X和Y是原子序数大于4的短周期元素,Xm+和Yn-两种离子的核外电子排布相同,下列说法中,正确的是( )
A:X的原子半径比Y小
B:X和Y的核电荷数之差为m-n
C:电负性:X>Y
D:第一电离能:X<Y
—I haven't worked the problem out yet. What am I supposed to do?
—Try again! It’s only difficult.
我国的第一架飞机是1909年制造的。 ( )
The green sweater is only 50 ____ (dollar).