Give the symbols for 4 species that are isoelectronic with the bromide ion, br-

The species isoelectric to are Kr, .

Isoelectronic species are the ones that have the same number of electrons. By gaining the electrons or losing electrons, the same electronic configuration can be achieved.

The Bromide ion, has 36 electrons. The Krypton (Kr) also has 36 electrons.

Selenium has 34 electrons and a gain of 2 electrons will form 36 electrons. Thus has 36 electrons.

The Rubidium has 37 electrons and losing 1 electron will result in 36 electrons. Thus has 36 electrons.

Strontium has 38 electrons, the loss of 2 electrons will result in 36 electronic configurations.  Thus has 36 electrons.

The species isoelectric to are Kr, .

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your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.