Period 4 element be dangerous in large enough quantities due to its radioactivity. Nickel (Ni) is an element in period 4, between cobalt and copper. Nickel is rare in the Earth's crust, mainly due to the fact that it reacts with oxygen in the air, with most of the nickel on Earth coming from nickel iron meteorites. However, nickel is incredibly common in the Earth's core; along with iron it is one of the two main components. Nickel is an important component of stainless steel, and in many superalloys. Copper (Cu) is an element in period 4, between nickel and zinc. Copper is
CU convoys CU 23, CU 24, CU 28, CU 30, CU 33, and CU 35 included troopships and are sometimes identified with a TCU prefix. Convoy CU 24 sailed in two sections with troopships in TCU 24A and ammunition ships in TCU 24B. A total of 2255 ships crossed the Atlantic in CU convoys until convoy CU 73 made the last departure from New York on 30 May 1945. Submarines sank three loaded United States tankers from CU convoys: Empty ships travelling westbound on similar routes were designated UC convoys. Seventeen ballasted tankers departed from Liverpool on 15 February 1943 as convoy