Washington Master Esthetics Practice Exam

How many energy shells does an atom with 12 protons and 12 electrons have?

1. 1

2. 2

3. 3

4. 4

The correct answer is: 3

An atom with 12 protons and 12 electrons corresponds to the element magnesium, which has an atomic number of 12. The arrangement of electrons around the nucleus of an atom is organized into energy shells or electron shells. For magnesium, the electron configuration is determined by how many energy shells can accommodate its 12 electrons. The first shell can hold a maximum of 2 electrons, the second shell can hold up to 8 electrons, and the third shell can hold up to 18 electrons (though only a total of 12 electrons are present in magnesium). The distribution of electrons in the shells for magnesium would be: - The first shell has 2 electrons. - The second shell can accommodate the next 8 electrons, making a total of 10 electrons. - The remaining 2 electrons will go into the third shell. Thus, with 12 electrons distributed across three shells (2 in the first, 8 in the second, and 2 in the third), magnesium has a total of 3 energy shells. This is why the correct answer is that an atom with 12 protons and 12 electrons has 3 energy shells.