Analysis of the Periodic Table's organization and the predictive power of periodic trends.
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Dmitri Mendeleev is credited with the development of the first widely accepted periodic table. What was the primary property he used to arrange the elements?
Atomic mass
Mendeleev arranged elements in order of increasing atomic mass. It was later, through the work of Henry Moseley, that the periodic table was reorganized by atomic number.
Which scientist modified the Periodic Law to state that the properties of elements are periodic functions of their atomic numbers rather than atomic masses?
Henry Moseley
Henry Moseley's experiments with X-ray spectra showed that the atomic number (number of protons) is the fundamental property determining an element's identity, correcting Mendeleev's mass-based arrangement.
Elements in the s-block and p-block (excluding the noble gases) are collectively known as:
Representative elements
The elements in Groups 1, 2, and 13–18 (s and p blocks) are often called representative or main-group elements because their outer shells are being filled.
Which general electronic configuration represents the d-block elements (Transition Metals)?
(n−1)d1−10ns0−2
Transition metals are characterized by the filling of inner d-orbitals. The general configuration is (n−1)d1−10ns0−2.
An element has the atomic number 35. To which block and group does it belong?
p-block, Group 17
Atomic number 35 corresponds to Bromine (Br). Its configuration ends in 4s23d104p5. Since the last electron enters a p-orbital, it is in the p-block. With 7 valence electrons (s2p5), it belongs to Group 17 (Halogens).
Which of the following sets of atomic numbers belongs to the Alkali Metals (Group 1)?
3, 11, 19, 37
Group 1 elements are Li (3), Na (11), K (19), Rb (37), etc. Option (b) is Alkaline Earth Metals, (c) is Halogens, and (d) is Noble Gases.
The metalloids (semi-metals) are located along a 'staircase' line on the periodic table. Which of the following elements is a metalloid?
Silicon (Si)
Silicon (Si) is a classic metalloid, exhibiting properties intermediate between metals and nonmetals. Aluminum is a metal; Phosphorus and Sulfur are nonmetals.
Which of the following correctly describes the trend of atomic radius across a period (left to right) and its cause?
Decreases due to increasing effective nuclear charge (Zeff)
Across a period, electrons are added to the same shell while protons are added to the nucleus. This increases the effective nuclear charge (Zeff), pulling electrons closer and shrinking the radius.
Arrange the following species in order of increasing radius: Mg2+, Na+, Ne, F−.
Mg2+<Na+<Ne<F−
These species are isoelectronic (all have 10 electrons). The size is determined by the nuclear charge. Mg has 12 protons, Na 11, Ne 10, F 9. Higher nuclear charge pulls the same number of electrons tighter. Thus, Mg2+ is smallest and F− (with the fewest protons) is largest. (Note: F− is typically larger than Ne due to lower Z).
Why is the radius of a cation always smaller than its neutral parent atom (e.g., Na+<Na)?
Loss of valence electrons increases effective nuclear charge per electron and may remove a shell
Removing an electron reduces electron-electron repulsion and, in many cases (like Na→Na+), removes the outermost shell entirely. The remaining electrons are held more tightly by the nucleus.
