxygen has an atomic number of 8, meaning each oxygen atom has 8 protons in its nucleus.
Treebeard, called by Gandalf the oldest living Ent and the oldest living thing that walks in Middle-earth, [T 1] is described as being around 14 feet (4 m) tall, "Man-like, almost Troll-like", and clad in something that might have been tree-bark, with seven toes, a bushy, "almost twiggy" beard and deep penetrating eyes.
Also in 2016, Quizlet launched "Quizlet Live", a real-time online matching game where teams compete to answer all 12 questions correctly without an incorrect answer along the way. [15] In 2017, Quizlet created a premium offering called "Quizlet Go" (later renamed "Quizlet Plus"), with additional features available for paid subscribers.
The periodic table, also known as the periodic table of the elements, is an ordered arrangement of the chemical elements into rows ("periods") and columns ("groups"). It is an icon of chemistry and is widely used in physics and other sciences.
This number is subject to change if new shorter-lived primordials are identified on Earth. [ citation needed ] One of the primordial nuclides is tantalum-180m , which is predicted to have a half-life in excess of 10 15 years, but has never been observed to decay.
The earliest evidence for life on Earth includes: 3.8 billion-year-old biogenic hematite in a banded iron formation of the Nuvvuagittuq Greenstone Belt in Canada; [30] graphite in 3.7 billion-year-old metasedimentary rocks in western Greenland; [31] and microbial mat fossils in 3.48 billion-year-old sandstone in Western Australia.
The ancient Greeks assumed the literal truth of stars attached to a celestial sphere, revolving about the Earth in one day, and a fixed Earth. [9] The Eudoxan planetary model , on which the Aristotelian and Ptolemaic models were based, was the first geometric explanation for the "wandering" of the classical planets . [ 10 ]
The first eon in Earth's history, the Hadean, begins with the Earth's formation and is followed by the Archean eon at 3.8 Ga. [2]: 145 The oldest rocks found on Earth date to about 4.0 Ga, and the oldest detrital zircon crystals in rocks to about 4.4 Ga, [34] [35] [36] soon after the formation of the Earth's crust and the Earth itself.