Difference between serendipity and Lagrange elements in the Finite Element Method

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As an engineer I often use the finte element method to solve solid mechanics problems, both by commercial software and by my own (matlab) code. When using quadrilateral and hexahedral elements there are two types called Lagrange elements and 'serendipity' elements (typically missing the midside nodes). I have previously encountered convergence issues when using serendipity elements, so I was wondering what the practical differences between the two is?