The periodic table isn't just a list of elements — its layout encodes real chemistry. Elements are arranged in order of increasing atomic number, but the table wraps into rows and columns specifically so that elements with similar properties line up in the same column. That's the entire point of the design. When Dmitri Mendeleev built the first successful table in 1869, he trusted that pattern so much that he left gaps for elements nobody had found yet and predicted their properties. His "eka-silicon", predicted to sit below silicon with an atomic mass near 72 and a high melting point, was discovered in 1886 as germanium, with almost exactly the properties he forecast. The modern periodic law says that when elements are arranged by atomic number, their properties repeat in a regular (periodic) way — and the rest of this lesson explains why they repeat.
Periods (rows)
A period is a horizontal row. The period number tells you how many electron shells (energy levels) an atom of that element uses. Elements in Period 3 (Na through Ar) all have electrons occupying shells 1, 2, and 3. Moving left to right across a period, protons and electrons are added one at a time to the same outer shell. That also explains the lengths of the first rows. Period 1 has only two elements (H and He) because shell 1 holds just 2 electrons. Periods 2 and 3 each have eight elements because their outer shells fill with 8. When a shell is full, the next element has to start a new shell — and a new row — which is why every period begins with a very reactive metal holding a single valence electron and ends with an unreactive noble gas.
Groups (columns) and families
A group (or family) is a vertical column. Elements in the same group have the same number of valence (outer-shell) electrons, which is why they show similar chemical behaviour — valence electrons are what control bonding. For main-group elements, the group number (1, 2, 13–18) tells you the number of valence electrons directly (for groups 13–18, subtract 10).
| Family | Group # | Valence e⁻ | Key trait |
|---|---|---|---|
| Alkali metals | 1 | 1 | Extremely reactive, soft metals; never found free in nature |
| Alkaline earth metals | 2 | 2 | Reactive metals, less so than Group 1 |
| Transition metals | 3–12 | varies | Form multiple ion charges, often coloured compounds |
| Halogens | 17 | 7 | Very reactive nonmetals; form salts with metals |
| Noble gases | 18 | 8 (2 for He) | Full outer shell; essentially unreactive |
You can watch the family resemblance in the lab. Every alkali metal reacts with water to make hydrogen gas and a metal hydroxide, and the reaction gets more violent down the group: lithium fizzes steadily, sodium melts into a ball and skates across the surface, and potassium bursts into a lilac flame. The halogens all exist as diatomic molecules (, , , ), and their state changes smoothly down the group — fluorine and chlorine are gases, bromine is a red-brown liquid and iodine is a grey-black solid at room temperature. The noble gases are so unreactive that they are used where nothing must react: argon fills incandescent bulbs and shields welds, and helium lifts balloons safely.
| Group | Valence e⁻ | Easiest change | Typical ion | Example |
|---|---|---|---|---|
| 1 | 1 | Lose 1 | 1+ | Na⁺ |
| 2 | 2 | Lose 2 | 2+ | Mg²⁺ |
| 13 | 3 | Lose 3 | 3+ | Al³⁺ |
| 15 | 5 | Gain 3 | 3− | N³⁻ |
| 16 | 6 | Gain 2 | 2− | O²⁻ |
| 17 | 7 | Gain 1 | 1− | Cl⁻ |
| 18 | 8 | None | No ion | Ar |
Metals, nonmetals, and metalloids
- Metals (left/centre of the table): shiny, malleable, ductile, good conductors, tend to lose electrons and form cations.
- Nonmetals (upper right): dull, brittle if solid, poor conductors, tend to gain electrons and form anions.
- Metalloids (along the staircase line, e.g. B, Si, Ge, As, Sb, Te): properties in between; several are semiconductors used in electronics.
- About four out of every five elements
- All solids at SATP except mercury (a liquid)
- Loosely held valence electrons move freely, so they conduct heat and electricity
- Atoms slide past each other, so they bend (malleable) and draw into wires (ductile)
- Lose electrons to form positive ions
- Only about 17 elements: hydrogen plus the upper right, including the noble gases
- Eleven are gases at SATP, bromine is a liquid, the rest are solids
- Valence electrons are held tightly, so they are insulators
- Solid nonmetals such as sulfur shatter when struck (brittle)
- Gain electrons to form negative ions, or share electrons with other nonmetals
Reading the table like a chemist
- Find the element's period: that is how many electron shells it uses.
- Find its group: for main-group elements, that gives the valence electrons (groups 1–2 directly; groups 13–18 subtract 10).
- Combine the two to write the electron arrangement: fill the inner shells completely, then put the valence electrons in the last shell.
- Check its region: left of the staircase means metal (forms a cation), right means nonmetal (forms an anion), on the line means metalloid.
- Predict its ion from the fewest electrons it must lose or gain to match the nearest noble gas.