Order the following elements according to increasing Z_eff: Ca, Se, Kr, K.

Verified step by step guidance

1. Step 1: Understand the concept of effective nuclear charge (Z_eff), which is the net positive charge experienced by an electron in a multi-electron atom. It accounts for the shielding effect caused by inner-shell electrons.
2. Step 2: Identify the atomic numbers of the given elements: Ca (20), Se (34), Kr (36), and K (19). The atomic number indicates the number of protons in the nucleus, which contributes to the nuclear charge.
3. Step 3: Consider the electron configuration of each element to determine the number of inner-shell electrons that contribute to the shielding effect. For example, Ca: [Ar] 4s², Se: [Ar] 3d¹⁰ 4s² 4p⁴, Kr: [Ar] 3d¹⁰ 4s² 4p⁶, K: [Ar] 4s¹.
4. Step 4: Calculate the effective nuclear charge (Z_eff) for each element using the formula Z_eff = Z - S, where Z is the atomic number and S is the shielding constant, which is approximately equal to the number of inner electrons.
5. Step 5: Compare the Z_eff values for each element and order them from lowest to highest. Remember that elements with more protons and similar shielding will have a higher Z_eff.

Key Concepts

Effective Nuclear Charge (Z_eff)

Effective nuclear charge (Z_eff) is the net positive charge experienced by an electron in a multi-electron atom. It accounts for the shielding effect, where inner electrons repel outer electrons, reducing the full nuclear charge. Understanding Z_eff is crucial for predicting trends in atomic size, ionization energy, and electron affinity across the periodic table.

Periodic Trends

Periodic trends refer to the predictable patterns observed in the properties of elements as you move across a period or down a group in the periodic table. Key trends include atomic radius, ionization energy, and electronegativity. Recognizing these trends helps in ordering elements based on their Z_eff, as elements in the same group or period exhibit similar behaviors.

Shielding Effect

The shielding effect occurs when inner-shell electrons partially block the attraction between the nucleus and the outer-shell electrons. This results in a lower effective nuclear charge felt by the outer electrons. The extent of shielding varies among elements, influencing their Z_eff and thus their chemical properties, making it essential for understanding the ordering of elements.