Atkins, P. W., & De Paula, J. (2006). Physical Chemistry. Oxford University Press.
In a gas, the atoms or molecules are widely spaced and are free to move in any direction. Gases have neither a definite shape nor a definite volume, and will expand to fill their container. The intermolecular forces in a gas are very weak, and are often negligible. states of matter goodstein pdf
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Goodstein, D. L. (2002). States of Matter. Dover Publications. Atkins, P