Periodic Classification of Elements

• How Mendeleev arranged elements and where his table fell short
• The Modern Periodic Law and how the modern periodic table is organised
• How atomic size, metallic character and valence electrons change across a period and down a group
• How valency of elements relates to their group number

1. Mendeleev's Periodic Table: Dmitri Mendeleev arranged the known elements in order of increasing atomic mass and grouped elements with similar chemical properties together. He left gaps for elements not yet discovered and successfully predicted their properties (e.g. eka-aluminium = gallium, eka-silicon = germanium).
2. Limitations of Mendeleev's table:
   • Position of hydrogen was uncertain (resembles both alkali metals and halogens).
   • Isotopes of the same element have different atomic masses but were not given separate positions.
   • At some places, elements with higher atomic mass were placed before elements with lower atomic mass to keep similar properties together (e.g. cobalt before nickel, tellurium before iodine) — this could not be explained by atomic mass ordering.
3. The Modern Periodic Law: "The physical and chemical properties of elements are periodic functions of their atomic number." This was proposed by Henry Moseley, resolving the anomalies of Mendeleev's table (e.g. cobalt, Z=27, correctly comes before nickel, Z=28).
4. Structure of the Modern Periodic Table: Elements are arranged in 18 vertical columns called groups and 7 horizontal rows called periods.
   • Elements in the same group have the same number of valence electrons and hence similar chemical properties.
   • Elements in the same period have the same number of occupied electron shells; the number of valence electrons increases by one on moving from left to right.
5. Trend across a period (left to right):
   • Atomic size (radius) decreases — because the nuclear charge increases while electrons are added to the same shell, pulling electrons closer to the nucleus.
   • Number of valence electrons increases from 1 to 8 (across a typical period).
   • Metallic character decreases (metals on the left, non-metals on the right, metalloids/semi-metals along the zig-zag dividing line); tendency to lose electrons decreases and tendency to gain electrons increases.
   • Valency first increases from 1 to 4 (going to the middle of the period) and then decreases to 0 (noble gas).
6. Trend down a group (top to bottom):
   • Atomic size increases — because a new electron shell is added at each step, increasing the distance between the nucleus and the outermost electrons, and the effect of increased nuclear charge is outweighed by this extra shell (shielding effect).
   • Metallic character increases — atoms lose their outermost electrons more easily as they are farther from the nucleus, so elements become more metallic on going down a group (e.g. carbon at the top of Group 14 is a non-metal, lead at the bottom is a metal).
   • Number of valence electrons remains the same down a group, so valency stays constant down a group.
7. Valency and group number: For groups 1–2 and 13–17, valency = number of valence electrons (for groups 1-2, 13-14) or (8 − number of valence electrons) for groups 15-17. Elements in the same group therefore show the same valency (e.g. Group 1: valency 1 for Li, Na, K; Group 17: valency 1 for F, Cl, Br, I).

An element X is in Period 3 and Group 1 of the modern periodic table. Predict its valency, whether it is a metal or non-metal, and how its atomic size compares with the element directly below it in Group 1.

Group 1 elements have 1 valence electron, so valency = 1. Group 1 elements are alkali metals, so X is a highly reactive metal (this is sodium, Na). Moving down Group 1, atomic size increases because a new shell is added, so the element below X (potassium, K) has a larger atomic size than X.

• Thinking atomic size increases across a period — it actually decreases due to increasing nuclear charge pulling the same shell closer.
• Confusing "valence electrons increase across a period" with "valency increases across a period" — valency rises only up to the middle of the period (to 4) and then falls back to 0 at the noble gas.
• Believing Mendeleev used atomic number — he actually used atomic mass; it was Moseley's Modern Periodic Law that switched the basis to atomic number.
• Assuming metallic character increases across a period — it is the opposite: metallic character decreases left to right and increases top to bottom.

• Why does atomic size decrease across a period but increase down a group?
• State the Modern Periodic Law and explain how it fixed the cobalt-nickel anomaly in Mendeleev's table.

Open the Practice tab for graded questions on Periodic Classification of Elements.