First things first, the mark scheme avaialable does not contai the full range of answers. The mark scheme is a rough guide to marking, and it does not mean that answers that are not mentioned in mark scheme r the only possible answers.
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Following is an extract from an edexcel mark scheme:
General Guidance on Marking
Examiners should look for qualities to reward rather than faults to penalise. This does not mean giving credit for incorrect or inadequate answers, but it does mean allowing candidates to be rewarded for answers showing correct application of principles and knowledge, and for critical and imaginative thinking. Examiners should therefore read carefully and consider every response; even if it is not what is expected it may be worthy of
credit.
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There are many questions which have multiple possibilities for answers, all of which recieve equal credit in most cases. The mark scheme is simple a list of very concise, short bullet-points. To get full marks, u have to imply those key points through effective communication.
There are some questions, especially those like definitions of electron affinity, inoisation energy etc, which require the key points for any marks to be gained. Still, there r alternatives. For example, if the examiner asks:
Define first electron affinity......then u can either write it descrptively , or u can say "the enery change per mole for : and then give the equation".
To sum up, the mark scheme is important, but it is in no way exhaustive.