4. Lấy bao nhiêu phần khối lượng của nguyên tử cacbon làm đơn vị cacbon? Nguyên tử khối là gì? 5. Hãy so sánh nguyên tử magie nặng / nhẹ bao nhiêu lần so với: cacbon, lưu huỳnh, nhôm. 6. Nguyên tử `X` nặng gấp 2 lần nguyên tử nitơ. Tính nguyên tử khối của X và cho biết X thuộc nguyên tố nào. Viết kí hiệu hoá học của nguyên tử đó. 7. a) Theo giá trị khối lượng tính bằng gam của nguyên tử cacbon cho trong bài học hãy tính xem một đơn vị cacbon tương ứng với bao nhiêu gam. b) Khối lượng tính bằng gam của nguyên tử nhôm: `A. 5,342 . 10^{-23} g` `B. 6,023.10^{-23}g` `C. 4,482. 10^{-23}`g` `D. 3,990. 10^{-23}`g`

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