Final answer:
Cesium, because of its low electronegativity of 0.79, is the least likely to hold on to its valence electrons in the presence of a highly electronegative atom. Electronegativity increases from left to right and decreases down a group in the periodic table, making group 1 metals like cesium more prone to losing electrons.
Step-by-step explanation:
In the presence of a highly electronegative atom, the element least likely to hold on to its valence electrons would be one that has a low electronegativity. According to Pauling's electronegativity values, cesium is the least electronegative nonradioactive element, with a value of 0.79. Electronegativity tends to increase across a period from left to right and decrease down a group in the periodic table. Given these trends, metals, and specifically group 1 metals like cesium, are less electronegative and therefore more likely to lose their valence electrons when interacting with more electronegative elements such as fluorine or oxygen.
Moreover, as elements increase diagonally from the lower left to the upper right of the periodic table, their electronegativity values generally increase. This means elements like fluorine, being the most electronegative with a value of 4.0, will likely attract electrons from other atoms, while less electronegative elements will not retain their electrons as strongly. Compounds formed between elements of differing electronegativities will often result in polar covalent bonds, with the electron density localized around the more electronegative atom.