They are the hydrogen and oxygen of the water itself. After that it's chlorine and sodium.
sodium and chlorine
You think probable to sodium chloride (NaCl).
When the two most abundant elements in seawater, sodium and chloride ions, combine, they form sodium chloride salt (NaCl), which is commonly known as table salt. This compound makes up the majority of the dissolved solids in seawater.
You think probable to sodium chloride (NaCl).
These elements are transported by rivers to the sea. Another source is the volcanic activity.
The most common element dissolved in seawater is chlorine, primarily in the form of chloride ions (Cl⁻). Chloride ions account for about 55% of the total dissolved salts in seawater. Sodium, in the form of sodium ions (Na⁺), is the second most abundant, and together, these two elements contribute to the salinity of ocean water.
potassium and sulfate
Basically all of them. However, most stars contain large amounts of hydrogen (since that's the most common element in the Universe), as well as helium, as well as smaller amounts of other elements.
Hydrogen is not made up of any other elements but hydrogen. The hydrogen atom has 1 proton, 1 electron, and the most common isotope has no neutrons. The other two uncommon isotopes have one or two neutrons.
Sodium chloride, potassium chloride, magnesium chloride
Chlorides salts of Na, Mg, Ca.
The two most common elements in the Universe, and in most stars are - in that order - hydrogen and helium (elements #1 and #2).