Author’s note. This page links to a site that requires Java. The animations on that site are very good. If the student has Java installed, this is the more graphic page including animations regarding electron configuration. Since there have been security concerns with Java, the author cannot recommend installing Java just to see this page. The sister page, How to Read Periodic Table (electron Configuration) non-Java is an alternative.
To understand why elements exhibit the physical and chemical properties they do, one has to understand the arrangement of electrons in atoms of the elements. Review the Charge section of the Basics Tab before continuing if you don’t remember the fundamentals of charges.
Recall from the Introduction section, elements in a Family (column) have similar properties and elements in a Period (row) exhibit recurring properties. The objective of the following discussion is to understand electron arrangement and then relate this to properties of the elements.
Due to the attraction between protons in the nucleus and electrons outside the nucleus, the electrons will be as close to the nucleus as possible to provide the most energetically stable arrangement, referred to as lower energy levels. In elements with higher atomic numbers, all the electrons cannot be located close to the nucleus and will be placed at greater distances from the nucleus. These electrons will be at higher energy levels. The greater the distance between the nucleus and an electron, the higher the energy level of the electron.
Using your current Internet browser, open a second window. In Chrome, Firefox, and Safari, under the “File” drop down menu there is a ”New Window” choice. Click this to open a new window. Copy and paste the following address, http://www.colorado.edu/physics/2000/applets/a2.html, to access David’s Wizzy Periodic Table. Depending on your Internet connection, this may take a bit of time to load. This site will not display if your computer does not have Java installed.
This window has an abbreviated Periodic Table with only the first four Periods. There is an emission spectum below the Periodic Table. On the left side of the window is an animation of an electron moving about a nucleus. The graphic on the right is an energy scale in units of electron volts with a dot the same color as the moving electron. The energy scale indicates the energy levels of electrons in an atom.
The animation being displayed is that of hydrogen. Hydrogen (atomic number 1) has only one proton in the nucleus and one electron moving around the nucleus.
Click on the following elements (He. Li. O. Ne. Na. S, Ar, K, Se, Kr ) to get a sense of how electrons behave in atoms of elements with higher atomic numbers. As you move from element to element, note the atomic number, take time to observe the placement of electrons in the animation and match this to the graphic on the right. Also observe the emission spectrum of each element.
In the current model of electron configuration each Period corresponds to a principal energy level (also called shell or principal quantum number). The principal energy level is where electrons are located. There will be no electrons in the spaces between the energy levels.
Period one has 2 elements in it. Therefore the the first principal energy level will contain 2 electrons. Click on H and then He to see the electrons in these two elements
Period two has 8 elements in it. Therefore the second principal energy level has 8 electrons. Start with Li in Period two and click on each element in the row and observe the placement of the electrons and the corresponding energy levels. Note the last element in this row has a total of 10 electrons. Two from the first energy level and eight from the second energy level.
Period three has 8 elements in it therefore 8 electrons will be added to the third principal energy level. Start with Na in Period three and click on each element in the row and observe the placement of the electrons and the corresponding energy levels. Period 3 can accommodate 18 electrons that occurs in elements with higher atomic numbers, but this only occurs in higher atomic number elements.
Period four has 18 elements in it therefore 18 electrons will be added to the fourth principal energy level. Period 4 can accommodate 32 electrons that occurs in elements with higher atomic numbers.
The animations shown for each element provide a visual to show the approximate placement and movement of the electrons. The animation does not and cannot show the exact location and motion of the electrons. There is a phenomenon called the Heisenburg Uncertainity Principle that states it is not possible to know the exact position and momentum of a particle such an electron.
In the current model of electron configuration, electrons do not have a fixed orbit about the nucleus as shown in the animations. They move in a space about the nucleus defined by a combination of positive-negative attractions and negative-negative repulsions between electrons. This space is called an orbital. An orbital can hold two electrons and take different shapes depending on the number of electron-electron repulsions. Orbitals are sublevels of the principal energy level.
A new principal energy level is associated with the first element on the left side of a period. The first electron in this principal energy level moves in a spherical shape. The second electron in the principal energy level also moves in this same spherical shape, but with an opposite spin. The spherical shape these electrons occupy is identified as an “s” orbital and the two electrons are called “s” electrons. There is only 1 s orbital per principal energy level and it can accommodate 2 electrons
If a principal energy level has more than two electrons, the added electrons take on an elliptical shape to minimize the electron-electron repulsions. These electrons are identified as “p” orbital electrons. They are farther from the nucleus than s orbital electrons so they will have higher energy levels. There are three p orbitals per principal energy level that can accommodate 6 electrons, two electrons per orbital.
If the principle energy level has more than 8 electrons, the electrons also have an elliptical shape again to minimize electron-electron repulsion. These electrons are identified as “d” orbital electrons. They are farther from the nucleus than p orbital electrons so they will have a higher energy level. There are 5 d orbitals per energy level that can accommodate 10 electrons.
If the principal energy level has more than 18 electrons, the electrons also have an elliptical shape to again to minimize electron-electron repulsion. These electrons are identified as “f” orbital electrons. They are farther from the nucleus than d orbital electrons so they will have a higher energy level. There are 7 f orbitals per energy level that can accommodate 14 electrons.
The preceding discussion has introduced orbitals and energy levels where electrons are found. A system of expressing the relative energy levels for electrons in the atoms of an element exists. This is referred to as the electron configuration of an atom (or ion). Open the interactive periodic table. Use the table as you read through the following.
The system starts with the lowest atomic number element hydrogen. Atoms of hydrogen have 1 electron. The electron configuration for hydrogen is: 1s1
The break down of the symbol is:
The number 1 indicates the primary energy level
s is the orbital type
the superscript is the number of electrons in the s orbital
The electron configuration for He is 1s2. There are two s orbital electrons in the first primary energy level.
The electron configuration of B is 1s22s2 2p1 . There are two s orbital electrons in the first primary energy level, two s orbital electrons in the second primary energy level, and 1 p orbital electron in the second primary energy level.
The electron configuration for S is s22s22p63s23p4. There are two s orbital electrons in the first primary energy level, two s orbital electrons and six p orbital electrons in the second primary energy level, two s orbital electrons and 4 p orbitals in the third primary energy level.
The electron configuration for Sc is: 1s22s22p63s23p63d14s2 . The d electrons of Sc are in the third primary energy level. Evidence shows 4s orbital electrons have a higher energy level than the 3d orbital electrons.
The electron configuration for Cr is: 1s22s22p63s23p63d54s1. Why is the 3d orbital at a lower energy than than the 4s orbital? The real answer is quite long. The short answer is that electrons want to be apart if they can. This configuration places 1 electron in each of the five 3d orbitals and one in the 4s orbital
Observation: The sum of the superscripts in an electron configuration is equal to the atomic number.
The preceding discussion provides an introduction into electron configuration that is a system of identifying the energy level of each electron in an atom. Visit the links below to get more information regarding electron configuration.
The web site Electron configurations of the elements (data page) has the electron configuration for every element.
Introduction to Electron Configuration
This video shows the relative energy levels of electrons for elements H (hydrogen) to Ne (neon) with lower energy levels near the bottom of the graph and higher energy levels arranged higher on the graph.
The primary energy levels are indicated by the values of n shown near the vertical axis. Notice the primary energy levels get closer together as the number gets larger.
The rectangular boxes represent orbitals. As the video indicates an orbital holds two electrons with the spins of the electrons being in opposite directions. Opposite spins are indicated by one arrow pointing up and the other pointing down.
It would be helpful as you watch this video to have a periodic table. As the video progresses from H to Ne the atomic number increases. This is a good reinforcement to information given in the Introduction section of Elements. As the atomic number increases more protons (+) and electrons (-) are in the atoms of the elements. The number in the rectangular graphic is the atomic number for that element.
Notice the s, p, and d orbitals of a primary energy level are grouped close together, but still different in energy.
The electron configuration is given below the rectangular graphic.
Another observation is that He in family 18 has a full outside orbital as does Ne that is also in family 18. All family 18 elements have a full outer orbital.
Applet: Electron Configuration
This site allows the student to add electrons and visualze what principal energy level and the sublevel (s, p, d) the electrons fill. This interactive site gives the student the opportunity to write electron configurations for each element as the slider is moved farther to the right.
Animation of Atomic orbitals – Electron configuration of Scandium
This animation provides a visual of the orientation of orbitals in an atom of Scandium. This animation gives a good visual of the shapes of the orbitals (s are spherical, p are elliptical, d are more elliptical) and their orientation about the nucleus (located where the three axes intersect). It also gives a perspective on the relative size of the orbitals as the primary energy level increases. This animation is a good visual to show that higher energy levels get closer to each other. This helps with understanding how orbitals of atoms of elements can be altered when they react with other elements to form chemical bonds between the two atoms. This will be discussed in the section of Covalent Compounds: (Hybridization).
The interactive Periodic table still needs to be open. If use your current Internet browser, open a second window. In Chrome, Firefox, and Safari, under the “File” drop down menu there is a ”New Window” choice. Click this to open a new window. Copy and paste the following address, http://www.ptable.com/#. Keep both windows open as you read the following including the Valence electrons section.
Make the Periodic Table wide enough to see names of the elements. Click on the Orbitals tab.
Near the top in the center are stacks of boxes with symbols. The first stack on the left represents s orbital electrons, the second stack of 3 columns represents p orbital electrons, the third stack of 5 columns represents d orbital electrons, and the fourth stack of 7 columns represents f orbital. (We will probably not discuss f orbital electrons in this class).
The lower the number in the stack, the lower the energy of those electrons.
Start with a period or two or three, click on the first element in the period. and click on all the other elements and observe the boxes where electrons are appearing. The electron configuration for the selected element appears to the right of the stacks.
Valence electrons are the electrons of an atom that participate in forming chemical bonds with other atoms and are on the exterior of atoms.
Valence electrons are identified by their orbital type, s, p, d or f. On the Interactive Periodic Table, the type of valence electrons associated with the elements is identified by color. Elements with an s valence electron are identified by the fushia color. Elements with p valence electrons are show by the tan color, d valence electrons are shown by the green color, and f valence electrons are identified by the bluish/purple color.
The number of valence electrons the atom of an element has is very important in determining the properties of that element. Click on the element Li that is in the s valence group. The number 1 appears below the details window to indicate the atoms of Li have 1 valence electron with an s orbital shape. Click on the remaining elements in Period 2 to observe the number of valence electrons and the orbital type of the valence electrons in the atoms of each of the elements.
Click on the element, Li, of the Alkali Metals Family (column 1) and observe the number of valence electrons for the elements in this family. Start with the first element of Family 13 and observe the number of valence electrons for the elements in this family. Continue with Families 14 through 18. Knowing the number of valence electrons and the orbital type of those electrons is very important to discussions in the remaining course.
To determine the number of valence electrons for elements in the s and p valence groups, one can count the number of columns starting from the left. N is in the fifth column because it has 5 valence electrons (2s and 3p). This works for elements in Periods 4 and above if one ignores the d valence electrons in that period.
In addition to knowing the number and orbital type of valence electrons, the size of the atoms of the elements is also important. The site Elements, Atomic Radii ,and the Periodic Table provides a graphic representation of the atomic radii of atoms of the elements.
Observe the trend of sizes within a Family. Atoms get larger as the Atomic Number increases within a Family.
Observe the trend of sizes within a Period. Atoms get smaller as the Atomic Number increases within a Period.
Why should these two trends exist?
Within a Family as the atomic number increases, there are more principal energy levels present to accommodate more electons as the atomic number increases. Each higher principal energy level is farther from the nucleus than a lower principal energy level. So an atom of Li with 2 principal energy levels will have a smaller radius than an atom of K with 4 principal energy levels.
Within a Period as the atomic number increases, electrons are being added to the same principal energy level. As the atomic number increases within a Period, more protons and electrons are being added into the same approximate space. Therefore as atomic number increases with in the same principal energy level, there will be an increase in plus and minus attraction that results in a reduction in the radii of the atoms of the elements.
From this point forward, the student will be expected to be able to tell the number and orbital type of valence electron(s) for the atoms of any elements in Periods 1 through 4 using a bare bones Periodic Table with only the symbol, atomic number, atomic weight, period number, and family number. The student will be expected to also know the relative sizes of atoms of the elements.
The arrangement of elements in the Periodic Table is a result of the electron configuration of the atoms of the elements. Elements in a family (column) have similar physical and chemical properties because they have the same number and orbital type of valence electrons. For example, Alkali Metals Family (column 1) all have 1 s electrons. They differ in properties because they have different atomic radii. Table salt is composed of sodium ion (Na+) and chloride ion (Cl–). One can purchase a salt substitute that is composed of potassium ion (K+) and chloride ion (Cl–). KCl has a less “salty ” taste due to the difference in the radius of the Na+ and K+ ions. The important message here is that elements in families have similar chemical properties.
Mercury is the only metal that is a liquid at room temperature. The electron configuration for mercury atoms is:
Mercury atoms have an electron configuration similar to the Noble gases and have a high attraction for their valence electron. This reduces the interaction between mercury atoms. The magnitude of the interaction is large enough to hold them in a liquid state, but not high enough to hold them in solid state.
Home work Assignment
Go to Electron Configuration under the Homework tab and complete.