Q: What is the effective nuclear charge ($$Z_{\text{eff}}) $$and how does it generally change across a period?

$$Z_{\text{eff}} is $$the actual positive charge experienced by an electron, and it increases across a period.

Question 1

Which of the following statements best explains why Mendeleev’s periodic table had inconsistencies, such as argon appearing before potassium, and how were these inconsistencies resolved?

Accepted Answer

Mendeleev arranged elements by atomic mass, but inconsistencies were resolved when Moseley showed that ordering by atomic number explained the periodic trends.

Question 2

Which of the following electron configurations represents a ground state atom of nitrogen (Z = 7)?

Accepted Answer

$$1s^2$$\ $$2s^2$$\ $$2p^3$$

Question 3

Which principle states that electrons fill the lowest energy orbitals first before occupying higher energy orbitals?

Accepted Answer

Aufbau Principle

Question 4

Which of the following elements would you expect to have chemical properties most similar to chlorine?

Accepted Answer

Bromine

Question 5

What is the effective nuclear charge ($$Z_{	ext{eff}}) $$and how does it generally change across a period?

Accepted Answer

$$Z_{	ext{eff}} is $$the actual positive charge experienced by an electron, and it increases across a period.

Question 6

Arrange the following elements in order of increasing atomic radius: O, S, P.

Accepted Answer

O \u003c S \u003c P

Question 7

Which of the following best describes the trend in first ionization energy ($$IE_1) $$across a period from left to right?

Accepted Answer

$$IE_1$$ increases due to increasing effective nuclear charge.

Question 8

Which of the following ions is isoelectronic with neon ($$Ne$$)?

Accepted Answer

$$F^-$$

Question 9

Why does the ionic radius of a cation decrease compared to its atomic radius?

Accepted Answer

The loss of electrons reduces electron–electron repulsion and the remaining electrons are drawn closer to the nucleus.

Question 10

Which of the following electron configurations represents the ground state of chromium (Cr, Z = 24)?

Accepted Answer

$$[Ar]\ 4s^1$\ 3d^5$

Question 11

Which of the following best explains why the second ionization energy ($$IE_2) of $$sodium is much higher than its first ionization energy ($$IE_1$$)?

Accepted Answer

The second electron is removed from a core shell, which is closer to the nucleus and experiences a higher effective nuclear charge.

Question 12

Which of the following statements about electron affinity is correct?

Accepted Answer

Electron affinity is the negative of the energy released when an atom in the gas phase accepts an electron.

Question 13

Which of the following elements is most likely to form a cation with a $$+2$$ charge?

Accepted Answer

Magnesium

Question 14

Which of the following best describes the trend in metallic character across the periodic table?

Accepted Answer

Metallic character increases from right to left across a period and from top to bottom within a group.

Question 15

Which of the following ions has the largest radius in the isoelectronic series $$O^{2-}$$, $$F^-$$, $$Na^+$$, $$Mg^{2+}$$?

Accepted Answer

$$O^{2-}$$

Question 16

Which rule states that the most stable arrangement of electrons in degenerate orbitals is the one with the maximum number of unpaired electrons with the same spin?

Accepted Answer

Hund’s Rule

Question 17

Which of the following is the correct electron configuration for the $$Fe^{2+}$$ ion?

Accepted Answer

$$[Ar]\ 3d^6$

Question 18

Which of the following best explains why halogens have high electron affinities?

Accepted Answer

They have a nearly full valence shell and a high effective nuclear charge, making it energetically favorable to gain one electron.

Question 19

Which of the following statements about the electron configuration of ions of d-block elements is correct?

Accepted Answer

Electrons are removed first from the ns subshell before the (n-1)d subshell.

Periodic Trends of the Elements: Electron Configurations and Properties

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