# Why is potassium cation harder than copper?

According to HSAB theory, the smaller radius and the less polarizable an acid is, the harder it is. According to Wikipedia, the ionic radii of $\ce{ K+}$ is much larger than $\ce{Cu+}$, which makes sense because d-electrons are poor shielders. Also, $\ce{Cu+}$ has larger electron density but it has a higher effective nuclear charge in comparison to potassium ion because of the poor shielding ability of d-electrons. Yet why is $\ce{K+}$ ion considered hard while $\ce{Cu+}$ ions are considered soft?

• I have misspelt harder in the title can any privileged user/OP take a look. – Avyansh Katiyar Mar 20 '18 at 15:18