Vitamin A is an essential vitamin that the body needs for growth and development. It is also vital for cell recognition, vision, immune function, and reproduction.
It also helps the heart, lungs, kidneys, and other organs function correctly.
Vitamin A deficiency is rare in the United States. If it happens, it can lead to eye problems, including night blindness.
In this article, learn more about the types of vitamin A, what vitamin A does, and some good sources of it.
A. new ocean ridges and seafloor
<span>All metals have similar properties BUT, there can be wide variations in melting point, boiling point, density, electrical conductivity and physical strength.<span>To explain the physical properties of metals like iron or sodium we need a more sophisticated picture than a simple particle model of atoms all lined up in close packed rows and layers, though this picture is correctly described as another example of a giant lattice held together by metallic bonding.</span><span>A giant metallic lattice – the <span>crystal lattice of metals consists of ions (NOT atoms) </span>surrounded by a 'sea of electrons' that form the giant lattice (2D diagram above right).</span><span>The outer electrons (–) from the original metal atoms are free to move around between the positive metal ions formed (+).</span><span>These 'free' or 'delocalised' electrons from the outer shell of the metal atoms are the 'electronic glue' holding the particles together.</span><span>There is a strong electrical force of attraction between these <span>free electrons </span>(mobile electrons or 'sea' of delocalised electrons)<span> (–)</span> and the 'immobile' positive metal ions (+) that form the giant lattice and this is the metallic bond. The attractive force acts in all directions.</span><span>Metallic bonding is not directional like covalent bonding, it is like ionic bonding in the sense that the force of attraction between the positive metal ions and the mobile electrons acts in every direction about the fixed (immobile) metal ions of the metal crystal lattice, but in ionic lattices none of the ions are mobile. a big difference between a metal bond and an ionic bond.</span><span>Metals can become weakened when repeatedly stressed and strained.<span><span>This can lead to faults developing in the metal structure called 'metal fatigue' or 'stress fractures'.</span><span>If the metal fatigue is significant it can lead to the collapse of a metal structure.</span></span></span></span>
Answer: If the intermolecular forces are weak, then molecules can break out of the solid or liquid more easily into the gas phase. Consider two different liquids, one polar one not, contained in two separate boxes. We would expect the molecules to more easily break away from the bulk for the non-polar case. If the molecules are held tightly together by strong intermolecular forces, few of the molecules will have enough kinetic energy to separate from each other. They will stay in the liquid phase, and the rate of evaporation will be low. ... They will escape from the liquid phase, and the rate of evaporation will be high. To make water evaporate, energy has to be added. The water molecules in the water absorb that energy individually. Due to this absorption of energy the hydrogen bonds connecting water molecules to one another will break.
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