16 ， 3 

试题分析：8.4 g N₂含有的N原子为8.4/28×2=0.6mol，设该单质的摩尔质量为M，则
0.3/9.6/M=" 3:2" ，M=48g/mol，x=0.6/（9.6/48）=3，R的相对原子质量=48/3=16.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
点评：本题考查了化学的基本计算，给出的数字较多，要合理利用上述数字，该题难度适中。