Predicting Properties of Elements
- The periodic table is a tool that can be used to predict or estimate chemical properties when information is missing.
- The predictions need to be based in the position of the element in the table
Worked example
According to periodic trends, which of the following elements has the smallest atomic radius?
- Potassium (K)
- Calcium (Ca)
- Bromine (Br)
- Strontium (Sr)
Answer:
- The correct answer is Bromine is in period 3 at the far right side of the periodic table. Bromine’s nuclear charge is greater than potassium and calcium, and it has less shielding than Strontium
- Strontium is in period 4. Therefore, the shielding effect is bigger, the coulombic attraction to the valence electrons weaker, and the atom is bigger. That is why C is also incorrect
- B and A are not correct since they are in the same period but at the far left. Even if the shielding effect is the same, the nuclear charge of Potassium and Calcium is smaller compared to Bromine. Therefore, the coulombic attraction from the nucleus to the valence electrons is smaller, meaning bigger atoms
Exam Tip
Expect to be assessed in the prediction of periodic trends in both sections of your AP Chemistry Exam. Make sure to have the chemical principles behind the properties really clear
Worked example
Using your knowledge of atomic structure, predict if the ionization energy of Chlorine (Cl) would be greater, smaller or equal than the ionization energy of fluorine (F).
Answer:
- Chlorine has a larger atomic size and more electron shells.
- This leads to weaker coulombic attraction between the nucleus and outer electrons
- Therefore, less energy is required to remove an electron compared to fluorine
- Fluorine has less shielding effect and the coulombic attraction to its valence electrons are stronger
