Periodic trends explained
The periodic table organizes elements so that recurring (periodic) patterns line up into columns and rows. These are the trends every chemistry student is expected to know — and each one falls straight out of electron structure.
Atomic radius
r: → decreases · ↓ increases
Atomic radius decreases across a period from left to right, because each added proton increases the nuclear charge pulling on electrons that are all in the same shell. It increases down a group, because each new period adds an entirely new, larger electron shell. Caesium and francium are among the largest atoms; helium is the smallest.
Electronegativity
χ: → increases · ↓ decreases (max: F = 3.98)
Electronegativity — an atom's pull on shared bonding electrons — increases across a period and decreases down a group, mirroring atomic radius in reverse. Fluorine is the most electronegative element (3.98 on the Pauling scale); the alkali metals at the bottom-left are the least. Large electronegativity differences between two bonded atoms produce ionic bonds; small differences give covalent bonds.
Ionization energy
IE₁: → increases · ↓ decreases
First ionization energy is the energy needed to remove the outermost electron. It rises across a period (electrons are held more tightly by the growing nuclear charge) and falls down a group (outer electrons are farther from the nucleus and shielded by inner shells). Noble gases sit at the peaks; alkali metals at the valleys, which is why alkali metals lose an electron so easily to form +1 ions.
Electron affinity
EA: highest at the halogens
Electron affinity is the energy released when an atom gains an electron. Halogens have the highest values — chlorine tops the list — because adding one electron completes their outer shell. Noble gases have essentially zero affinity: their shells are already full, so an extra electron would have to start a new, high-energy shell.
Metallic character
metallic character: → decreases · ↓ increases
Metallic character — the tendency to lose electrons and form positive ions — increases down a group and decreases across a period, the opposite of electronegativity. The most metallic elements sit at the bottom-left; the most nonmetallic at the top-right. The metalloids (boron, silicon, germanium, arsenic, antimony, tellurium) form a diagonal staircase marking the fuzzy border between metals and nonmetals.
s, p, d, and f blocks
s=2 · p=6 · d=10 · f=14 electrons
The table is divided into blocks by which subshell is being filled. The 2-wide s-block and 6-wide p-block make up the main-group elements; the 10-wide d-block holds the transition metals; and the 14-wide f-block — the lanthanides and actinides — is pulled out below the table to keep it readable. The block widths are exactly the electron capacities of the s (2), p (6), d (10), and f (14) subshells.
The element families
Coloring the table by family groups elements that behave alike. Here is what each color means.
- Alkali metal — Soft, silvery, extremely reactive metals with one valence electron; they react vigorously with water.
- Alkaline earth metal — Reactive metals with two valence electrons; harder and less reactive than the alkali metals.
- Transition metal — Hard, dense metals that form colored compounds and multiple oxidation states; they fill the d-subshell.
- Post-transition metal — Softer metals to the right of the transition block, such as aluminium, tin, and lead.
- Metalloid — Elements along the staircase with in-between properties; silicon and germanium are the basis of semiconductors.
- Reactive nonmetal — Reactive nonmetals including the building blocks of life — carbon, nitrogen, oxygen, phosphorus, and sulfur.
- Halogen — Highly reactive nonmetals of group 17 that readily gain one electron to form −1 ions and salts.
- Noble gas — Inert gases of group 18 with full outer shells; they rarely react at all.
- Lanthanide — The 4f-block rare-earth metals, essential to magnets, lasers, and phosphors.
- Actinide — The 5f-block metals, all radioactive, including uranium and plutonium.
- Unknown properties — Superheavy synthetic elements whose chemical properties are predicted but not yet fully measured.
Frequently asked questions
How do you read the periodic table?
Read it left to right, top to bottom, in order of increasing atomic number (the number of protons). Each row is a period and each column is a group. Elements in the same group share the same number of outer-shell electrons and therefore behave chemically alike. An element's box shows its atomic number, symbol, and atomic mass.
What is an electron configuration?
An electron configuration lists how an atom's electrons are distributed among its shells and subshells, for example carbon is 1s² 2s² 2p². It follows the aufbau principle — electrons fill the lowest-energy subshells first. This tool shows the noble-gas shorthand configuration and a Bohr shell diagram for every element.
Why do periodic trends happen?
Because chemical behaviour is controlled by the outermost electrons and how strongly the nucleus holds them. Moving across a period adds protons without adding a new shell, so atoms get smaller and hold electrons more tightly. Moving down a group adds whole new shells, so atoms get bigger and hold their outer electrons more loosely.
What is the difference between a group and a period?
A group is a vertical column (there are 18) and a period is a horizontal row (there are 7). Groups gather elements with similar chemistry; periods track the progressive filling of a new electron shell. The number at the top of a main-group column equals the count of valence electrons.
Where are the metals and nonmetals?
Metals fill the left and center of the table — the vast majority of elements. Nonmetals cluster at the top-right, with the noble gases in the last column. A diagonal staircase of metalloids (boron, silicon, germanium, arsenic, antimony, tellurium) separates the two, showing gradually blended properties.
Which elements are liquid at room temperature?
Only two elements are liquid at 25 °C (298 K): mercury and bromine. Gallium and caesium melt just above room temperature (around 30 and 28 °C). Use the State at T mode to see how the count of solids, liquids, and gases changes as you raise or lower the temperature.
Problem solved in full
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Carbon at 12.011 u rebuilt from 12 C and 13 C isotopes 5 steps
The element card gives carbon an atomic mass of 12.011 u, and no carbon atom weighs that. This is the Carbon detail state. Rebuild the figure from the two isotopes a terrestrial sample contains — 12C at 98.93% abundance, 13C at 13.00335 u and 1.07% — then work out what that 1.07% does to a molecule carrying 60 carbon atoms.
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Start with the unit, because one of these two masses was never measured. The unified atomic mass unit is defined as one twelfth of a 12C atom, so 12C weighs 12 u exactly, to as many decimals as you care to write. Only the 13C figure came from an experiment.
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The tabulated mass is an average over isotopes weighted by how many atoms of each a sample contains. Weight by mass fraction instead and you get a different, wrong number.
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That is the value on the card, rebuilt from two isotope masses and one abundance measurement.
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Now look at what the average is not. 98.93% of carbon atoms sit 0.011 u below the printed figure and the remaining 1.07% sit 0.992 u above it. The mean lands in a gap where no atom exists.
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Put 60 of those atoms in one molecule. The expected number of 13C is 60 × 0.0107 = 0.642, which invites the conclusion that a typical buckyball carries one heavy atom. It does not: the binomial gives 52.4% carrying none against 34.0% carrying exactly one.
Answer
12.011 u — a mass no carbon atom has. C60 therefore does not have a mass, it has a spectrum: 52.4% of the molecules weigh 720.000 u exactly, 34.0% weigh 721.003 u, 10.9% weigh 722.007 u, and the ladder runs on with rungs 1.00335 u apart. Multiplying 60 by 12.011 predicts 720.66 u, which is not the mass of a single molecule in the sample. Scale the same calculation up to a protein with 1000 carbon atoms and 0.98931000 = 2.1 × 10⁻⁵ of the molecules are free of 13C — 2 in 100 000, so the all-12C line has effectively vanished and the envelope is the only thing left to weigh. Every mass on this table is a statement about a population, and the larger the molecule you build, the more the population is all you can see.
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References (2)
- Insight block 4 credits the table with predicting elements. Three of them, and how they were found: M. V. Orna & M. Fontani, "Discovery of Three Elements Predicted by Mendeleev’s Table: Gallium, Scandium, and Germanium", in Perspectives on the History of Chemistry, 227–257. Springer, 2021.
- And the other half of that record, which the story usually leaves out: G. Lente, "Where Mendeleev was wrong: predicted elements that have never been found." ChemTexts 5, article 17, 2019.