Question

How many core electrons does an atom of beryllium (Be) contain? 

A) 0 B) 1 C) 2 D)3 E) 4

Which of the following metals would be expected to have the smallest atomic radius? 

A) Cesium (Cs) B) Barium (Ba)  C) Lanthanum (La)  D) Europium (Eu) E) Tantalum (Ta)

Rank the following atoms in order of decreasing size (i.e., largest to smallest): Rb, F, Mg, B, N. 

A) Rb > Mg > B>N>F B) N > F >B> Mg > Rb C) Rb > Mg > F>N>B D) Mg > Rb > B>N>F 11 E) Rb > Mg >N>B>F

Answer 1

(1)Be : Atomic number = 4

EC = [He] 2s²

Electrons per shell : 2,2

Therefore Be has 2 valence electrons.

Core electrons are electrons which don't take part in bonding, electrons except valence electrons.

Core electrons in Be :: 2

(2)Along period atomic radius trend decry from left to right.

All these elements are in same period.

La ,Eu are in lanthanide series in lanthanide series there is slight decrease in atomic radius along period .

Clearly Ta being on rightmost side Ta is smallest.

(3) Along period atomic radius trend decreases from left to right.

Keeping that in mind we can compare B, N,F being in same period B>N>F.

Along group atomic radius trend increases from top to bottom

Therefore Rb is much bigger than Mg

Rb>Mg>B>N>F