Potassium has many isotopes, but the half life of K40 is listed as 1.28x109 years.
This is a much used radioisotope.
Bananas which are rich in potassium have been known to set off radiation alarms at the shipping port, because of the K40 - Ar40 decay.
And another potassium trivia; you'll accumulate about as much radiation by sleeping alongside your partner (who has K40 decay proceeding) as you will get from the ionization type smoke detector.
Chromium-50 is the only naturally radioactive isotope of chromium. It has a half-life of 1.8x1017years. It decays to Titanium-50.
chlorides half life is either 308,000 or 300,000
The half-life of iron is near infinity. Iron is a stable element.
For the half lives of all curium isotopes see the link below.
Each isotope of technetium has another half life.
See the list of Tc isotopes at this link.
98.5h
120 days
108 h
infinite