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Magnesium is an alkaline earth metal with an atomic number of 12. It's in the third row in the periodic table. Which expression shows the number of valence electrons in one atom of magnesium?

A. [tex][tex]$12 \times 3$[/tex][/tex]
B. [tex][tex]$12 \div 3$[/tex][/tex]
C. [tex][tex]$(12-8)-2$[/tex][/tex]
D. [tex][tex]$(12-2)+8$[/tex][/tex]

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GinnyAnswer
Let's determine the correct number of valence electrons in one atom of magnesium (Mg) using the periodic table and its properties.

Magnesium (Mg) is an alkaline earth metal with an atomic number of 12. It is located in the third period (row) of the periodic table. Each period in the periodic table represents the principal energy level, or shell, that is being filled with electrons.

- In the first period, the maximum number of electrons is 2 (filling the 1s orbital).
- In the second period, the maximum number of electrons is 8 (filling the 2s and 2p orbitals).
- In the third period, the electrons will start to fill the 3s and 3p orbitals.

The electronic configuration of magnesium can be determined step-by-step:
1. Fill the 1s orbital: 2 electrons.
2. Fill the 2s orbital: 2 electrons.
3. Fill the 2p orbitals: 6 electrons.
4. The remaining electrons will occupy the 3s orbital: 2 electrons.

So, the electronic configuration of magnesium (Mg) is [tex]\(1s^2 2s^2 2p^6 3s^2\)[/tex].

The electrons in the outermost shell (in this case, the third shell) are known as the valence electrons. Since magnesium has 2 electrons in its 3s orbital, it has 2 valence electrons.

Given the options:

A. [tex]\(12 \times 3\)[/tex] is incorrect because it multiplies the atomic number by the period number, which does not represent the valence electrons.
B. [tex]\(12 \div 3\)[/tex] is incorrect because dividing the atomic number by the period number is not the correct method for determining valence electrons.
C. [tex]\((12-8)-2\)[/tex] is the correct expression. Here’s why:
- First, subtract 8 from 12 (this represents the electrons that fill the first and second energy levels: [tex]\(2 + 6 = 8\)[/tex]).
- Then subtract 2 more because this represents the subtraction from 12 to account for the two electrons in the 3s orbital.
- This leaves us with 2 valence electrons.
D. [tex]\((12-2)+8\)[/tex] is also incorrect because adding 8 to the calculation of the number of remaining electrons after subtracting 2 gives an incorrect result.

Thus, the correct expression for calculating the number of valence electrons in magnesium is:
C. [tex]\((12-8)-2\)[/tex]
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