A stable electronic configuration requires elements with valency 1 to gain one electron or lose one electron. The atomic number 17 of chlorine is an example. In terms of electronic configuration, it has a 2,8,7 configuration. As a result, it accepts one electron to form Cl. The octet configuration becomes stable. In this case, the element is a non-metal. The electronic configuration of sodium with atomic number 11 is 2,8,1, in contrast to elements like mercury with atomic number 12. Losing the outermost electron makes it easier to form Na+. Metals are such elements. https://www.pw.live/question-answer/elements-with-valency-1-are-17904