1) for the 2 bit adder why was a half-adder and full-adder used instead of 2 full adders?

2) how many IC's (and types) would you need to use to contruct an 8-bit adder?

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I haven't researched it, but it seems to me that a full adder produces a carry bit, which is used to propagate to the next bit. The MSB just overflows, so no carry is needed, making a half-adder the "logical" choice.

Of course, using a full adder for the MSB would enable detection of overflow.