# How to Use a Periodic Table

It’s important to know how to use a periodic table. The periodic table organizes the elements in a way that allows you to predict element properties and chemical reactions, even if you don’t know anything about an element except what you see on the table. Here’s a look at the information you can find on a standard periodic table of the elements and how to use these facts:

### Periodic Table Organization

The key to knowing how to use a periodic table is understanding its organization:

• Elements are listed in order of increasing atomic number. The atomic number is the number of protons in all atoms of an element. If the number of electrons in an atom changes, it becomes a different ion, but the same element. If the number of neutrons in an atom changes, it becomes a different isotope of the element. (Note: Mendeleev’s original table organized element by order of increasing atomic weight.)

• Elements are grouped according to periodic properties or trends. On a colored periodic table, the element groups usually are different colors from each other. The main element groups are: alkali metals, alkaline earth metals, transition metals, basic metals, rare earth metals (lanthanides and actinides), metalloids (semimetals), nonmetals, halogens, and noble gases. There are different group numbering methods. The most common method lists Arabic numbers above the top of the table, from 1 to 18. But, some periodic table use Roman numerals.

• A row of the periodic table is called an element period.  A period indicates the highest energy level occupied by the electrons of that element at its ground state. There are 7 periods on the periodic table. Hydrogen (H) and helium (He) are in the same period as one another. Scandium (Sc) and titanium (Ti) are in the same period. Francium (Fr) and actinium (Ac) are in the same period, even though it isn’t immediately obvious they are in the same row.

• A column of the periodic table is called an element group. Members of an element group have the same number of valence electrons. For example, lithium (Li) and sodium (Na) are in the same element group (alkali metals or group 1). Both lithium and sodium each have one valence electron.

• The two rows separated from the main body of the table are the rare earth elements, which consist of the lanthanides and actinides. These elements may be considered special transition metals. If you look at their atomic numbers, you see the lanthanides actually fit between barium (Ba) and hafnium (Hf). The actinides fit between radium (Ra) and rutherfordium (Rf).

### How to Read an Element Cell

Each element cell or tile offers important information about that element. The organization of the information varies, but you can expect certain key facts:

• The one- or two-letter symbol is the element’s symbol. Usually, the symbol includes the first letter of an element’s name, although there are some exceptions. For example, H is the element symbol for hydrogen. Br is the element symbol for bromine. Yet, Hg is the symbol for mercury. Element symbols are recognized and used internationally, even though countries may use different names for elements.

• Some periodic tables list each element’s full name.

• The integer number is the element atomic number. It is the unique identifier for a particular element. This is the number of protons in each atom of that element. For example, each bromine atom has 35 protons. Atoms of different elements may have the same number of electrons and neutrons, but never the same number of protons. At present, there are 118 elements, so atomic numbers range from 1 (hydrogen) to 118 (oganesson).

• The decimal number is the element’s relative atomic mass. The relative atomic mass (sometimes called atomic weight) is a weighted average of the mass of the isotopes of that element. Atomic mass is given in atomic mass units (amu). You can also consider the number as the grams per mole of each element. For example, one mole of bromine atoms would have a mass of 79.904 grams.

### How to Use a Periodic Table to See Periodic Table Trends

The table is organized to display trends or periodicity of element properties:

Atomic Radius: one-half the distance between the nuclei of two atoms that are just touching each other.

Ionization Energy: energy needed to completely remove an electron from an atom or ion in the gas phase.

Electron Affinity: measure of an atom’s ability to accept an electron.

Electronegativity: measure of an atom’s ability to form a chemical bond

### Summary of Periodic Table Trends

Part of learning how to use a periodic table means understanding trends in element properties. The periodic table organization shows trends in atomic radius, ionization energy, electron affinity, and electronegativity.

Moving Left → Right Across a Row of the Periodic Table

• Ionization Energy Increases
• Electron Affinity Generally Increases (except Noble Gas Electron Affinity Near Zero)
• Electronegativity Increases

Moving Top → Bottom Down a Column of the Periodic Table