In a covalent bond two atoms put electrons in and share the pair. That does not mean the pair sits halfway between them. It leans toward whichever atom pulls harder, and how hard an atom pulls is its electronegativity.
The panel names at the top right open and close each panel. Choose the two elements to bond in the element panel. Drag a panel by its header to move it.
Choose two elements and the pair leans by the difference between them. Two atoms of the same element differ by zero and the pair stays in the middle: a nonpolar covalent bond.
With a difference, the more electronegative atom carries a slight negative charge and the other a slight positive one. Those are the δ− and δ+ ends, and the bond is polar covalent.
Make the difference large enough and the electrons do not lean so much as move. What is left is a cation and an anion attracting each other, which is an ionic bond. There is no line drawn between covalent and ionic; it shades from one into the other as the difference grows.