Bonds between atoms break and new ones form and energy is either given out or taken in
Hydrogen H weight: 81
Non-metal
Hydrogen is the simplest element; an atom consists of only one proton and one electron. It is also the most plentiful element in the universe. Despite its simplicity and abundance, hydrogen doesn't occur naturally as a gas on the Earth--it is always combined with other elements.
period 1 group 1
Hydrogen is easily the most abundant element in the universe. It is found in the sun and most of the stars, and the planet Jupiter is composed mostly of hydrogen. On Earth, hydrogen is found in the greatest quantities as water.
In an exothermic reaction, heat is produced along with products. But in endothermic reaction, heat is absorbed. If the number of products is less than the sum of the enthalpies of the reactants, the reaction will be exothermic. If the number of products is more than the sum of the enthalpies of the reactants, the reaction is endothermic.
