Write a symbol for each of the following neutral isotopes. include the atomic number and mass number for each.
(a) the chalcogen with a mass number of 125
(b) the halogen whose longest-lived isotope is radioactive
(c) the noble gas, used in lighting, with 10 electrons and 10 neutrons
(d) the lightest alkali metal with three neutrons

Explanation:

a) chalcogens are elements of group 16 of periodic table.

Tellurium have mass no 125. atomic no 52 so  ¹²⁵Te₅₂ is the symbol .

b) halogen ( group 17 elements) radioactive isotope is astatine

atomic no is 85 and mass no may be 210 ( most stable)   ²¹⁰At₈₅

c) neon gas is used in lighting .It is a noble gas .

²⁰Ne₁₀ is the symbol

d) lightest alkali metal Beryllium ( Be) .Isotope having 3 neutrons is

⁷Be₄ .

(a)

(b)

(c)

(d)

Explanation:

Write a symbol for each of the following neutral isotopes. Include the atomic number and mass number for each.

(a) the chalcogen with a mass number of 125

Chalcogens are the elements in the Group 16 in the Periodic Table. Tellurium has an average atomic mass of 127.6 u so it must be an isotope of it. The atomic number of Te is 52. The symbol is .

(b) the halogen whose longest-lived isotope is radioactive

Halogens are the elements in the Group 17 of the Periodic Table. The atom whose longest-lived isotope is radioactive  is astatine, with a mass number = 210 and an atomic number = 85. The symbol is .

(c) the noble gas, used in lighting, with 10 electrons and 10 neutrons

The noble gas used in lighting is neon, with 10 protons (atomic number = 10), 10 electrons and 10 neutrons (mass number = 10 p⁺ + 10 n⁰ = 20). The symbol is .

(d) the lightest alkali metal with three neutrons

Alkali metals are the elements in the Group 1 in the Periodic Table. The lightest one is lithium with 3 protons (atomic number = 3). The isotope with 3 neutrons has a mass number = 3 p⁺ + 3 n⁰ = 6. The symbol is .