Sulfur has electronegativity of 2.5 which is higher than that of 2.1 for hydrogen. Its electronic configuration is: [Ne] 3s2 2p4. Since sulfur can attract only two electrons to complete the 3rd level (octet rule) it forms H2S with hydrogen. On contrary, the fluorine atom shows the highest electronegativity of 4.0. It attracts electrons from Sulfur to finish its [He] 2s2 2p5 level. Since sulfur can donate 6 electrons and fluorine can accept only one, they form SF6.