1) How did Rutherford's experiments with alpha particles suggest that most of the mass of an atom is concentrated in a small positively charged nucleus?
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because the alpha particles (positively charged) mostly passed right through without deflecting, but some of them were ricocheted right back in the direction they came from like they hit a brick wall. Ernest could calculate the size of the hard center (nucleus) based on the proportion that bounced back and it accorded extremely well with his experimental results. Amazing!