An element has three naturally occuring isotopes: 90.48% is element-20 (mass = 19.992 amu), 0.27% is element-21 (mass - 20.994 amu), and the remainder is element-22 (mass = 21.991 amu). Calculate the average atomic mass of the element, rounded to three decimal places.

Average atomic mass  = 20.180 amu.

Explanation:

Given data:

Abundance of X-20 = 90.48%

Atomic mass of X-20 = 19.992 amu

Abundance of X-21 = 0.27%

Atomic mass of X-21 = 20.994 amu

Abundance of X-22 =  9.25%

Atomic mass of X-22 = 21.991 amu

Average atomic mass = ?

Solution:

Average atomic mass  = (abundance of 1st isotope × its atomic mass) +(abundance of 2nd isotope × its atomic mass) +(abundance of 3rd isotope × its atomic mass)   / 100

Average atomic mass = ( 90.48×19.992)+(0.27×20.994)+(9.25+ 21.991 ) /100

Average atomic mass = 1808.88  + 5.668+203.42 / 100

Average atomic mass  = 2017.968 / 100

Average atomic mass  = 20.180 amu.

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.