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How to Read the Periodic Table

Work through the steps in order, from a single cell to whole-table trends, and you will be able to predict how an element behaves just from where it sits.

  1. The information shown in each cell

    Each cell typically shows the atomic number at the upper left or top, the element symbol prominently in the center (C for carbon, O for oxygen), and the element name and atomic weight (average mass) below. Because atomic weight is an average reflecting the natural proportions of an element's isotopes, it is usually a decimal rather than a whole number. Example: the cell for sodium shows 11, Na, sodium and about 22.99. The 11 tells you it has 11 protons, and, in a neutral atom, 11 electrons.

  2. What a period (horizontal row) means

    The horizontal rows are called periods, and elements in the same period have their outermost electrons in the same electron shell. Period 1 has one shell, period 2 has two, and so on; the current table has 7 periods. The rows are not the same length: they contain 2, 8, 8, 18, 18, 32 and 32 elements, because higher shells have more room for electrons. Moving across a period, elements change gradually from reactive metals to nonmetals and finally to a noble gas.

  3. What a group (vertical column) means

    The vertical columns are groups, numbered 1 to 18. Elements in the same group tend to have similar chemical properties because they have the same number of outermost (valence) electrons. For example, group 1 (lithium, sodium, potassium and others) consists of highly reactive alkali metals, while group 18 (helium, neon, argon and others) consists of noble gases that rarely react at all. For the main groups 1, 2 and 13 to 18, the last digit of the group number equals the number of valence electrons, except for helium, which has two.

  4. From group number to valence electrons and typical charge

    For the main-group elements, the group number points to the usual ion charge. Group 1 elements tend to lose one electron and form +1 ions, group 2 elements form +2 ions, group 17 elements tend to gain one electron and form -1 ions, and group 16 elements form -2 ions. This is why sodium (group 1) and chlorine (group 17) combine one to one in sodium chloride, NaCl, while magnesium (group 2) and chlorine form MgCl2. The transition metals are less predictable because many can form several different charges.

  5. The metal, nonmetal, and metalloid regions

    A staircase-shaped line splits the table into metals, which make up most of the left side and center, and nonmetals, mostly in the upper right. Near that line sit metalloids, or semimetals, such as boron, silicon and germanium, which have some properties of both. Metals are generally shiny and conduct electricity and heat well and can be shaped without breaking. Nonmetals often have the opposite properties. Hydrogen is a nonmetal even though it is placed at the top of group 1.

  6. The concept of the s/p/d/f blocks

    The table can also be divided into s, p, d and f blocks according to the type of orbital being filled. Groups 1 and 2 belong to the s block; groups 13 through 18 belong to the p block; the transition metals in the middle belong to the d block; and the lanthanides and actinides, set apart at the bottom, belong to the f block. This reflects the type of orbital each element's last electron enters. The block label also helps you read an element's electron configuration, such as 1s2 2s2 2p6 3s1 for sodium.

  7. Why elements in the same group are similar

    An element's chemical properties are determined mainly by its outermost electrons. Elements in the same group may differ in the number of shells, but they share the same number of outermost electrons, giving them similar ways of bonding and similar reactivity. This was the core principle that let Mendeleev arrange the elements by periodic properties, and it allowed him to predict undiscovered elements from the gaps in his table.

  8. Practice: reading three elements

    Take chlorine, iron and neon. Chlorine is in group 17 and period 3, in the p block: a nonmetal with seven valence electrons that readily gains one electron to form Cl-. Iron is in group 8 and period 4, in the d block: a transition metal that can form more than one ion, such as Fe2+ and Fe3+. Neon is in group 18 and period 2: a noble gas with a full outer shell that is chemically inert. Each description comes directly from position, without memorizing anything about the element itself.

How was the periodic table created?

The periodic table originated in 1869, when the Russian chemist Mendeleev arranged the elements known at the time by atomic weight and noticed that properties repeat at regular intervals. He left gaps and predicted that missing elements would be found; later discoveries of elements such as gallium and germanium matched his predictions closely. In the early twentieth century, Moseley showed that atomic number, not atomic weight, is the right ordering principle, and the table was refined into its current form. All 118 elements are now filled in, and in 2016 IUPAC approved the names of the last four: nihonium, moscovium, tennessine and oganesson.

Reading the table in a few seconds

A fast routine works well. Look at the group first, since it tells you the number of valence electrons and the family of the element. Then look at the period, which tells you how many electron shells it has and roughly how large it is. Next, check the color or position to see whether it is a metal, nonmetal or metalloid. Finally, use the trends to compare it with its neighbors: is it more or less electronegative than the element above it, and is it larger or smaller than the element to its left. With practice you can answer these questions without looking anything up.

Common mistakes when reading the table

A frequent mistake is to confuse rows and columns: periods run horizontally and groups run vertically. Another is to assume the table is ordered by atomic weight; it is ordered by atomic number, which is why a few pairs, like argon and potassium, appear in an order that looks reversed by mass. Learners also treat the group number as the number of valence electrons for every column, which holds for the main groups but not for the transition metals. Keep in mind that textbooks differ in how they label groups and which elements they place in group 3. This page is general educational information. For atomic structure basics, see our guide to atoms.

Frequently Asked Questions

Where can I look up detailed information on all 118 elements myself?

If you are curious about detailed information for individual elements, try our interactive periodic table tool, which contains data for all 118 elements. This page is a guide focused on how to read the periodic table itself.

Which block do the transition metals belong to?

The transition metals, groups 3 to 12, mostly belong to the d block. These elements commonly share the ability to take on multiple oxidation states and often form colored compounds, as seen in copper sulfate solutions or the many colors of chromium compounds.

Why do some elements have atomic weights in brackets?

Elements with no stable isotopes, such as technetium or the heavy synthetic elements, do not have a natural isotope mixture to average. Many tables therefore show the mass number of their longest-lived or most important isotope in brackets instead of an average.

Why is hydrogen at the top of group 1 if it is not a metal?

Hydrogen has one valence electron like the alkali metals, so it is placed above them, but its properties are very different, and it is a nonmetal gas. Some tables place it separately or above group 17 instead, and textbooks vary.

How can I predict the formula of a simple ionic compound from the table?

Use the typical charges from the group numbers. Sodium (+1) and chlorine (-1) give NaCl. Calcium (+2) and chlorine give CaCl2, as two chloride ions are needed to balance one calcium ion. Aluminum (+3) and oxygen (-2) give Al2O3, because two Al3+ ions give +6 and three O2- ions give -6.