A se ion has a mass number of 77 and a charge of −2
. Determine the number of neutrons, protons, and electrons in this ion.

1. Neutron number = 43

2. Proton number = 34

3. Electron number = 36

Explanation:

From the question given above, the following data were obtained:

Mass number = 77

Charge of ion = –2

1. Determination of the number of neutron.

Mass number of Se = 77

Atomic number = 34

Neutron number =?

NOTE: Atomic number = proton number

Mass number = Proton + Neutron

77 = 34 + Neutron

Collect like terms

77 – 34 = Neutron

Neutron number = 43

2. Determination of the proton number.

The atomic number of an element is simply defined as the number of protons in the atom of the element. Mathematically,

Atomic number = Proton number

Atomic number of Se = 34

Therefore, the proton number of Se²¯ is 34.

NOTE: The atomic number does not change be it atom, ion or molecule. It remains the same.

3. Determination of electron number.

Charge of ion = –2

Proton number = 34

Electron number =?

Proton – Electron = Charge

34 – Electron = –2

Rearrange

34 + 2 = Electron

36 = Electron

Electron number = 36

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