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The Lewis Theory predicts the way atoms form bond based on their numbers of valence electrons. Atoms gain or lose electrons as they seek to achieve either a complete eight-electron valence shell or an empty shell, exposing the inner eight-electron shell. Both electron configurations are stable and are known as "octets". However, atoms tend to prefer the configuration with the minimum number of electron transfers.
Strontium (Sr) is found in group 2 (II A) of a modern periodic table. A neutral atom of strontium has two valence electrons in its valence shell.
Selenium (Se) is found in group 16 (VI A) of a modern periodic periodic table. A neutral atom of selenium contains six electrons in its valence shell.
Strontium needs to either lose two electrons or gain six electrons to achieve a configuration with eight electrons in its valence shell. Losing two electrons take less energy. Strontium is thus expected to lose two electrons.
Similarly, selenium would prefer gaining two electrons to losing six electrons. It is expected to gain two electrons.
As a result, a strontium atom would supply exactly two electrons. At the same time, a As a result, a selenium atom would accept the two electrons, no more or no less. The two elements would combine at a one to one ratio. In other words:
Each electron carries one negative charge. As a result, atoms that have gained electrons demonstrate a negative charge, whereas atoms that have lost electrons appear to be positive. By convention, atoms with a positive charge shall be written in front of those with a negative charge in a formula.
Sr atoms have lost electrons. Accordingly, they carry positive charges and shall be written at the beginning of the formula.
Se atoms have gained electrons. Accordingly, they carry negative charges and shall be placed to the end of the formula.
Thus the chemical formula: