Introduction to the Periodic Table Layout
The periodic table is an arrangement of all known elements in order of increasing atomic number, in rows called periods and columns called groups, so that elements with similar properties fall in the same column. In brief: the table has 7 periods and 18 groups; metals are on the left and in the center, nonmetals are at the upper right, and the noble gases are in the last column on the right.
Learning how to read the layout of the periodic table is one of the most useful skills in chemistry. Position tells you the type of element, its number of electrons and often how it will react. This article explains periods, groups and blocks, shows how to read an element box, lists the lengths of the periods in a table, works through finding information from position and gives the history of the table.
How is the periodic table arranged?
The elements are listed by atomic number, which is the number of protons in the nucleus. Hydrogen (1) is first, and the table continues through to oganesson (118). The sequence is cut into rows so that elements with similar chemical behavior line up in columns. This repeating pattern of properties is why the table is called periodic.
What are periods?
A period is a horizontal row. The period number equals the number of occupied electron shells in the atoms of that row. Across a period from left to right, atomic number increases and properties change gradually from metallic to nonmetallic, ending in a noble gas.
| Period | Number of elements | Atomic numbers | First element | Last element |
|---|---|---|---|---|
| 1 | 2 | 1 to 2 | Hydrogen | Helium |
| 2 | 8 | 3 to 10 | Lithium | Neon |
| 3 | 8 | 11 to 18 | Sodium | Argon |
| 4 | 18 | 19 to 36 | Potassium | Krypton |
| 5 | 18 | 37 to 54 | Rubidium | Xenon |
| 6 | 32 | 55 to 86 | Cesium | Radon |
| 7 | 32 | 87 to 118 | Francium | Oganesson |
The total is 2 + 8 + 8 + 18 + 18 + 32 + 32 = 118 elements. In printed tables, the lanthanides (57 to 71) and actinides (89 to 103) are usually placed in two rows below the main table to keep it a reasonable width.
What are groups?
A group is a vertical column, numbered 1 to 18. Elements in the same group have the same number of outer (valence) electrons for the main groups, and therefore similar chemical properties. Going down a group, the atoms get larger because they have more electron shells.
| Group | Name | Examples | Outer electrons |
|---|---|---|---|
| 1 | Alkali metals | Li, Na, K | 1 |
| 2 | Alkaline earth metals | Mg, Ca, Ba | 2 |
| 3 to 12 | Transition metals | Fe, Cu, Zn | varies |
| 13 | Boron group | B, Al | 3 |
| 14 | Carbon group | C, Si, Sn | 4 |
| 15 | Nitrogen group | N, P | 5 |
| 16 | Oxygen group (chalcogens) | O, S | 6 |
| 17 | Halogens | F, Cl, Br, I | 7 |
| 18 | Noble gases | He, Ne, Ar | 8 (2 for helium) |
How do you read an element box?
Each box usually gives the atomic number (whole number, often at the top), the symbol, the name and the relative atomic mass (a decimal number). For example, the box for carbon shows 6, C, carbon and 12.011.
- Atomic number (6): the number of protons, and in a neutral atom also the number of electrons.
- Symbol (C): the one- or two-letter abbreviation.
- Relative atomic mass (12.011): the weighted average mass of the atoms of the element, in atomic mass units, which takes isotopes into account.
What are the main regions of the table?
Metals fill the left side and center. Nonmetals fill the upper right. A zigzag line from boron to astatine marks the position of the metalloids. Chemists also speak of the s-block (groups 1 and 2, plus helium), the p-block (groups 13 to 18), the d-block (the transition metals) and the f-block (lanthanides and actinides). The block depends on the type of orbital that is being filled by the last electron.
Worked example: reading information from position
- Where is sulfur? Sulfur has atomic number 16. It is in period 3, since it falls between sodium (11) and argon (18), and group 16. It is a nonmetal in the p-block with 6 outer electrons.
- Predict similar elements. Oxygen is in the same group, 16. Sulfur and oxygen both tend to form ions with a 2− charge, such as S2− and O2−, so sodium sulfide is Na2S, like sodium oxide Na2O.
- Number of electron shells. Calcium (atomic number 20) is in period 4, so its atoms have 4 occupied shells and 2 outer electrons, because it is in group 2.
- Count from a position. The element in period 2, group 17 is fluorine (atomic number 9): period 2 starts at 3 and group 17 is the seventh of the 8 elements in the period, so 3 + 6 = 9.
Who created the periodic table?
In 1869, Dmitri Mendeleev arranged the 63 elements known at the time by atomic mass and chemical properties and left gaps for elements that had not been discovered yet. He predicted the properties of some of them, such as eka-aluminum, which was discovered in 1875 as gallium. Lothar Meyer independently produced a similar table. In 1913, Henry Moseley showed that the elements are better ordered by atomic number than by atomic mass, which explained a few places where Mendeleev’s order by mass did not fit the pattern of properties.
Which mistakes are common?
Mistake 1: Confusing periods and groups
Periods are rows, and groups are columns. Remember that groups go “down” like pillars.
Mistake 2: Thinking that the relative atomic mass is a whole number
It is an average over isotopes, so it usually has decimals. The atomic number is the whole number.
Mistake 3: Expecting hydrogen to behave like the alkali metals
Hydrogen sits at the top of group 1 but is a nonmetal gas with unique properties.
Mistake 4: Forgetting the two rows below the table
The lanthanides and actinides belong in periods 6 and 7 and are shown separately only for space.