Question

an element with the following electron configuration will gain ____________ electrons to form a common ion. 1s² 2s² 2p⁶ 3s² 3p³
4
2
1
3
question 4
1 pts
the electron configuration for se²⁻ is __________ which is isoelectronic with __________.
ar5s² 4d¹⁰ 5p⁶, xenon
ar4s² 3d¹⁰ 4p⁴, krypton
ar4s² 3d¹⁰ 4p⁶, krypton
ar4s² 3d¹⁰ 4p⁶, argon

Explanation:

Step1: Analyze the first electron - configuration

The electron - configuration is \(1s^{2}2s^{2}2p^{6}3s^{2}3p^{3}\). The outermost shell is the third shell (\(n = 3\)) with \(2 + 3=5\) electrons in the \(3s\) and \(3p\) sub - shells. Elements tend to gain or lose electrons to achieve a noble - gas electron configuration. The nearest noble gas is argon (\(1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}\)), so this element will gain \(3\) electrons to fill the \(3p\) sub - shell.

Step2: Analyze the electron - configuration of \(Se^{2-}\)

Selenium (\(Se\)) has an atomic number of \(34\), so its neutral electron - configuration is \([Ar]4s^{2}3d^{10}4p^{4}\). When it forms \(Se^{2-}\), it gains \(2\) electrons. The electron - configuration of \(Se^{2-}\) is \([Ar]4s^{2}3d^{10}4p^{6}\). Krypton (\(Kr\)) has an electron - configuration of \([Ar]4s^{2}3d^{10}4p^{6}\), so \(Se^{2-}\) is isoelectronic with krypton.

Answer:

1. D. 3
2. C. \([Ar]4s^{2}3d^{10}4p^{6}\), krypton