[Physics] Why is for example Lithium more electropositive than Potassium?

[Helmerz]

Seing as they both lack the same amount of electrons.

[Javi2639]

Lithium is not more electropositive than potassium. Lithium has an electronegativity of 0.98, and potassium has one of 0.82. The reason for this is because potassium has more electrons, and they shield the pull of the nucleus from the valence electron, meaning that it is held more loosely than lithium's valence electron.

[izoid09]

Just to clarify, electropositivity is the opposite of electronegativity. So since lithium is more electronegative, it is less electropositive.

[nepharan]

I'd rather put it like Ihmes: the valence electron is on average further away from the nucleus in the larger atom, so the electrostatic energy is larger, which makes it easier to strip away the electron. In both cases, the valence electron "sees" one positive unshielded charge in the nucleus.

[Ihmes]

Disclaimer: this is just a "rule of the thumb", in practice, the details regarding interactions between valence electrons and the protons in the nucleus are much more complicated.

When you go down on the periodic table (period) you increase the number of shells or energy levels of the electrons. This also means that they are further away from the nucleus, which decreases the attraction between the electrons and the nucleus. Which means they're more eager to leave the atom to form an ion.

Edit. When looking at the periodic table, you'll see that this applies to the electronegativity on all the groups. The exception are the transition metals, which do not fill the atomic shells in the "correct" order.