---
title: The Periodic Table
subtitle: 118 elements, one organising principle
author: md2any examples
date: auto
theme: light
aspect: 16:9
layout: clean
toc: true
---
# Why the table looks the way it does
## Mendeleev's gamble
Dmitri Mendeleev published the first recognisable periodic table in 1869. He had 63 known elements, ordered by **atomic mass**, arranged so that elements with similar chemistry sat in the same column.
His move was the gaps. Mendeleev left empty slots and **predicted** the properties of the missing elements — including atomic mass, density, and reactivity. Eka-aluminium (gallium, discovered 1875), eka-boron (scandium, 1879) and eka-silicon (germanium, 1886) all turned up with properties within a few percent of his predictions.
The table got refined twice afterwards:
- 1913: Henry Moseley showed the right ordering is **atomic number** ($Z$), not atomic mass. Fixed a handful of out-of-order pairs (Ar/K, Co/Ni, Te/I).
- 1944: Glenn Seaborg moved the actinides into their own row below the lanthanides, giving the table its modern shape.
## What "periodic" means
Properties repeat at regular intervals along the rows because electrons fill orbitals in a predictable order. The "period" is how long each row is — and the row lengths reflect the size of each new electron shell:
| 1 | 2 | 1s only |
| 2 | 8 | 2s + 2p (2 + 6) |
| 3 | 8 | 3s + 3p |
| 4 | 18 | 4s + 3d + 4p (2 + 10 + 6) |
| 5 | 18 | 5s + 4d + 5p |
| 6 | 32 | 6s + 4f + 5d + 6p (2 + 14 + 10 + 6) |
| 7 | 32 | 7s + 5f + 6d + 7p |
The "f-block" elements (lanthanides + actinides) get pulled out below the main table to keep the layout printable. They're really part of periods 6 and 7.
# The 18 groups
## Reading the columns
Elements in the same column share their **outermost electron configuration** and therefore most of their chemistry.
| 1 | Alkali metals | s¹ | Li, Na, K, Rb, Cs |
| 2 | Alkaline earth | s² | Be, Mg, Ca, Sr, Ba |
| 3-12 | Transition metals | (n−1)d, ns | Fe, Cu, Ag, Au, Pt |
| 13 | Boron group | p¹ | B, Al, Ga |
| 14 | Carbon group | p² | C, Si, Ge, Sn |
| 15 | Pnictogens | p³ | N, P, As, Sb |
| 16 | Chalcogens | p⁴ | O, S, Se, Te |
| 17 | Halogens | p⁵ | F, Cl, Br, I |
| 18 | Noble gases | p⁶ (full) | He, Ne, Ar, Kr |
The noble gases have full outer shells — chemically inert under most conditions. The alkali metals are one electron *over* a full shell — desperate to give it away — which makes them the most reactive metals. Halogens are one electron *short* — equally desperate to take one — which makes them the most reactive non-metals.
# Period 1 and 2 (the small ones)
## Hydrogen and helium
| 1 | H | Hydrogen | 1.008 | Most abundant element in the universe (≈75% by mass) |
| 2 | He | Helium | 4.003 | Second most abundant; second highest ionisation energy |
Hydrogen sits on the table awkwardly — it has 1 electron like the alkali metals (group 1), but its chemistry is more like the halogens (group 17). Different tables put it in different places.
## Period 2: the building blocks of life
| 3 | Li | Lithium | 6.94 | Mood stabilisers, EV batteries |
| 4 | Be | Beryllium | 9.012 | Aerospace alloys; toxic dust |
| 5 | B | Boron | 10.81 | Borosilicate glass, neutron absorbers |
| 6 | C | Carbon | 12.011 | Every organic molecule |
| 7 | N | Nitrogen | 14.007 | 78% of Earth's atmosphere |
| 8 | O | Oxygen | 15.999 | 21% of atmosphere; most abundant in Earth's crust |
| 9 | F | Fluorine | 18.998 | Toothpaste; the most reactive non-metal |
| 10 | Ne | Neon | 20.180 | Signs that say OPEN |
Carbon, hydrogen, nitrogen and oxygen account for ~96% of human body mass. The chemistry of life is mostly the chemistry of the second row.
# Period 3 and 4 (the workhorses)
## Period 3
| 11 | Na | Sodium | 22.990 | Half of table salt |
| 12 | Mg | Magnesium | 24.305 | Chlorophyll's central atom |
| 13 | Al | Aluminium | 26.982 | Most abundant metal in Earth's crust |
| 14 | Si | Silicon | 28.085 | Second most abundant in crust; semiconductors |
| 15 | P | Phosphorus | 30.974 | DNA backbone; matches |
| 16 | S | Sulfur | 32.06 | Vulcanised rubber; smell of struck matches |
| 17 | Cl | Chlorine | 35.45 | The other half of table salt; pool sanitiser |
| 18 | Ar | Argon | 39.948 | 1% of atmosphere; welding shield gas |
## Period 4 (highlights)
The first row with transition metals. 18 elements; here are the famous ones:
| 19 | K | Potassium | 39.098 | Bananas; nerve impulse pumps |
| 20 | Ca | Calcium | 40.078 | Bones, teeth, intracellular signalling |
| 22 | Ti | Titanium | 47.867 | Aerospace; medical implants |
| 24 | Cr | Chromium | 51.996 | Stainless steel; emerald colour |
| 26 | Fe | Iron | 55.845 | Haemoglobin; structural alloys |
| 27 | Co | Cobalt | 58.933 | Battery cathodes; vitamin B₁₂ |
| 28 | Ni | Nickel | 58.693 | Stainless steel; coinage |
| 29 | Cu | Copper | 63.546 | Electrical wiring; brass and bronze |
| 30 | Zn | Zinc | 65.38 | Brass; immune function |
| 33 | As | Arsenic | 74.922 | Semiconductors; historically a poison |
| 35 | Br | Bromine | 79.904 | Flame retardants; one of two liquid elements at room temp |
| 36 | Kr | Krypton | 83.798 | Defined the metre, 1960-1983 |
# The transition-metal jewels
## Where we get our coinage and catalysts
The 10-element-wide transition-metal block contains most metals familiar from everyday objects, currency, and chemistry catalysts.
| 42 | Mo | Molybdenum | 95.95 | Steel alloying; nitrogen fixation enzymes |
| 44 | Ru | Ruthenium | 101.07 | Pen nibs; catalysis |
| 45 | Rh | Rhodium | 102.906 | Catalytic converters; most expensive precious metal as of 2023 |
| 46 | Pd | Palladium | 106.42 | Catalytic converters; hydrogen storage |
| 47 | Ag | Silver | 107.868 | Coinage; antimicrobial; best electrical conductor |
| 74 | W | Tungsten | 183.84 | Highest melting point of any element (3422°C); incandescent filaments |
| 76 | Os | Osmium | 190.23 | Densest naturally occurring element (22.59 g/cm³) |
| 77 | Ir | Iridium | 192.217 | Second-densest; Cretaceous-Paleogene boundary layer marker for the asteroid impact |
| 78 | Pt | Platinum | 195.084 | Catalysis; jewellery; defined the kilogram 1889-2019 |
| 79 | Au | Gold | 196.967 | Jewellery; reserve currency; electronics contacts |
| 80 | Hg | Mercury | 200.59 | Only metal liquid at room temperature; thermometers; toxic |
# Lanthanides and actinides
## The "f-block"
These 30 elements (atomic numbers 57-71 and 89-103) get displayed below the main table so the layout fits on a page. Chemically they're remarkable: the lanthanides ("rare earths") are essential to almost every modern electronic device.
| 57 | La | Lanthanum | Camera lenses; battery electrodes |
| 58 | Ce | Cerium | Catalytic converters; glass polishing |
| 59 | Pr | Praseodymium| Magnets; aircraft engine alloys |
| 60 | Nd | Neodymium | The strongest permanent magnets ever made |
| 62 | Sm | Samarium | High-temperature magnets |
| 63 | Eu | Europium | Red and blue phosphors in OLED displays |
| 64 | Gd | Gadolinium | MRI contrast agents |
| 66 | Dy | Dysprosium | EV motor magnets |
| 67 | Ho | Holmium | Strongest magnetic moment of any element |
| 68 | Er | Erbium | Fibre-optic amplifiers |
| 70 | Yb | Ytterbium | Atomic clocks |
Despite the name, rare-earth elements aren't actually rare in the crust — they're just rarely concentrated in mineable deposits, and their chemistries are so similar that separation is the expensive step.
## The actinides
Atomic numbers 89-103. Only thorium (90) and uranium (92) occur in nature in meaningful quantities; everything else is synthesised in reactors or accelerators.
| 90 | Th | Thorium | 14 billion years (Th-232) |
| 92 | U | Uranium | 4.5 billion years (U-238) |
| 93 | Np | Neptunium | 2.1 million years (Np-237) |
| 94 | Pu | Plutonium | 80 million years (Pu-244) |
| 95 | Am | Americium | 7,370 years (Am-243); in smoke detectors |
| 96 | Cm | Curium | 16 million years (Cm-247) |
| 98 | Cf | Californium | 898 years (Cf-251) |
| 99 | Es | Einsteinium | 472 days (Es-252) |
| 102| No | Nobelium | 58 minutes (No-259) |
| 103| Lr | Lawrencium | 11 hours (Lr-266) |
# Synthesised elements
## Beyond uranium
Every element with $Z > 92$ has been made in a particle accelerator. The first transuranium element was neptunium in 1940 (Berkeley). The most recent additions completed period 7 in 2016:
| 113 | Nh | Nihonium | 2003 (RIKEN)| ~10 seconds (Nh-286) |
| 114 | Fl | Flerovium | 1999 (JINR/LLNL)| ~2.6 seconds (Fl-289) |
| 115 | Mc | Moscovium | 2003 (JINR/LLNL)| ~0.65 seconds |
| 116 | Lv | Livermorium | 2000 (JINR/LLNL)| ~60 milliseconds |
| 117 | Ts | Tennessine | 2010 (JINR/ORNL)| ~80 milliseconds |
| 118 | Og | Oganesson | 2002 (JINR/LLNL)| ~0.7 milliseconds |
Oganesson is the heaviest element produced to date and the only synthetic element named after a living person (Yuri Oganessian) at the time of its naming.
The hunt for the **island of stability** — a predicted region around $Z = 114-126$ where superheavy nuclei should be unusually long-lived — continues.
# Trends across the table
## Atomic radius
Atoms get **smaller** moving left-to-right across a period (more protons pulling the same shell closer) and **larger** moving top-to-bottom down a group (each new period adds a shell).
$$r_{\text{Cs}} \approx 260\ \text{pm} \quad \gg \quad r_{\text{He}} \approx 31\ \text{pm}$$
Caesium is roughly 8× the diameter of helium. The largest stable atom vs. the smallest.
## Ionisation energy
Energy needed to remove the outermost electron. **Opposite trend**: smaller, tighter atoms hold their electrons more strongly.
| Cs | 376 |
| K | 419 |
| Na | 496 |
| Mg | 738 |
| H | 1312 |
| F | 1681 |
| He | 2372 |
Helium is the hardest atom to ionise. The alkali metals are the easiest — which is why they react explosively with water (the water happily takes the loose electron).
## Electronegativity (Pauling scale)
How strongly an atom pulls electrons toward itself in a bond. Linus Pauling's 1932 scale runs from 0.7 (caesium) to 3.98 (fluorine).
| 3.98| 3.44| 3.04| 3.16| 2.55| 2.20| 0.93| 0.79|
The electronegativity *difference* between two bonded atoms predicts bond type:
- $\Delta\chi < 0.5$ → non-polar covalent (C-H, C-C)
- $0.5 \leq \Delta\chi < 1.7$ → polar covalent (H-O, H-N)
- $\Delta\chi \geq 1.7$ → ionic (Na-Cl is 2.23)
# Isotopes and abundance
## Same element, different mass
Atoms of one element can have different numbers of neutrons. The element is fixed by proton count; the isotope by total nucleon count.
| Hydrogen | ¹H (99.98%), ²H deuterium, ³H tritium | Deuterium in heavy water |
| Carbon | ¹²C (98.9%), ¹³C, ¹⁴C | ¹⁴C dating (half-life 5730 y) |
| Oxygen | ¹⁶O, ¹⁷O, ¹⁸O | ¹⁸O/¹⁶O ratio reconstructs paleoclimate |
| Uranium | ²³⁸U (99.27%), ²³⁵U (0.72%) | ²³⁵U is the fissile one |
The "atomic mass" on a periodic table is the weighted average of natural isotope abundances. That's why hydrogen reads 1.008 (mostly ¹H but with some ²H) rather than exactly 1.
## Elements in the universe
The Big Bang made hydrogen, helium, and trace lithium. Stars made everything up to iron via fusion. Supernovae and neutron-star mergers made everything heavier.
| H | 10¹² |
| He | 10¹¹ |
| C | 8.4 × 10⁸ |
| O | 6.8 × 10⁸ |
| Fe | 3.2 × 10⁷ |
| Au | 1.0 |
Carl Sagan: "The nitrogen in our DNA, the calcium in our teeth, the iron in our blood — were all made in the interiors of collapsing stars. We are made of star-stuff."
# What md2any showed off in this deck
## Feature index
- **Tables, tables, tables** — 14 of them, ranging from 3 columns to 6, with right-aligned numerics, mixed text + scientific notation, and back-to-back layouts
- **Unicode coverage** — element symbols (Th, U), Greek symbols ($\Delta\chi$), superscripts (¹²C, ²³⁸U), subscripts (B₁₂), degree sign (°C)
- **Math** — inline ($r_{\text{Cs}} \approx 260$ pm) and display ($\Delta\chi$ thresholds)
- **Block quotes** with attribution (Sagan)
- **Comparison rows** with $\gg$ relations
- **Front-matter TOC** auto-generates the contents slide
All facts are textbook periodic-table data. Sources: Royal Society of Chemistry periodic table; NIST atomic weights; IUPAC element discovery summaries.
