An atom becomes a positive ion when it loses a valence electron.

An atom becomes a positive ion, also known as a cation, when it loses one or more of its valence electrons. This process occurs because the loss of negatively charged electrons results in an overall positive charge in the atom. Valence electrons are the outermost electrons of an atom and are crucial for defining its chemical properties and interactions.

When an atom loses a valence electron, it not only changes its charge from neutral to positive but also typically alters its reactivity and bonding capabilities. For example, alkali metals like sodium lose one valence electron to form Na⁺ ions, becoming more stable in this ionic form.

The other options introduce ambiguity or conditions that do not apply in the general context of atomic behavior. The statement provided holds true across all elements that can lose electrons to become positive ions, rendering the notion of needing more information or specific conditions unnecessary. Thus, stating that an atom becomes a positive ion when it loses a valence electron is fundamentally accurate.