The trend of the electronegativy in the periodic table is that it grows in a row from left to right, and decreases in a column from up to bottom.
That leads to the fact the F is the most electronegative element (noble gases do not count for electronegativity because they have the valence electron sehll full).
So, you can compate electronegativities of elements in a same row or in a same column. Also if an element is upper and more to the right than other you know that it is more electroneative.
From that, you cannot compare P, C and Br, because they are more or less in the same diagonal. and you cannot infere from that the correct order of their relative electronegativities.
You can compare Mg elements in the same row and column:
In its row, Mg is more electronegative than Na and less electronegative than Al, Si, P, S and Cl
In its column, Mg is less electronegative than Be and more electronegative than Ca, Sr, Ba and Ra.
That is how you can compare: elements to the right are more electronetatives than other elements in the same row that are more to the left; elements upper are more electronegative than other elements in the same column.
