How do you order the elements #K, Cs,# and #P# in increasing electronegativity?
Electronegativity, is conceived to be the ability of an atom involved in a chemical bond to polarize electron density towards itself. And as we face the Periodic Table, electronegativity INCREASES across the Period from left to right, and DECREASES down a Group. This is best rationalized on the basis of nuclear charge versus shielding by other electrons. Incomplete electron shells shield the nuclear charge very ineffectively, and this is also abundantly manifested in the well-known decrease in ATOMIC radii across the Period from left to right.
And so we gots 2 alkali metals,