respiratory c's a series of nerve centers (the apneustic, pneumotaxic, and medullary respiratory centers) in the medulla and pons that coordinate respiratory movements. satiety center a group of cells in the ventromedial hypothalamus that when stimulated suppress the desire for food.. a point from which a process starts, especially a plexus or ganglion giving off nerves that control a function. 2. nerve center. 3. an agency or other site where services are offered to the public. accelerating center the vasomotor center in the brainstem involved in acceleration of heart action.
Noble Gases. The noble gases are the elements at the far right-hand column of the periodic table: helium, neon, argon, xenon, and radon. Because of the group they belong to, they each have a full valence shell or level. In the case of helium, a full valence shell has two electrons, and in the remaining noble gases, eight electrons make up a full valence shell.
Valence electrons are the electrons present in the outermost shell of an atom. You can easily determine the number of valence electrons an atom can have by looking at its Group in the periodic table. For example, atoms in Groups 1 and 2 have 1 and 2 valence electrons, respectively. Atoms in Groups 13 and 18 have 3 and 8 valence electrons, respectively. Valence electrons are responsible for the reactivity of an element. They determine how willing the elements are to bond with each other to form new compounds. If the valence shell of an element is full, such as with a noble gas, then the element does not want to gain or lose an electron.