Question

an atom has an atomic mass of 7u. it has 2 electrons in K-shell and one electron in L-shell. how many neutrons does it have?

Answer 1

Mass of atom = 7u

No. of electrons = 2+1 = 3

As we know that,

No. of proton = No. of electron

∴No. of protons = 3

let the no. of neutrons be 'n'

As we know,

Mass of atom = no.of protons + no. of neutrons

substituting the values, we get,

7 = 3 + n

⇒ 7 - 3 = n

⇒ 4 = n

or, n = 4

∴ No. of neutrons in the atom is 4

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Answer 2

It has 3 electrons
As number of:
protons=electrons
So, it has 3 protons
Now,

Atomic mass=number of protons+number of neutrons
So,
7=3 +no. of neutrons

Number of neutrons =7-3=4
So, the atom has 4 neutrons
This atom may be Lithium