The Periodic Table

How it was developed, changed, and is now.

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Contributions of Dmitri Mendeleev

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Dimitri Mendeleev put elements into their correct places in the table. In some cases the relative atomic mass had been wrongly calculated by others. By correcting the relative atomic mass he put the element in the correct place.

At the time, relative atomic masses was called atomic weight.

The combining (or equivalent) weights were generally accurate but sometimes an element was given the wrong valency. Thus beryllium, combining weight 4.6, was given the valency 3 because it was chemically similar to aluminium. This gave an atomic weight of 13.8, placing it between carbon and nitrogen where there was no space. Mendeleev said the valency was 2; the problem was solved - it fitted into the space between lithium and boron.

Mendeleev sometimes decided that atomic weights must be wrong because the elements simply appeared in the wrong place. For example he placed tellurium before iodine although its atomic weight is greater simply because iodine’s properties are so similar to those of fluorine, chlorine and bromine and tellurium’s to those of oxygen, sulfur and selenium rather than the other way round.

We now know that it is atomic number, not relative atomic mass that governs an element’s position in the Periodic Table, but in most cases the two result in the same order.

The greatness of Mendeleev was that not only did he leave spaces for elements that were not yet discovered but he predicted properties of five of these elements and their compounds. How foolish he would have seemed if these predictions had been incorrect but fortunately for him three of these missing elements were discovered by others within 15 years (within his lifetime). These discoveries established the acceptance of the Russian's table, although two other elements whose properties were predicted were not discovered for 50 years.

One thing that Mendeleev did not predict was the discovery of a whole new Group of elements, the noble gases, by the Scot William Ramsay and co-workers during the last decade of the 19th century. Mendeleev was at first dismayed by this but before he died in 1907 realized that Ramsay's discoveries were further proof of the Periodic Table, not a contradiction. Ramsay was awarded a Nobel Prize for discovering five elements.

Mendeleev never received that honor. However, an element (atomic number 101) has been named after Mendeleev, an even rarer distinction. This is surely deserved by the original formulator of the Periodic Table.

The genius of Mendeleev's periodic table - Lou Serico


Contributions of Henry Moseley

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By 1907, when Mendeleev died, chemists were sure that iodine followed tellurium in the Periodic Table and that there was something odd about their relative atomic masses. However no-one was able to measure atomic number, it was just the position of an element in the Periodic Table sequence. For example lithium was known to be the third element but this number three was only because its properties meant that it slotted in between helium and beryllium.

Henry Moseley found and measured a property linked to Periodic Table position. Hence atomic number became more meaningful and the three pairs of elements that seemed to be in the wrong order could be explained.

Moseley used what was then, brand-new technology in his experiments. A device now called an electron gun had just been developed. He used this to fire a stream of electrons (like machine gun bullets) at samples of different elements. He found that the elements gave off X-rays. (This is how the X-rays used in hospitals are produced.)

Moseley measured the frequency of the X-rays given off by different elements. Each element gave a different frequency and he found that this frequency was mathematically related to the position of the element in the Periodic Table – he could actually measure atomic number.


Organization of Elements Into Periods & Groups

You've got Your Periods...

Even though they skip some squares in between, all of the rows read left to right. When you look at the periodic table, each row is called a period (Get it? Like PERIODic table.). All of the elements in a period have the same number of atomic orbitals. For example, every element in the top row (the first period) has one orbital for its electrons. All of the elements in the second row (the second period) have two orbitals for their electrons. As you move down the table, every row adds an orbital. At this time, there is a maximum of seven electron orbitals.

Your Groups...

Now you know about periods going left to right. The periodic table also has a special name for its vertical columns. Each column is called a group. The elements in each group have the same number of electrons in the outer orbital. Those outer electrons are also called valence electrons. They are the electrons involved in chemical bonds with other elements.

Every element in the first column (group one) has one electron in its outer shell. Every element in the second column (group two) has two electrons in the outer shell. As you keep counting the columns, you'll know how many electrons are in the outer shell. There are exceptions to the order when you look at the transition elements, but you get the general idea. Transition elements add electrons to the second-to-last orbital.

...and the Two at the Top

Hydrogen (H) and helium (He) are special elements. Hydrogen can have the electron traits of two groups: one and seven. For chemists, hydrogen is sometimes missing an electron like the members of group IA, and sometimes has an extra one as in group VIIA. When you study acids and bases you will regularly work with hydrogen cations (H+). A hydride is a hydrogen anion and has an extra electron (H-).

Helium (He) is different from all of the other elements. It is very stable with only two electrons in its outer orbital (valence shell). Even though it only has two, it is still grouped with the noble gases that have eight electrons in their outermost orbitals. The noble gases and helium are all "happy," because their valence shell is full.


Chemical Characteristics of Elements in the Same Periods and Groups


A group, or family, is a vertical column in the periodic table. Elements in the same group show patterns in atomic radius, ionization energy, and electronegativity. From top to bottom in a group, the atomic radii of the elements increase: since there are more filled energy levels, valence electrons are found farther from the nucleus. From top to bottom, each successive element has a lower ionization energy because it is easier to remove an electron since the atoms are less tightly bound. Similarly, from top to bottom, elements decrease in electronegativity due to an increasing distance between valence electrons and the nucleus. There are exceptions to these trends however – in Group 11, for example, the electronegativity increases down the group.


Elements in the same period show trends in atomic radius, ionization energy, electron affinity, and electronegativity. Moving left to right across a period, from the alkali metals to the noble gases, atomic radius usually decreases. This is because each successive element has an additional proton and electron, which causes the electrons to be drawn closer to the nucleus. The additional proton increases the effective nuclear charge to a greater extent than the addition of an extra electron to an already partially-filled shell. The decrease in atomic radius also causes ionization energy to increase from left to right across a period: the more tightly bound an element is, the more energy is required to remove an electron. Electronegativity increases in the same manner as ionization energy because of the pull exerted on the electrons by the nucleus. Electron affinity also shows a slight trend across a period: metals (the left side of a period) generally have a lower electron affinity than nonmetals (the right side of a period), with the exception of the noble gases which have an electron affinity of zero.


Comparisons and Differences in the Periodic Properties of Elements

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  • Elements that form cations when compounds of it are in solution and oxides of the elements form hydroxides rather than acids in water. Most metals are conductors of electricity, have crystalline solids with a metallic luster and have a high chemical reactivity. Many of these elements are hard and have high physical strength.


  • A chemical element that lacks the characteristics of a metal and that is able to form anions, acidic oxides, acids, and stable compounds with hydrogen

Metalloid Behavior

Definition of Metalloid

  • The Metalloids, or semi-metals, are elements with properties intermediate between metals and non-metals.

Physical Properties
  • State of Matter: Solid
  • Luster: Metallic lustre
  • Elasticity: Brittle
  • Conductivity: Semi-conductive. (Average transmission of heat.)

Chemical Properties

  • Oxidation: Readily form glasses
  • Alloys: Form alloys with metals
  • *Allotropic: Several metallic and non-metallic allotropic forms
  • Melting: Some metalloids contract on melting
  • Compounds: Reacts with the halogens to form compounds

  • *Allotropic - Allotropes are forms of an element with different physical and chemical properties occurring in two or more crystalline forms in the same physical state.

Electrical/Heat Conductivity

  • The degree to which a specified material conducts electricity, calculated as the ratio of the current density in the material to the electric field that causes the flow of current. It is the reciprocal of the resistivity.

  • The rate at which heat passes through a specified material, expressed as the amount of heat that flows per unit time through a unit area with a temperature gradient of one degree per unit distance.


  • Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons.

  • The Pauling scale is the most commonly used.

Electron Affinity

  • Electron affinity is defined as the change in energy (in kJ/mole) of a neutral atom (in the gaseous phase) when an electron is added to the atom to form a negative ion.

  • In other words, the neutral atom's likelihood of gaining an electron.

Ionization Energy

  • The ionization energy is qualitatively defined as the amount of energy required to remove the most loosely bound electron of an isolated gaseous atom to form a cation.


Now you know how the periodic table was developed, how it has changed, and how it is organized now.


If you want to learn more, here are some really cool sites to check out!