The first element whose atomic number is 21 is Scandium and the second element whose atomic number is 7 is Nitrogen. Based on electronic configuration
Electronic configuration is a systematic way of distributing the electrons in their orbitals Electron configurations of atoms follow a standard notation in which all electron-containing atomic subshells (with the number of electrons they hold written in superscript) are placed in a sequence.
Electron Configurations are useful for:
Determining the valency of an element.
Predicting the properties of a group of elements (elements with similar electron configurations tend to exhibit similar properties).
Interpreting atomic spectra.
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