How many of the following species are paramagnetic? sc3+ br- mg2+ se?

           One: Selenium is Paramagnetic

Explanation:
                   Those compounds which have unpaired electrons are attracted towards magnet. This property is called as paramagnetism. Lets see why remaining are not paramagnetic.

Electronic configuration of Scandium;

Sc  =  21  = 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹

Sc³⁺  =  1s², 2s², 2p⁶, 3s², 3p⁶ 

Hence in Sc³⁺ there is no unpaired electron.

Electronic configuration of Bromine;

Br  =  35  = 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰, 4p⁵

Br⁻  =  1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰, 4p⁶

Hence in Br⁻ there is no unpaired electron.

Electronic configuration of Magnesium;

Mg  =  12  = 1s², 2s², 2p⁶, 3s²

Mg²⁺  =  1s², 2s², 2p⁶

Hence in Mg²⁺ there is no unpaired electron.

Electronic configuration of selenium;

Se  =  34  = 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰, 4p⁴

Or,

Se  =  34  = 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰, 4px², 4py¹, 4pz¹

Hence in Se there are two unpaired electrons hence it is paramagnetic in nature.

You use the atomic number to find out how many protons and neutrons there are in the atom. you use the atomic mass to find out how many protons and electrons there are in the atom. 

Electrons are emitted