Final answer:
The order of decreasing electronegativity for the given elements is Oxygen, Arsenic, Gallium, and Calcium. Electronegativity increases across a period and decreases down a group in the periodic table.
Step-by-step explanation:
The electronegativity of elements can be arranged in order of decreasing values as follows: Element O (Oxygen), Element As (Arsenic), Element Ga (Gallium), and Element Ca (Calcium). Oxygen, with the highest electronegativity among the given elements, is the most capable of attracting electrons towards itself, while calcium, with the lowest electronegativity, is the least capable in this group.
Electronegativity tends to decrease as we move down a group in the periodic table because the atomic size increases, thus electron attraction decreases. Conversely, it increases across a period from left to right due to increasing nuclear charge with the same valence shell, which better attracts electrons. As a result, Oxygen, being in the top right of the periodic table among the listed elements, has the highest electronegativity, followed by arsenic, while calcium and gallium are lower as they are to the left and downwards compared to arsenic in the periodic table.