A chemist needs to know the mass of a sample of to significant digits. She puts the sample on a digital scale. This is what the scale shows: 0 0 7 6 . 2 1 g If this measurement is precise enough for the chemist, round it to significant digits. Otherwise, press the "No solution" button.

The mass of the sample to 2 significant figures = 76 g

Explanation:

In any type of scientific measurement, the precision of the measurement is expressed in the significant digits of that measurement. It also used to express measurement to the required degree of accuracy.

Significant digits include every digit except the leading zero(s).

And if the number after the required significant digits is not up to 5, it is rounded down and the required significant digits is written as is. But if the number after the required significant digits is at least 5, it rounded up, and the last number on the significant digits requirement is increased by a factor of 1.

Examples include

0.3547 = 0.355 to 3 s.f (0.0007 is rounded up to increase the next digit, 4, by a factor of 1)

7.399 = 0.40 to 2 s.f

Hence, the given mass, 76.21 g becomes 76 g to 2 significant figures since the next digit after the required significant digits, 2, is less than 5, it is rounded down.

76.21 = 76.2 to 3 s.f

76.21 = 76 to 2 s.f

76.21 = 80 to 1 s.f

I hope you get it now!

atomic mass = 39

number of neutrons = 20

number of protons= 19

electronic configuration = 1s2 2s2 2p6 3s2 3p6 4s1

the number of electron in first shell = 2

number of electrons in n=3 shell = 8