The periodic table is a map, and its best feature is that the neighbourhoods mean something. Move one square to the right and an element grips its electrons harder. Move one row down and it lets them go more easily. Those two moves explain a surprising amount of chemistry.
To make the pattern visible, I built the Periodic Table Trends app. Pick a property, and every element is coloured on that scale. Click an element and the side panel plots the same property across its period and down its group.
Across a period: electrons held tighter
Take period 2, lithium to fluorine. Electronegativity, the tendency of an atom to pull bonding electrons toward itself, climbs from 0.98 for lithium to 3.98 for fluorine on the Pauling scale. Period 3 does the same, from 0.93 for sodium to 3.16 for chlorine. The standard explanation is that each step right adds a proton to the nucleus without adding an inner shell, so the outer electrons feel a stronger pull.
Atomic size follows the same logic in reverse. In the dataset behind the app, the period-2 radii shrink from 145 pm for lithium to 50 pm for fluorine. A stronger pull means a tighter atom.
Down a group: electrons held looser
Group 1 shows the opposite trend. First ionization energy, the energy needed to remove one electron, falls from 520.2 kJ/mol for lithium to 495.8 for sodium, 418.8 for potassium, 403 for rubidium and 375.7 for caesium. Radius rises over the same stretch, from 145 pm to 260 pm. Each row down adds a shell, which puts the outer electron farther from the nucleus and shields it from the charge inside.
Caesium has the lowest first ionization energy in the dataset, and fluorine the highest electronegativity. At the other extreme, helium has the highest first ionization energy, 2372.3 kJ/mol, followed by neon and fluorine.
Where the pattern breaks
The best part of a trend is the exception. Across period 2, first ionization energy mostly rises, but beryllium (899.5 kJ/mol) is higher than boron (800.6), and nitrogen (1402.3) is higher than oxygen (1313.9). The usual textbook reasons involve the energy of the sub-shell that boron’s outer electron sits in, and the extra repulsion when oxygen has to pair two electrons in one orbital. Click through period 2 in the app, switch to ionization energy, and the dips jump out of the line chart.
Use the app also for the gaps. Not every element has a published value for every property: helium has no Pauling electronegativity in the data, and some of the heaviest elements have no measured values at all. Some of the listed densities for the heaviest elements, in the range of 37 to 41 g/cm³, are predictions rather than measurements, so I would not treat them as established.
Open Periodic Table Trends in its own tab, or use its Full screen button.
States of matter at any temperature
The last view in the property menu colours each element as solid, liquid or gas at a temperature you choose with a slider. At 300 K, the data lists eleven gases: hydrogen, helium, nitrogen, oxygen, fluorine, neon, chlorine, argon, krypton, xenon and radon. Bromine and mercury are the two familiar liquids; copernicium also shows up as a liquid in the data, which I would read as a prediction. Push the slider up to 1,800 K and the first transition row splits: manganese, cobalt, nickel and copper have melted, zinc has boiled away, and iron, with a melting point of 1,811 K, is about to go.
The extremes are worth a look. In the data, helium melts at 0.95 K and hydrogen at 13.99 K. At the hot end, tungsten melts at 3,695 K and rhenium at 3,459 K; carbon is listed at 4,600 K, which is a different kind of number because carbon does not melt at ordinary pressure.
Limits of the data
The app is only as good as its source, an open dataset called Periodic-Table-JSON, which in turn derives from Wikipedia, and I have not checked each of the thousands of values against a primary reference. Atomic radius is the empirical value where one exists and the calculated value otherwise, so comparisons between elements measured different ways, especially across the noble gases, deserve caution. If you need a number for real work, check it against a primary source such as the NIST or CRC handbook data.
Other simulators in this series: double-slit interference, chaos attractors and Orbit Lab.
Sources
- Periodic-Table-JSON by Bowserinator, CC BY-SA 3.0, derived from Wikipedia: github.com/Bowserinator/Periodic-Table-JSON. All numbers quoted above were read from this dataset.
