Answer (score: 1): Z-Which two atoms of elements lack two electrons to complete the external level? (1) Br (2) Se 3) 0 4) F 5) Li Answer (score: 1):

The question is asking which two atoms of elements lack two electrons to complete their outermost electron shell. Atoms strive to have a full outermost electron shell, which typically consists of eight electrons. This is known as the octet rule. <br /><br />1. Br (Bromine) has an atomic number of 35, which means it has 35 electrons. Its electron configuration is [Ar] 3d10 4s2 4p5. It has seven electrons in its outermost shell and needs one more electron to complete it.<br /> <br />2. Se (Selenium) has an atomic number of 34, which means it has 34 electrons. Its electron configuration is [Ar] 3d10 4s2 4p4. It has six electrons in its outermost shell and needs two more electrons to complete it.<br /><br />3. O (Oxygen) has an atomic number of 8, which means it has 8 electrons. Its electron configuration is 1s2 2s2 2p4. It has six electrons in its outermost shell and needs two more electrons to complete it.<br /><br />4. F (Fluorine) has an atomic number of 9, which means it has 9 electrons. Its electron configuration is 1s2 2s2 2p5. It has seven electrons in its outermost shell and needs one more electron to complete it.<br /><br />5. Li (Lithium) has an atomic number of 3, which means it has 3 electrons. Its electron configuration is 1s2 2s1. It has one electron in its outermost shell and needs one more electron to complete it.<br /><br />From the above analysis, Se and O are the two elements that lack two electrons to complete their outermost electron shell.