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Explanation:

Isotope notation is written as:

where A is the mass number and Z is the atomic number. A charge can be added to the right subscript if needed.

1. Find A

A is the mass number, or the sum of protons and neutrons.

There are 19 protons and 20 neutrons.

2. Find Z

Z is the atomic number of number of protons.

There are 19 protons.

3. Find a possible charge

If this is a neutral atom, the protons equal the neutrons.

There are 19 protons and 18 electrons, which is obviously not equal. This is an ion and there is a charge.

There is 1 more proton. A proton has a positive charge, so the charge must be +1.

4. Write the symbol.

A= 39, Z= 19, and the charge is +1

So, the correct answer must be choice A.

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.