The periodic table we use today is based on the one devised and published through Dmitri Mendeleev in 1869.

Mendeleev found he can arrange the 65 elements then well-known in a network or table so the each element had:

1. A higher atomic weight than the one ~ above its left. Because that example, magnesium (atomic load 24.3) is inserted to the best of salt (atomic load 23.0):


Mendeleev realized that the table in former of the lay in ~ the an extremely heart that chemistry.And much more than that, Mendeleev experienced that his table was incomplete - there were spaces where aspects should be, however no-one had uncovered them.

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Just together Adams and Le Verrier can be claimed to have found the planet Neptune on paper, Mendeleev can be stated to have discovered germanium top top paper.He called this brand-new element eka-silicon, after observing a space in the regular table in between silicon and also tin:


Similarly, Mendeleev discovered gallium (eka-aluminum) and scandium (eka-boron) ~ above paper, since he predicted their existence and also their properties before their yes, really discoveries.

Although Mendeleev had actually made a an important breakthrough, he made little further progress.With the advantage of hindsight, we understand that Mendeleev"s regular table to be underpinned through false reasoning.Mendeleev believed, incorrectly, that chemical nature were figured out by atomic weight.Of course, this to be perfectly reasonable when we think about scientific knowledge in 1869.

In 1869 the electron itself had actually not been discovered - that occurred 27 year later, in 1896.

In fact, it take it 44 years for the exactly explanation the the consistent patterns in Mendeleev"s regular table to it is in found.

The Explanation is FoundThe explanation came in 1913 native Henry Moseley, that fired electron at atoms, causing the emission of x-rays.Moseley uncovered that each facet he studied emitted x-rays at a distinct frequency.

When he looked at the frequencies emitted by a collection of elements, he uncovered a pattern that was best explained if the confident charge in the nucleus enhanced by precisely one unit from aspect to element.


In various other words, Moseley had uncovered that elements are different from one one more because your atoms have different numbers that protons.He uncovered that elements" positions in the periodic table are better predicted by your atomic numbers than their atomic weights.(An element"s atomic number is same to the number of protons, and also hence electrons, in one of its atoms.)

Moseley"s discovery settled the cobalt-nickel and argon-potassium problems.

Considering the argon-potassium problem, that was recognized that argon has a higher atomic weight 보다 potassium.According to Mendeleev"s reasoning, argon should therefore be placed after potassium in the routine table.But, doing this made no sense in regards to chemical properties.


Moseley"s occupational showed that argon"s atom number is 18 and also potassium"s is 19.Therefore argon must be placed before potassium in a periodic table based upon atomic numbers.Chemists approximately the human being breathed a collective sigh that relief, since this agreed v the observed chemical nature of these elements.


Moseley likewise emulated Mendeleev"s accomplishment of discovering brand-new elements ~ above paper, finding four atomic numbers through no matching elements.He predicted the existence of elements with atom numbers 43, 61, 72, and 75.These elements were indeed discovered; us now call them technetium, promethium, hafnium and rhenium.

Today the chemical facets are still arranged in order of raising atomic number (Z) together we look native left come right throughout the periodic table.We call the horizontal rows periods. For example, below is period 4:


We speak to the vertical rows groups.

because that example, here is team 2:


We likewise now know that one element"s chemistry is figured out by the means its electrons are arranged - that is electron configuration.

Electrons in atoms deserve to be pictured as occupying layers or shells neighboring the atom nucleus.This is displayed in the diagram of a lithium atom on the left next of this page.We picture electrons as small planets whizzing approximately a sun-like nucleus, which is where the protons and also neutrons room located.This is called a Bohr representation of an atom.This is in reality an approximation, yet it"s a an excellent starting allude for expertise the chemistry properties that the elements.

If us exclude the transition metal elements, we deserve to say that atoms which occupy the same group of the routine table have the same number of outer electrons.For example, all of the aspects in team 2, displayed left, have actually two outer electrons.

These outer electrons are called valence electrons.

Valence Electrons

It is the valence electrons which reason chemical reactivity.

All that the elements in team 1 have actually one valence electron; group 2, two valence electrons; team 13, 3 valence electrons; team 14, 4 valence electrons; group 15, five valence electrons;Group 16, six valence electrons; team 17, 7 valence electrons; and Group 18, eight valence electrons, except for helium, which has two.

Group 18 is the noble gas group, a team of unreactive elements.The reluctance of the noble gases to reaction chemically is the vital that unlocks our expertise of why other facets do react.


Unreactive Species: If one atom has actually the electron construction of a noble gas it will certainly be chemically unreactive, or only react v difficulty.


For example, in the diagram above, a sodium atom loser its single valence electron to a chlorine atom.When an atom loses or benefit one or much more electrons, it have the right to no much longer be described as one atom - the is called an ion.

All ions space either positively or negatively charged.


Since our sodium atom has lost a negatively fee electron, it i do not care a positively charged sodium ion: Na+.This salt ion, through one electron fewer 보다 the sodium atom, has actually the very same electron configuration as the noble gas neon and also is chemically stable.

The chlorine atom, which starts with seven valence electrons, profit a solitary electron and becomes a negatively charged chlorine ion: Cl-.This ion has actually the very same electron configuration together the noble gas argon, and therefore this ion is chemically secure too.

The positive charged salt ion and negatively charged chlorine ion lure one an additional electrostatically, developing a secure chemical compound, sodium chloride.These electrostatic binding are called ionic bonds.

For the transition metals the instance is a tiny more complex than those defined above, since electrons indigenous the lower shells in change metal atoms can come to be valence electrons.This is why, because that example, us can obtain different species of copper ions, Cu+ and Cu2+, and also iron ions, Fe2+ and also Fe3+.

The variety of valence electron in atom is the communication of the continuous patterns observed by Mendeleev in 1869, patterns which eventually have given us our modern periodic table.

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Ionization energies of the facets - the power required to remove one electron.These measurements were not possible in Mendeleev"s time.They highlight the regular (regular pattern) behavior of the elements.