Questions: The Periodic Table

Question 1

An unknown element is in Group 2, Period 4 of the periodic table. Without looking it up, which property can you most confidently predict from its position alone?

• A. It has 4 protons and 2 neutrons, based on its period and group numbers
• B. It is a nonmetal with high electronegativity because it is to the right of the transition metals
• C. It has 2 valence electrons in its outermost s orbital and will tend to form 2+ ions by losing them
• D. Its chemical properties will match those of the element directly above it because they share a period

Answer: (C)

Group 2 (alkaline earth metals) are s-block elements, each with 2 valence electrons in their outermost s orbital. These elements readily lose both electrons to achieve a noble gas configuration, forming 2+ cations. You cannot infer proton count from the group-period designation alone (that requires knowing atomic number). Group 2 elements are metals, not nonmetals. And elements in the same *group* (not period) share chemical similarities — period-mates have very different properties. The table's predictive power comes from group and block position encoding valence electron configuration.

Question 2

Mendeleev occasionally swapped elements in his periodic table so that chemical group similarities were preserved, even when this violated strict increasing atomic mass order. Why was this later vindicated by atomic theory?

• A. Atomic masses were poorly measured in Mendeleev's time; modern measurements now restore strict mass ordering
• B. Chemical properties are determined by the number of protons (atomic number), not mass; isotopes of the same element prove that mass can vary while chemical behavior stays the same
• C. Group similarities are coincidental patterns with no structural explanation, so Mendeleev's swaps were equally arbitrary
• D. Mendeleev was wrong to swap elements, and modern tables return them to strict mass ordering

Answer: (B)

The number of protons (atomic number) determines the electron configuration, which in turn determines chemical reactivity. Atomic mass includes neutrons, which do not participate in bonding. Isotopes of an element have the same atomic number, nearly identical chemistry, and different masses — directly proving that mass is not the organizing principle. Mendeleev's 'swaps' (e.g., placing tellurium before iodine despite tellurium's higher average mass) put elements with matching chemical character in the same group. Once atomic number was understood, these swaps were shown to correspond to correct atomic number ordering, confirming Mendeleev's chemical intuition.

Question 3

The periodic table is called 'periodic' because elements arranged by atomic number show repeating patterns of chemical properties at regular intervals.

• T. True
• F. False

Answer: True

Periodicity is the table's defining feature. As atomic number increases, valence electron configurations repeat in a predictable cycle: each new period begins with an alkali metal (one valence electron, very reactive) and ends with a noble gas (full valence shell, very unreactive). Properties like electronegativity, atomic radius, ionization energy, and typical ion charge all show periodic patterns — decreasing or increasing across a period, then resetting at the start of the next. This periodicity is not a coincidence; it is a direct consequence of electrons filling successive orbitals in the Aufbau sequence.

Question 4

Because transition metals occupy the d-block, the group number of any d-block element directly equals its number of valence electrons, just as it does for s- and p-block elements.

• T. True
• F. False

Answer: False

This is one of the explicitly noted misconceptions in the topic. For s- and p-block elements, group number reliably gives valence electron count. Transition metals are different: their electron configurations involve both d and s electrons, and exceptions are numerous — chromium (Group 6) has [Ar]3d⁵4s¹ instead of the expected [Ar]3d⁴4s², and copper (Group 11) has [Ar]3d¹⁰4s¹. Additionally, how many d electrons count as 'valence' depends on the bonding context. The simple group = valence-electron rule that works well for the main-group elements does not transfer cleanly to the d-block.

Question 5

Why does the periodic table have exactly 2 elements in the first row, 8 in the second and third rows, and 18 in the fourth row — and what determines these numbers?

Model answer: The row lengths follow directly from orbital filling (the Aufbau principle). The first row fills only the 1s subshell (1 orbital × 2 electrons = 2 elements). Rows 2 and 3 fill one s subshell and one p subshell (1 + 3 orbitals × 2 electrons each = 2 + 6 = 8 elements). Row 4 adds the 3d subshell (5 orbitals × 2 = 10 more elements) for a total of 2 + 6 + 10 = 18. The f-block rows accommodate 14 elements (7 orbitals × 2). Each subshell type (s, p, d, f) can hold 2, 6, 10, or 14 electrons respectively, directly determining the width of the s-, p-, d-, and f-blocks.

Understanding row length as a consequence of orbital capacity transforms the table from an arbitrary arrangement to a map of electron configuration. The s-block is always 2 columns wide (one s orbital), the p-block is 6 columns wide (three p orbitals), the d-block is 10 columns wide, and the f-block 14. The 'staircase' shape of the table is therefore a direct visual representation of quantum mechanical orbital structure — not a historical accident.