its easy to get confused with the Metal Chart, because its not a Pure Chart, its been Changed many times. First, you have the Natural 16 Metals, which are then changed by Ruin and Preservation by replacing two of the Metals with Atium and Larasium. After the Rise of The Lord Ruler, he suppreses public knowledge of the chart, and the Noble families, start keeping secrets from each other, resulting in the Devolution of the chart to 10 metals, 8 Basic Metals, and 2 Time Metals. As the books progress, new Metals are rediscovered by Vin, and through experimentation(mostly on Herself), new Alloys, which brings the chart up to 15 at the end of the Final book. (note: 8 basic, gold + alloy, atium + alloy, aluminum + alloy, larasium) At the End of the Book, Sazed withdraws the two God Metals + alloys, and restores the two Natural Metals, + alloys.
A quick rundown of the metals remain the same. Iron, Tin, Zinc, Copper, Aluminum, and Gold. and their Alloys, Steel, Pewter, Brass, Bronze, Duralumin, and Electrum. Atium is not a Natural Metal, or a Natural part of the Chart, it was Stapled onto it (by Preservation) as a way to weaken Ruin. It, and its Alloy Replaced one of the time metals on the Chart. Larasium is also Not a Natural Metal, and it Replaces one of the Enhancement metals.
Once Sazed gathered all the Ruin/Preservation powers to himself, he stopped the Release of Atium/Larasium into the world, and restored the Original Two Metals, Chromium, and Cadmium to their spots. With their alloys, this brings the chart back up to 16 Natural Metals. If in a later book Brandon returns, their will remain the possibility of Sazed Releasing the two God Metals back. but we don't know if he will, or will do something different and cool.
this is a bit of rambling explanation.