EQB5: Electrons and the Periodic Table

Which statement best describes how electrons fill orbitals in the periodic table?

• Electrons fill orbitals in order of their increasing energy from left to right.
• Electrons fill orbitals in order of their increasing energy from right to left.
• Elements fill orbitals in order of increasing energy from top to bottom in each group.
• Elements fill orbitals in order of increasing energy from bottom to top in each group.

Answer explanation

- Explanation: Electrons fill the orbitals starting with the lowest energy level to the highest, according to the Aufbau principle. This typically means filling from the 1s orbital upwards, not in a left-to-right or top-to-bottom manner as in the periodic table's layout. Thus, options B, C, and D are incorrect because they describe sequences that do not represent the actual order of increasing orbital energy.

Which of the following symbols would be included in the noble gas notation for an element that had valence electrons in the 5s subshell?

• Ar
• Kr
• Xe
• Rn

Answer explanation

- Explanation: Noble gas notation simplifies electron configuration by starting with the nearest noble gas of lower atomic number, then adding the remainder. Since the 5s subshell is just after xenon (Xe) in the periodic table, Xe is the correct notation start for elements with valence electrons in the 5s subshell. Ar and Kr come before 5s elements, and Rn comes much later.

In which orbitals would the valence electrons for selenium (Se) be placed?

• s orbital and f orbital
• s orbital only
• s orbital and p orbital
• d orbital only

Answer explanation

- Explanation: Selenium is in the p-block of the periodic table, specifically in group 16, which means its valence electrons fill the s and p orbitals of its highest energy level. The f and d orbitals do not get filled by elements in this group until after the 4th period, making A and D incorrect. B is incorrect because selenium uses both s and p orbitals for its valence electrons.

How many valence electrons does aluminum (Al) have available for bonding?

• 1
• 2
• 3
• 4

Answer explanation

- Explanation: Aluminum is in group 13 of the periodic table, which means it has three valence electrons. These are the electrons available for bonding. Options A, B, and D are incorrect because they do not correspond to the group number for aluminum, which directly indicates the number of valence electrons.

The diagram below shows that the periodic table is divided into different blocks. Elements that have complete valence electron shells are mostly found in the

• s block.
• d block.
• p block.
• f block.

Answer explanation

- Explanation: The elements that have complete valence electron shells, also known as the noble gases, are found in the p-block of the periodic table at the far right in group 18. The s, d, and f blocks do not contain elements with completely filled valence shells as their defining characteristic.

In which orbitals would the valence electrons for carbon (C) be placed?

• s orbital and d orbitals
• s orbital only
• p orbitals only
• s orbital and p orbitals

Answer explanation

- Explanation: Carbon has four valence electrons, two in the 2s orbital and two in the 2p orbitals. It does not have valence electrons in the d orbitals, making A incorrect. B and C are incorrect because carbon's valence electrons are in both s and p orbitals, not just in one.

The electron configuration of nitrogen (N) is

• 1s22s22p3
• 1s22s22p4
• 1s22s22p5
• 1s22s22p6

Answer explanation

- Explanation: Nitrogen, with an atomic number of 7, has its first two electrons fill the 1s orbital, the next two fill the 2s orbital, and the remaining three fill the 2p orbitals, following the principle of lowest energy level filling first. The other options suggest incorrect fillings of the 2p orbitals.

Which element does the electron configuration 1s22s22p2 represent?

• B
• C
• N
• K

Answer explanation

- Explanation: This configuration has six electrons total, corresponding to carbon, which is the sixth element. The configuration describes two electrons in the 1s orbital, two in the 2s orbital, and two in the 2p orbitals, fitting carbon's position in the periodic table. Other options do not match this configuration.

How many valence electrons does lithium (Li) have available for bonding?

• 1
• 2
• 3
• 4

Answer explanation

- Explanation: Lithium, being in group 1 of the periodic table, has a single valence electron in its outermost shell, making it available for bonding. The other options are incorrect because they do not correspond to lithium's group placement, which directly indicates the number of valence electrons.

The noble-gas notation for tin (Sn) will contain the symbol

• [Ar].
• [Kr].
• [Xe].
• [Rn].

Answer explanation

- Explanation: Tin ends with electrons in the 5p subshell, following xenon in the periodic table. Xenon (Xe) is the nearest noble gas before tin, so its