Table of Contents
Basic Layout and Structure
The periodic table is not arranged arbitrarily: its layout reflects recurring patterns in the properties and electronic structures of the elements. Several structural features are central to its ordering:
- Periods: horizontal rows, numbered 1–7 in the modern table
- Groups: vertical columns, usually numbered 1–18
- Blocks: contiguous regions associated with the type of atomic orbital being filled ($s$, $p$, $d$, $f$)
- Zig-zag line: a boundary roughly separating metals from non-metals
Within this framework, each element has its own box, usually containing at least:
- atomic number $Z$ (ordering principle!)
- element symbol
- often: name, relative atomic mass, and sometimes other data (e.g. electron configuration, state at room temperature)
The crucial principle: elements are ordered primarily by increasing atomic number, and from this follow the rest of the structural rules.
Ordering by Atomic Number
Historically, elements were first arranged by increasing relative atomic mass, but this created inconsistencies for some pairs (e.g. tellurium–iodine). The modern periodic table instead uses:
- Atomic number $Z$: the number of protons in the nucleus
The elements are listed so that:
- $Z$ increases by 1 from one element to the next, if you “read” the table left to right, top to bottom.
- There are no exceptions: the order of atomic numbers is the fundamental ordering principle.
Consequences:
- Position of an element is fixed and unambiguous.
- Apparent “anomalies” in atomic mass (where a heavier element may appear before a lighter one) are resolved, because atomic number is the deciding criterion.
Periods: Horizontal Arrangement
Each period corresponds to a principal energy level being occupied by electrons.
General features:
- Period 1: 2 elements (H, He)
- Periods 2 and 3: 8 elements each
- Periods 4 and 5: 18 elements each
- Periods 6 and 7: 32 elements each (including the $f$-block, often shown separate)
Ordering rule within a period:
- Move from left to right: $Z$ increases by 1 at each step.
- The outer electron configuration changes systematically as you go across.
Ends of periods:
- Each period starts with an element having a single electron (or one more electron than a noble gas configuration) in a new main energy level.
- Each period ends with a noble gas (filled valence shell for that period).
The length of a period is determined by the number of available orbitals in that shell (this is explored in detail in the quantum mechanical model chapter).
Groups: Vertical Arrangement
Groups collect elements with broadly similar valence electron structures and thus related chemical behavior.
Modern group numbering (1–18) proceeds:
- from left to right
- aligning elements vertically when their outer electron configurations are similar
Essential ordering principle:
- Within a group, $Z$ increases from top to bottom, but the number of valence electrons stays largely the same for the main-group elements.
- This creates columns of chemically related elements (e.g. alkali metals, halogens, noble gases).
Important patterns in grouping (names and detailed properties are treated elsewhere):
- Group 1: “alkali metals” (except H, which is special)
- Group 2: “alkaline earth metals”
- Groups 3–12: transition elements (d-block)
- Groups 13–18: other main-group elements, ending with the noble gases in group 18
Some alternative notation systems (e.g. IUPAC vs old A/B notation like “VIIA”) exist, but the modern system 1–18 is now standard.
Blocks: s, p, d, and f
Beyond rows and columns, the table is divided into blocks according to which type of orbital is being filled as $Z$ increases.
- s-block: groups 1 and 2 (and helium by electron configuration)
- filling $ns$ orbitals
- p-block: groups 13–18
- filling $np$ orbitals
- d-block: groups 3–12, the “transition” region
- filling $(n-1)d$ orbitals
- f-block: lanthanoids and actinoids
- filling $(n-2)f$ orbitals
Ordering principle here:
- If you follow the increasing $Z$ while respecting the sequence in which orbitals are filled, you obtain the block pattern of the periodic table.
- The blocks appear as rectangular regions because the number of orbitals of each type is fixed:
- $s$ block: 2 columns (2 possible $s$ electrons)
- $p$ block: 6 columns (6 $p$ electrons)
- $d$ block: 10 columns (10 $d$ electrons)
- $f$ block: 14 columns (14 $f$ electrons)
Position of the f-Block (Lanthanoids and Actinoids)
The lanthanoids and actinoids (the $f$-block elements) present a special layout issue.
Ordering principles:
- In terms of $Z$, they fit into periods 6 and 7, between the $s$-block and $d$-block of those periods.
- To avoid making the table extremely wide, they are usually:
- placed in two separate rows below the main body of the table,
- but conceptually inserted between group 2 and group 3 in periods 6 and 7.
Their location is therefore:
- determined by atomic number and orbital filling (the start of $4f$ and $5f$ subshell occupation),
- displayed in a compactified way for practical reasons of layout.
Metals, Non-Metals, and Metalloids
Another ordering feature is the broad classification of elements based on physical and chemical type. The periodic table reflects:
- a large region of metals on the left and in the center,
- a smaller region of non-metals on the upper right,
- a zig-zag boundary with some elements often classified as metalloids (semimetals).
Ordering principle:
- The approximate diagonal line from boron (B) down to polonium (Po) (depending on the exact convention) marks:
- metals mainly below/left of the line,
- non-metals mainly above/right of the line,
- metalloids along the line.
This boundary is not perfect but reflects an underlying gradual change in properties across the table.
Special Placement Cases
A few elements have positions determined by atomic number and overall periodic trends, but their classification is not straightforward.
Hydrogen
Hydrogen ($Z = 1$):
- Placed in the top left, above group 1,
- Shares with alkali metals the feature of having 1 valence electron,
- But is a non-metal and behaves very differently in many reactions.
Ordering principle:
- Its position reflects a compromise between:
- similarity in valence electron count (group 1),
- and its unique character (often also compared with halogens or given special treatment).
Helium
Helium ($Z = 2$):
- Electron configuration corresponds to a filled $1s$ subshell.
- By electron configuration alone, it would fit in the $s$-block (above group 2).
- However, its complete valence shell and chemical inertness align it with the noble gases (group 18), so it is placed there.
Ordering principle:
- Helium’s column placement is chosen to reflect its chemical behavior more than its block membership.
Group 3 and the f-Block Connection
The precise boundaries between group 3 and the lanthanoid/actinoid series (which elements belong in group 3, how to display La/Lu and Ac/Lr) can vary in different tables.
Ordering principle:
- All variants still keep $Z$ in order,
- but differ in which elements are placed directly under scandium and yttrium based on detailed considerations of electronic structure and chemical similarity.
Synthetic Elements and Extension of the Table
The last rows of the table contain elements that do not occur naturally in significant amounts (or at all) and have been synthesized in laboratories.
Ordering principle:
- As new elements are created and confirmed, they are added by atomic number to the end of the table.
- Their predicted positions (group and period) follow from:
- atomic number,
- expected electron configuration (and therefore block),
- and emerging chemical properties.
The table thus can, in principle, be extended by continuing the sequence of $Z$, following the same structuring rules.
Summary of Core Ordering Principles
Collecting the key rules:
- Primary order by atomic number $Z$: increases by 1 from one element to the next.
- Periods: rows correspond to main energy levels; each begins with a new shell being occupied.
- Groups: columns gather elements with similar valence electron configurations and broadly similar chemistry.
- Blocks: ($s$, $p$, $d$, $f$) correspond to the type of orbital being filled as $Z$ increases.
- f-block placement: lanthanoids and actinoids belong in periods 6 and 7 but are usually shown separately to keep the table compact.
- Metal/non-metal division: a diagonal separation reflects gradual changes in properties.
- Special cases (H, He, group 3 borders): positioned by balancing electron structure with chemical behavior, while always respecting atomic number order.
- Synthetic elements: added sequentially in $Z$ and assigned to their positions according to predicted electronic and chemical patterns.
These structural principles make the periodic table a map in which an element’s position encodes rich information about its electronic structure and typical chemical behavior.