Why is this not a sound argument?

1. It is morally wrong to kill innocent human beings
2. Abortion is the act of killing an innocent human being
3. Abortion is morally wrong

The Answer....

The argument contains an equivocation over the term 'human being'.

We use 'human being' ambiguously sometimes meaning a biological concept, the species Homo sapiens, and other times meaning a psychological-moral concept, 'someone who has the characteristics that makes humans of special moral worth, such as rationality or rational self-consciousness.

We sometimes refer to this second concept by the term 'person'.

It is being human beings as persons, as having requisite psychological qualities, and not merely our membership in a biological group, that gives us a serious moral right to life.

But other beings may also have the required psychological qualities. perhaps apes, dolphins and Galacticans are also rationally self-conscious beings.

Then they would be persons.

And there are no doubt some Homo sapiens who do not possess minimally rational self-consciousness.

They would not be persons.

Applying this insight to our argument, we need to change the premise to read as follows:

1. It is morally wrong to kill innocent persons
2. Abortion is an act of killing an innocent member of the species Homo sapiens.
Therefore 3. Abortion is morally wrong.

If an attempt at clarification has succeeded, this argument is not sound for the original term 'human beings' was being used differently in the premises.

Further reading - very slowly - work through this.

Also read about personhood here...