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Because metallic bonding is non-localized, and extends throughout the metallic lattice. Metal nuclei can move with respect to other metal nuclei without disrupting the forces of attraction.
Beta decay by emitting an electron is called as β⁻ decay. when this happens, a neutron of the element converts into a proton by emitting an electron. Hence, the mass of daughter nucleus is same as parent atom but atomic number/number of protons is higher by 1 than atomic number of parent atom.
Sr has atomic number as 38.
After β⁻ decay, the daughter nucleus will have atomic number as 38 + 1 = 39.
Hence, the daughter nucleus should be Y (Yttrium). Formula is,
₃₈⁹⁰Sr → ₃₉⁹⁰Y + ₋₁⁰β + energy
