The Permian-Triassic extinction event<span> about 250 million years ago was the deadliest: More than 90 percent of all species perished. Many scientists believe an asteroid or comet triggered the massive die-off, but, again, no crater has been found. Another strong contender is flood volcanism from the Siberian Traps, a large igneous province in Russia. Impact-triggered volcanism is yet another possibility.</span>
the Bohr model, an electron's position is known precisely because it orbits the nucleus in a fixed path. In the electron cloud model, the electron's position cannot be known precisely. Only its probable location can be known.
