The term 'salt of element #53' refers to compounds containing the element iodine. The two crossword solutions for this term are iodate and iodide, both of which contain six letters. Iodate is an anion composed of one iodine atom and three oxygen atoms, while iodide is an anion composed of one iodine atom and one hydrogen atom. Both iodate and iodide are important in biochemistry, as they are essential components of thyroid hormones.