I was going through my chemistry textbook (IB Pearson), and it explicitly stated that the higher the absolute difference between the electronegativity of elements in a binary compound, the more 'ionic' that compound is, which does make intuitive sense given that a higher electronegativity difference leads to the compound being more 'polar.'
However, the book came up with the following example:
Which of the following pairs will form the most ionic compound?
1) Be and F (the electronegativity difference is $|4-1.6| = 2.4)$
2) Si and O (the electronegativity difference is $|3.4-1.9| = 1.5)$
3) N and Cl (the electronegativity difference is $|3.2-3| = 0.2)$
4) K and S (the electronegativity difference is $|2.6-0.8| = 1.8)$$\ce{K2S}$ will be the most ionic compound as the difference in electronegativity is the greatest.
I don't get how this works, the difference in electronegativity between beryllium and fluorine is visible greater, so why does the book say that the binary compound formed by K and S is more ionic?