Atomic Structure

The atomic mass unit is defined using the carbon-12 isotope ( ) as the reference. This isotope is assigned a mass of exactly 12 atomic mass units.

The number of neutrons in an isotope can be found using the formula: For , the atomic number is 92, so: Thus, the correct answer is 146.

The number of total electrons in an ion depends on its atomic number and the charge: - has 10 electrons (since Na has atomic number 11 and loses 1 electron). - has 10 electrons (since Al has atomic number 13 and loses 3 electrons). So, and both have 10 electrons, which makes this pair correct. Let's check the other pairs: - has 10 electrons (O has atomic number 8 and gains 2 electrons), and has 10 electrons (F has atomic number 9 and gains 1 electron). Thus, this pair has the same number of electrons. - has 10 electrons (Mg has atomic number 12 and loses 2 electrons), and Ar has 18 electrons. Therefore, this pair does not have the same number of electrons. - has 18 electrons (P has atomic number 15 and gains 3 electrons), and Ar has 18 electrons. This pair has the same number of electrons. Hence, the correct answer is and Ar.

The order of radii is determined by the nuclear charge and the number of electrons in each ion. Fe has the smallest radius, followed by Fe , and Fe has the largest radius.

To determine the molecular geometry using VSEPR theory, we follow these steps:

1. Identify the number of bonding pairs and lone pairs around the central atom for each molecule.
2. For NH₃ (ammonia):
3. For BF₃ (boron trifluoride):
4. For H₂O (water):
5. For CH₄ (methane):
6. Conclusion: Only BF₃ has a trigonal planar geometry.

Thus, the correct answer is: