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Session 1: Atoms – the building blocks of matter

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This diagram shows 5 boxes in a row, from left to right, the first is labelled 1s, the second is labelled 2s, and the three on the right hand side are positioned close together and collectively labelled 2p. Single headed arrows are arranged in each box. The boxes labelled 1s and 2s contain two arrows oriented in opposite directions. The 2p boxes each contain one arrow oriented in the same direction.