With what allotrope of oxygen is it isoelectronic in NO2- ?
The term isoelectronic means the same number of electrons. Then you need to count the number of electrons in the given species NO₂⁻. You have to count the number of electrons of N (nitrogen) atoms, and the number of electrons of two O (oxygen) atoms, and then adjust the negative charge of the ions.
The atomic number of N is 7, meaning that each atom has 7 protons and 7 electrons. The atomic number of O is 8, meaning that each atom has 8 protons and 8 electrons. The negative charge of the ion means that it has one extra electron, which is responsible for the negative charge. Therefore the total number of electrons of the ion NO₂⁻ is 7 + 2 (8) + 1 = 24.
Note that 24 = 3 x 8, so 24 is the number of electrons in 3 atoms of oxygen. The molecule O₃ is an allotrope of oxygen. (an allotrope is a form of an element with different structures and physical properties in the same physical state).
Therefore, the allotrope of oxygen isoelectronic with NO₂⁻ is O₃ (ozone)