boron has two naturally occurring isotopes: boron -10 (abundance = 19.8% , mass = 10.013 amu) boron -11 (abundance = 80.2%, mass= 11.009 amu) Calculate th eatomic mass of boron

Explanation:

eam=%abudance×mass+%abudance×mass

eam=19.8%×10.013/100+80.2%×11.009/100

eam=198.2574/100+882.9218/100

eam=1081.1792/100

eam=10.811792

eam=10.812

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.

the mantle

explanation:

earth is divided into three layers: the crust or the outermost layer on which we live; the underlying, middle layer - the mantle - and the core (the innermost layer of earth).