The Periodic Table

Early Attempts

Before the modern periodic table, scientists tried various ways to classify elements:

• Döbereiner's Triads (1829): Groups of three elements with similar properties where the middle element's atomic mass was the average of the other two (e.g. Li, Na, K)
• Newlands' Octaves (1866): Arranged elements by atomic mass and noticed properties repeated every 8 elements. However, this broke down after calcium and he was ridiculed

Mendeleev's Periodic Table (1869)

Dmitri Mendeleev made the breakthrough:

• Arranged elements by atomic mass (not number — protons weren't known yet)
• Left gaps for elements not yet discovered
• Predicted the properties of missing elements (e.g. "eka-silicon" = germanium)
• Swapped some elements out of mass order to keep similar properties in the same column

When his predicted elements were discovered with properties matching his predictions, his table was widely accepted.

The Modern Periodic Table

Today, elements are arranged by atomic number (number of protons), not atomic mass. This resolved the issues Mendeleev had with swapping elements.

Groups (Columns)

• Elements in the same group (vertical column) have the same number of outer-shell electrons
• This means they have similar chemical properties
• Group number = number of outer electrons (for main groups)

Periods (Rows)

• Elements in the same period (horizontal row) have the same number of occupied electron shells
• Period number = number of electron shells

Metals, Non-metals, and Metalloids

The periodic table can be divided into:

• Metals — on the left and centre

  • Good conductors of heat and electricity
  • Malleable and ductile
  • Usually form positive ions (cations)
  • Tend to have high melting points (except Group 1)

• Non-metals — on the right

  • Poor conductors (insulators)
  • Brittle as solids
  • Usually form negative ions (anions) or share electrons
  • Lower melting and boiling points

• Metalloids (semi-metals) — along the dividing line (e.g. silicon, germanium)

  • Properties between metals and non-metals
  • Used as semiconductors

The arrangement of electrons in an atom directly determines its position in the periodic table:

$\text{Group number} = \text{number of outer shell electrons}$ $\text{Period number} = \text{number of occupied shells}$

Example: Chlorine has electron configuration 2, 8, 7

• Outer shell electrons = 7 → Group 7
• Occupied shells = 3 → Period 3

Moving left to right across Period 3 (Na to Ar):

Moving down a group:

The transition metals are found in the central block of the periodic table (between Groups 2 and 3). They include iron (Fe), copper (Cu), zinc (Zn), gold (Au), and silver (Ag).

Properties of transition metals:

• High melting points (much higher than Group 1)
• High density
• Form coloured compounds (e.g. CuSO₄ is blue)
• Have variable oxidation states (e.g. Fe²⁺ and Fe³⁺)
• Often act as catalysts (e.g. iron in the Haber process)
• Good conductors of heat and electricity
• Hard, strong, and malleable

Comparison with Group 1:

• Always link properties to electron configuration — this is the underlying explanation
• When explaining reactivity trends, use the words: distance from nucleus, shielding, attraction, ease of losing/gaining electrons
• Mendeleev questions often ask about gaps and predictions — be specific
• Know the difference between groups (same outer electrons, similar properties) and periods (same number of shells)

• Elements are arranged by atomic number in the modern periodic table
• Groups = same outer electrons = similar properties
• Periods = same number of shells
• Metal reactivity increases down a group; non-metal reactivity decreases
• Transition metals have high melting points, variable oxidation states, form coloured compounds, and act as catalysts
• Mendeleev's success was in leaving gaps and making accurate predictions