Write the symbol for the elements that fit the following descriptions

1. 10 electrons in each -3 ion
2. 18 electrons in each +2 ion
3. The alkaline earth metal in the third period

1. -3 means 3 "extra" electrons

... this element is atomic number 7

2. +2 means 2 "lost" electrons
... this element is atomic number 20

3. column 2, row 3

To determine the symbols for the elements that fit the given descriptions, we need to refer to the periodic table. Here's how you can do it:

1. 10 electrons in each -3 ion:
First, we need to determine the element with 10 electrons in its -3 ion. To do this, we can subtract 3 from the atomic number of the element to find the number of electrons. Since the description doesn't specify a period or group, we'll consider all elements in the periodic table. Looking at the periodic table, we find that the atomic number 13 corresponds to aluminum (Al). Subtracting 3 from 13 gives us 10, which matches the given description. Therefore, the symbol for the element is Al.

2. 18 electrons in each +2 ion:
Similar to the previous question, we need to find an element with 18 electrons in its +2 ion. We'll use the same process by adding 2 to the atomic number to determine the total number of electrons. The atomic number 16 corresponds to sulfur (S). Adding 2 to 16 gives us 18, which matches the given description. Therefore, the symbol for the element is S.

3. The alkaline earth metal in the third period:
The alkaline earth metals are located in Group 2 of the periodic table. Since we need an element specifically in the third period, we'll look for the element in the third row. The third period contains the following elements: sodium (Na) (11), magnesium (Mg) (12), aluminum (Al) (13), silicon (Si) (14), phosphorus (P) (15), sulfur (S) (16), chlorine (Cl) (17), and argon (Ar) (18).
From this list, we find that the alkaline earth metal in the third period is magnesium (Mg). Therefore, the symbol for the element is Mg.