$\mathrm{Pt}$ and $\mathrm{Au}$ are almost unreactive. They are called noble metals.

Two of the least reactive, stable metals are gold and platinum. They are extremely stable and do not readily oxidise or react with other substances or instructions and they act inert in any reaction. Acids do not readily damage precious metals. Aqua regia, an acidic fluid, dissolves both platinum and gold.

Therefore, they are known as noble metals due to their inert behaviour like noble gases.