Home > Articles

  • Print
  • + Share This
This chapter is from the book

The Periodic Law

Although the arrangement of chemical elements in alphabetical order is convenient for inventory purposes, it is more reasonable to arrange them in the order of increasing atomic weights. In doing so, we find rather remarkable regularities which have led chemists to a rational classification of the elements. Arranging the elements in order of atomic weights,* we obtain the following sequence: H, He, Li, Be, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, A, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Br, Kr, Rb, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, I, Xe, Cs, Ba, La, etc. We notice, first of all, that there is a remarkable regularity in the distribution of noble gases, shown in italics, throughout the sequence: there is only 1 element preceding He, 7 elements between He and Ne, another 7 elements between Ne and A, 17 elements between A and Kr, and another 17 elements between Kr and Xe. Finally, there are 31 elements between Xe and Rn, which is the heaviest known noble gas.

*The careful student will notice that K, Ni, and I are out of order, but, as it was found later, the sequence of chemical properties has priority over atomic weights.

The elements immediately following the noble gases, lithium, sodium, potassium, rubidium, and cesium, are physically and chemically very similar to each other. They are all light, silvery-white metals with high chemical activity. If we drop a small piece of any of these elements in water, it will undergo a violent chemical reaction of the type:

Li + H2O → LiOH + H

Na + H2O → NaOH+ H


liberating hydrogen and forming the corresponding "hydroxide" with water (structural formula, Li—O—H, etc.). The hydrogen liberated in this reaction often becomes ignited and produces a flame which takes on the characteristics color of the vaporized metal (yellow for sodium, red for potassium, etc.). Uniting with hydrogen and oxygen, these elements form "hydrates" and "oxides" of the type LiH (Li—H), Li2O (Li—O—Li), etc., showing that their valency is 1. These elements are commonly known in chemistry as alkali metals.

The second neighbors to the right of the noble gases, beryllium, magnesium, calcium, strontium, barium, and radium, also form a homologous group known as alkali-earth metals. As their name indicates, they are similar to the alkali metals, but, as a rule, they are much harder and less reactive. Reacting with water, they produce compounds of the type Ca(OH)2 (H—O—Ca—O—H), while uniting with hydrogen and oxygen they give rise to compounds such as CaH2(H—Ca—H) and CaO (Ca=O), which indicates that their valency is 2. Similarly, we find that the third group to the right, boron, aluminum, etc., possesses a valency of 3 as demonstrated by such compounds as boron oxide, B2O3(O=B—O—B=O), and aluminum hydroxide, Al(OH)3.

Now if we look at the elements standing to the left of the noble gases, we will find that they are very similar to each other, but as different from metals as they could possibly be. This group comprises fluorine, chlorine, bromine, iodine, and astatine, and they are known as the halogens. They have a strong affinity for both alkali and alkali-earth metals, with which they form such compounds as NaCl (ordinary table salt) and CaBr2, indicating that they possess a single valency. The second neighbors to the left of the noble gases, oxygen, sulfur, etc., are also in some ways similar to each other and possess a valency of 2.

The existence of homologous groups and of a certain periodicity in the chemical properties of elements arranged in the order of increasing atom weights was noticed by several chemists during the nineteenth century, but the most important step of actually arranging the elements into a periodic table was made in 1869 by the Russian chemist, Dmitri Mendeleev (1834-1907). Mendeleev was handicapped in his studies because in his time the list of known chemical elements was rather incomplete and, in particular, the existence of the noble gases was not even suspected. From the sequence given above, Sc, Ga, Ge, Tc, and Rh were missing, making the sequence quite irregular except for the first two periods. Driven by a deep belief that there must be a regular periodicity in the natural sequence of elements, Mendeleev made the bold hypothesis that the deviations from the expected periodicity in his list were due to the failure of contemporary chemistry to have discovered some of the elements existing in nature. Thus, in constructing his table, he left a number of empty spaces to be filled in later by future discoveries. He gave to the "missing elements" names formed by adding the prefixes eka or dvi, meaning "first" and "second" in Sanskrit, to the names of neighboring homologous elements. In certain instances, he also reversed the atomic-weight order of elements in order to comply with the demands of the regular periodicity of their chemical properties. Using his table, shaky as it was, he was able to predict the physical and chemical properties of six "missing elements" on the basis of the known properties of their alleged neighbors. He called these elements eka-boron, eka-aluminum, eka-silicon, eka-manganese, dvi-manganese, and eka-tantalum. His predictions turned out to be in excellent agreement with the actually observed properties of the "missing elements" when they were finally found and named: scandium, gallium, germanium, technetium,* rhenium, and polonium. Just as an example, we give in Table 1-2 the comparison of Mendeleev’s predictions of the properties of his hypothetical element "eka-silicon," with the actually observed properties of this element, which was found fifteen years later by a German chemist, Winkler, and given the name germanium.

Pretty good for a prediction at this stage in the development of chemistry!

By enumerating the elements from 1 (for hydrogen) and up as they come in the periodic system of elements, we obtain what is known as the atomic numbers of the elements.

*Technetium, an unstable element normally non-existent in nature, was produced only recently in atomic piles.

Table 1-2

Mendeleev’s prediction for eka-silicon (Es) (1871)

Winkler’s data for germanium (Ge) (Discovered in 1886)

Atomic weight will be about 72

Atomic weight is 72.6

Will be obtained from EsO2 or K2EsF6 by reduction with Na

Was obtained from K2GeF6 reduction with Na

Will be a dark gray metal with high melting point and density about 5.5

Is a gray metal with melting point 958° C and density 5.36

On heating, Es will form the oxide EsO2 with high melting point and density 4.7

Reacts with oxygen forming and density 4.7 GeO2 with melting point 1,100°C

The sulfide EsS2 will be insoluble in water but soluble in ammonium sulfide

GeS2 is insoluble in water but readily soluble in ammonium sulfide

Thus, the atomic number of carbon is 6, that of mercury is 80, and that of mendelevium, 101. The atomic numbers of the six noble gases that form important landmarks of chemical periodicity are: 2, 10, 18, 36, 54, and 86. It is convenient to represent the periodic system of elements by a three-dimensional spiral structure that is shown in Figure 1-2. The backbone of the structure is the column containing the noble gases running all the way from He down to Rn. The next column to the right contains the alkali metals, with hydrogen placed at the top because its chemical properties are similar to those of the alkali metals. To the left and around the corner from the noble gas column is the one containing the halogens. The first two periods, from He to F and from Ne to Cl, contain 8 elements each and fall neatly into this scheme, but the next period contains 18 elements and constitutes a problem. On the basis of chemical properties, there seems to be no doubt that the 3 elements that follow A (K, Ca, and Sc) must be placed under the 3 corresponding elements (Na, Mg, and Al) of the previous period and that those preceding Kr (As, Se, and Br) should be under those preceding A (P, S, and Cl), but we do not seem to have places for the remaining 11 elements Ti to Ge). To dispose of this difficulty, we place Ti and Ge, which both resemble Si, under that element and make an extra loop to accommodate the remaining 9 elements (V to Ga). The same situation arises in the next and in all of the following periods so that the extra loop perpetuates itself all the way to the end of the known sequence of elements. In the beginning of the fifth period, we encounter further trouble of the same kind and are forced to build another extra loop to accommodate 14 extra elements (Ce to Lu), known as the rare earths. The sixth and last period runs in the same way with most of the natural and artificial radioactive elements forming a loop under that formed by the rare earths.

Things become quite complicated, and Dmitri Ivanovich Mendeleev would probably be horrified by the looks of it, but that’s how it is. Nevertheless, in spite of the complexity of the diagram (which reflects the complexity of the internal structure of the atom), the periodic system of elements in Figure 1-2 gives a very good representation of the properties of the different elements.

Figure 1.2

Figure 1-2. The periodic system of the elements represented as a wound ribbon. The diagram on the next page shows the other side of the second loop. At present the ribbon is cut at atomic number 101 (mendelevium). An asterisk indicates that the element is unstable (radioactive), and an asterisk in parenthesis indicates the presence of a radioactive isotope in the normally stable element. The properties of the underlined elements were predicted by Mendeleev.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020