My initial vote was Hydrogen, which readily loses its single electron. However my first punt was not correct: The element most likely to lose an electron is the element having the lowest ionization potential. That element is Caesium. This stands to reason, as Caesium is one of the largest elements in Group I of the Periodic Table of Elements.
Ionization potentials of Group I elements:
Hydrogen H is 13.598 eV (electron Volts)
Lithium Li is 5.39 eV
Sodium Na is 5.14 eV
Potassium K is 4.34 eV
Rubidium Rb is 4.18 eV
Caesium Cs is 3.89 eV
Francium Fr is 4.07 eV
Note that these are the ionization energies for losing the first electron. It takes considerably more energy to lose a second electron, and so on.
The ionization energy of Cesium is actually 0.79 eV but Francium has an ionization energy of 0.70 eV making it have the lowest energy on the chart. The reason why is because the outermost orbit is further away from the nucleus of the atom making it easier for another element to take it that would be in proportion to someone trying to protect a baby on the other side of the world I think someone in China or wherever would be able to take it no problem. The element with the highest ionization energy is Florine with a charge of 3.98 eV. Florine is very close to completing the octet rule so it is trying to take an electron from anything it can in order to complete that last orbit. A good way of putting this is that elements on the left side of the periodic table are more likely to lose and electron while the elements on the right side are trying hard to take whatever electrons they need to complete the octet rule.
All information came from "Chemistry: Matter and Chance" Copyright 2008 by The McGraw-Hill Companies, Inc.
The elements of first group of periodic table are known as alkali metals. Alkali metals have only one electron in their outer most shell. Due to the incomplete valence shell, alkali metals are unstable. Alkali metals can complete their outer most shell by losing one electron. When alkali metals lose their valence electron, alkali metal ions are formed. These ions get the electronic configuration of their nearest noble gases. So, alkali metals most likely to lose an electron.
Hope this helps.
mingo7 : )