Name: how valence electrons shape the periodic table
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Periodic Classification of the Elements

The electrons occupying the outermost shell of an atom are valence electrons, while electrons occupying the inner principal energy levels are core electrons. 

A sodium atom with a [Ne]3s1 electron configuration has three principal energy levels. The complete inner principal energy levels with [He]2s22p6 indicate that there are ten core electrons followed by the third outermost level containing the remaining one electron. Therefore sodium has one valence electron. Similarly, chlorine has ten core electrons and seven valence electrons.

Valence electrons are the farthest from the nucleus and are held most loosely. Hence, they are the easiest to lose or share and play an important role in chemical bonding.

Elements that have the same number of valence electrons exhibit similar chemical properties, as can be seen from the arrangement in the modern periodic table. 

For main-group elements, the lettered group number equals the number of valence electrons and the row number equals the highest principal quantum number of that element.

Each element in a group has the same number of electrons available for bonding. Down the group, the principal quantum number increases by one, whereas the number of valence electrons remains the same. 

The two far-left columns of the periodic table constitute the s-block. For these elements, the last electron enters an s-orbital. Group one elements except for hydrogen are called the alkali metals and are extremely reactive as they have only one valence electron. Group two elements are the alkaline earth metals with two electrons in the valence shell. 

The six far-right columns form the p-block. The valence shell of these elements has completely occupied s-orbitals and the last electron enters the p-orbital. From group three A to group eight A, the number of electrons in p orbitals increases by one. The noble gases have eight valence electrons except for helium, which belongs to the s-block, and only has two electrons.

The d-block consists of the ten columns placed between the s and p-block. These elements are referred to as the transition metals. The last electron enters the d-orbital of the principal shell number one less than the row number. In the fourth row, three d-orbitals fill, in the fifth row, four d-orbitals fill, and so on.

Inner transition elements constitute the f-block and have the last electron entering an f-orbital. The principal quantum number of the f-orbitals that fill across each row is two less than the row number. In the sixth row, the four f-orbitals fill, and in the seventh row, the five f-orbitals fill. Inner transition elements are arranged in the lanthanide series and the actinide series. 

The number of columns in each block indicates how many electrons can be filled in the sublevel of the block. Two columns in the s-block correlate to an s-orbital with two electrons, six columns in the p-block represent three p-orbitals with six electrons, while ten columns in the d-block correspond to five d-orbitals with two electrons each.

Lastly, the f-block comprises fourteen columns indicating the maximum capacity of seven f-orbitals.

periodic classification of the elements

The periodic table arranges atoms based on increasing atomic number so that elements with the same chemical properties recur periodically. When their electron configurations are added to the table, a periodic recurrence of similar electron configurations in the outer shells of these elements is observed. Because they are in the outer shells of an atom, valence electrons play the most important role in chemical reactions. The outer electrons have the highest energy of the electrons in an atom and are more easily lost or shared than the core electrons. Valence electrons are also the determining factor in some physical properties of the elements.

The horizontal rows are known as periods. Across a period, each consecutive element has an additional proton in the nucleus and an additional electron to the valence shell. The vertical columns are groups. Elements in any one group (or column) have the same number of valence electrons (Figure 1); the alkali metals lithium and sodium each have only one valence electron, the alkaline earth metals beryllium and magnesium each have two, and the halogens fluorine and chlorine each have seven valence electrons. It is the loss, gain, or sharing of valence electrons that defines how elements react. The similarity in chemical properties among elements of the same group occurs because they have the same number of valence electrons. 

It is important to remember that the periodic table was developed on the basis of the chemical behavior of the elements, well before any idea of their atomic structure was available. Now the arrangement of the periodic table is understood; elements whose atoms have the same number of valence electrons are in the same group. The colored sections of Figure 1 show the three categories of elements classified by the orbitals being filled.

<img alt="Periodic table diagram with s, p, d, f blocks; chemical elements categorized by properties." src="/files/core/CDNSource/lessons/1046/1046Image_1v2.png" data-altupdatedat="1754904545000" data-alt="Periodic table diagram categorizing elements into s, p, d, and f blocks." /> 
Figure 1: This version of the periodic table shows the configuration of each element. Note that down each group, the configuration is often similar.

Main group elements are sometimes called representative elements. These are the elements in which the last electron added enters an s or a p orbital in the outermost shell, shown in blue and red in Figure 1. This category includes all of the nonmetallic elements, as well as the metalloids and many metals. The valence electrons for main group elements are those with the highest n level. For example, gallium (Ga, atomic number 31) has the electron configuration [Ar]4s23d104p1, which contains three valence electrons (underlined). The completely filled d orbitals count as core, not valence, electrons.

The two far-left columns comprise the s-block and the six far-right columns constitute the p-block. The noble gases, which are a part of the p-block, all have eight valence electrons except for helium, which has two. These elements are highly stable and unreactive.

Transition elements or transition metals: These are metallic elements in which the last electron added enters a d orbital. The valence electrons (those added after the last noble gas configuration) in these elements include the ns and (n &#8211; 1) d electrons. The official IUPAC definition of transition elements specifies those with partially filled d orbitals. Thus, the elements with completely filled orbitals (Zn, Cd, Hg, as well as Cu, Ag, and Au in Figure 1) are not technically transition elements. However, the term is frequently used to refer to the entire d block (colored yellow in Figure 1).

The d-block consists of 10 columns. The principal quantum number of the d orbitals that fill across each row is equal to the row number minus one. In the fourth row, 3d orbitals fill, in the fifth row, 4d orbitals fill, and so on.

Inner Transition Elements: They are shown in green in Figure 1. The valence shells of the inner transition elements consist of the (n &#8211; 2)f, the (n &#8211; 1)d, and the ns subshells. Inner transition elements constitute f-block as the last electron enters an f orbital. The principal quantum number of the f orbitals that fill across each row is equal to the row number minus two. In the sixth row, the 4f orbitals fill, and in the seventh row, the 5f orbitals fill. There are two inner transition series:

<ol>
	<li>The lanthanide series: lanthanide (La) through lutetium (Lu)</li>
	<li>The actinide series: actinide (Ac) through lawrencium (Lr)</li>
</ol>

Lanthanum and actinium, because of their similarities to the other members of the series, are included and used to name the series, even though they are transition metals with no f electrons.

This text is adapted from <a href="https://openstax.org/books/chemistry-2e/pages/6-4-electronic-structure-of-atoms-electron-configurations">OpenStax Chemistry 2e, Section 6.4: Electronic Structure of Atoms.</a>

The periodic table arranges atoms based on increasing atomic number so that elements with the same chemical properties recur periodically. When their ...

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Periodic Properties of the Elements

KEY TERMS AND CONCEPTS

how valence electrons shape the periodic table

The periodic table organizes elements by increasing atomic number, and it also shows why elements in the same group behave alike. As the electron configurations are placed into the table, a repeating pattern appears in the outer shells of atoms. These outer electrons are called valence electrons, and they play the biggest role in chemical reactions. They are the highest-energy electrons in an atom, so they are easier to lose or share than core electrons. Valence electrons also help determine some physical properties of elements.

The horizontal rows on the periodic table are periods. Moving across a period, each next element has one more proton in the nucleus and one more electron in the valence shell. The vertical columns are groups. Elements in the same group have the same number of valence electrons, which is why they often show similar chemical behavior. For example, lithium and sodium each have one valence electron, beryllium and magnesium each have two, and fluorine and chlorine each have seven. Because elements react by losing, gaining, or sharing valence electrons, their group placement helps predict how they will behave.

It is important to remember that the periodic table was built from chemical behavior before atomic structure was understood. Today, the pattern makes sense because elements with the same number of valence electrons are placed in the same group. The colored regions of the table also show three categories based on which orbitals are being filled. Main group elements, also called representative elements, are the elements whose last added electron enters an s or p orbital in the outermost shell. This category includes all nonmetals, the metalloids, and many metals. For main group elements, the valence electrons are the electrons with the highest n level. Gallium, for example, has the electron configuration [Ar] 4s2 3d10 4p1, so it has three valence electrons. The filled d orbitals are counted as core electrons, not valence electrons.

The two far-left columns form the s-block, and the six far-right columns form the p-block. The noble gases are part of the p-block. They usually have eight valence electrons, except helium, which has two. This helps explain why noble gases are highly stable and unreactive. Transition elements, or transition metals, are metallic elements whose last added electron enters a d orbital. Their valence electrons include the ns and (n - 1)d electrons added after the last noble gas configuration. By the official IUPAC definition, transition elements have partially filled d orbitals, so Zn, Cd, Hg, and also Cu, Ag, and Au are not technically transition elements, even though the term is often used for the whole d-block.

The d-block has 10 columns. As you move across each row, the d orbitals that fill have a principal quantum number that is one less than the row number. In the fourth row, 3d orbitals fill; in the fifth row, 4d orbitals fill; and so on. Inner transition elements form the f-block, where the last electron added enters an f orbital. Their valence shells include the (n - 2)f, the (n - 1)d, and the ns subshells. The f orbitals that fill across each row have a principal quantum number that is two less than the row number. In the sixth row, 4f orbitals fill, and in the seventh row, 5f orbitals fill. The two inner transition series are the lanthanides, from lanthanum through lutetium, and the actinides, from actinium through lawrencium. Lanthanum and actinium are included in those series because they are similar to the other members, even though they are transition metals with no f electrons.

How Valence Electrons Shape the Periodic Table

how-valence-electrons-shape-the-periodic-table

The periodic table organizes elements by increasing atomic number, and it also shows why elements in the same group behave alike. As the electron configurations are placed into the table, a repeating pattern appears in the outer shells of atoms. These outer electrons are called valence electrons, and they play the biggest role in chemical reactions. They are the highest-energy electrons in an atom, so they are easier to lose or share than core electrons. Valence electrons also help determine...

atomic radii and effective nuclear charge

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Periodic Trends in Atomic Size

Periodic trends in atomic size help explain why atoms get larger or smaller across the periodic table. Elements in the same group show similar chemical behavior because they have the same number and arrangement of valence electrons. Across a period, each new element adds one proton to the nucleus and one electron to the valence shell. Down a group, the number of valence electrons stays the same, but the principal quantum number increases by one. Atomic radius is not easy to define with perfect...

ionization energy

Ionization Energy Trends in the Periodic Table

Ionization energy is the energy needed to remove an electron from a gaseous atom or ion in its ground state. The first ionization energy, or IE1, removes the most loosely bound electron and forms a 1+ cation. The second ionization energy, IE2, removes the next electron, and each later ionization energy removes another electron from the atom or ion. Ionization always requires energy, so these processes are endothermic. That means ionization energy values are always positive. In general, larger...

ionic radii

How Ionic Charge Changes Ion Size

Ionic radius describes the size of an ion. It changes when an atom gains or loses electrons. Cations are positive ions with fewer electrons than the parent atom, and they are smaller than the atoms they come from. Aluminum shows this pattern clearly. A neutral aluminum atom has a covalent radius of 118 pm, while Al3+ has an ionic radius of 68 pm. When electrons are removed from the outer valence shell, the remaining core electrons feel a stronger effective nuclear charge, or Zeff. That pull...

electron affinity

Electron Affinity Trends on the Periodic Table

Electron affinity is the energy change that happens when a gaseous atom gains an electron and becomes a negative ion, or anion. For many elements, this process releases energy, so the electron affinity value is negative. For some elements, energy must be added instead, so the value is positive. Electron affinity also applies when more than one electron is added. The second electron affinity is the energy change for adding an electron to an anion to make a 2− ion. Later electron affinities...

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Alkali Metals: Reactivity Trends

Alkali metals are Group 1 elements with one valence electron. They are soft, shiny metallic solids that are malleable, ductile, and good conductors of heat and electricity. Their melting points are unusually low for metals and drop as you move down the group. Density generally increases down the group, except for potassium. The trend in atomic size helps explain their behavior. Moving from lithium to cesium, each element has a higher principal quantum number, so the outer electron is farther...

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Halogens: Trends and Reactions

Halogens are Group 17 nonmetals that show clear trends in structure and reactivity. At room temperature, fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid. Astatine is highly unstable and radioactive, so most of its properties are still unknown because it has a very short half-life. Tennessine is a synthetic element that is also predicted to belong to this group. These elements do not usually exist as single atoms. Instead, they form diatomic molecules, which means...

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Noble Gases: Sources, Uses, and Reactions

Noble gases are the group 18 elements helium, neon, argon, krypton, xenon, and radon. They were called “noble” because they were once thought to be nonreactive. That idea changed in 1962, when Dr. Neil Bartlett at the University of British Columbia showed that this assumption was not fully correct. These gases occur in very small amounts in the atmosphere. Some natural gas contains 1–2% helium by mass. Helium is separated from natural gas by removing the condensable components, which leaves...

key terms and concepts

Learn how valence electrons explain periodic table groups, blocks, and element behavior, including main group, transition, lanthanide, and actinide elements.

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How Valence Electrons Shape the Periodic Table

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