The periodic table we use this day is based upon the one devised and published by Dmitri Mendeleev in 1869.

Mendeleev discovered he can arrange the 65 aspects then known in a grid or table so the each aspect had:

1. A greater atomic weight 보다 the one top top its left. For example, magnesium (atomic load 24.3) is placed to the ideal of salt (atomic weight 23.0):


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

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Just as Adams and also Le Verrier might be stated to have found the world Neptune ~ above paper, Mendeleev can be said to have uncovered germanium on paper.He referred to as this new element eka-silicon, after ~ observing a gap in the regular table in between silicon and also tin:


Similarly, Mendeleev found gallium (eka-aluminum) and also scandium (eka-boron) top top paper, because he predicted their existence and also their properties before their really discoveries.

Although Mendeleev had actually made a vital breakthrough, the made tiny further progress.With the benefit of hindsight, we know that Mendeleev"s periodic table to be underpinned through false reasoning.Mendeleev believed, incorrectly, that chemical properties were identified by atom weight.Of course, this to be perfectly reasonable as soon as we think about scientific knowledge in 1869.

In 1869 the electron itself had actually not been discovered - that taken place 27 years later, in 1896.

In fact, it take it 44 years because that the correct explanation that the regular patterns in Mendeleev"s periodic table to it is in found.

The Explanation is FoundThe explanation came in 1913 from Henry Moseley, who fired electrons at atoms, leading to the emission of x-rays.Moseley found that each element he learned emitted x-rays at a distinct frequency.

When the looked in ~ the frequencies emitted by a collection of elements, he uncovered a pattern that was ideal explained if the optimistic charge in the nucleus raised by precisely one unit from facet to element.


In other words, Moseley had uncovered that aspects are different from one an additional because their atoms have different numbers that protons.He discovered that elements" location in the periodic table are better predicted by your atomic numbers 보다 their atom weights.(An element"s atomic number is equal to the variety of protons, and hence electrons, in among its atoms.)

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

Considering the argon-potassium problem, that was well-known that argon has a higher atomic weight than potassium.According to Mendeleev"s reasoning, argon should therefore be put after potassium in the routine table.But, act this do no feeling in terms of chemical properties.


Moseley"s job-related showed the argon"s atom number is 18 and also potassium"s is 19.Therefore argon need to be placed prior to potassium in a periodic table based on atomic numbers.Chemists roughly the world breathed a cumulative sigh the relief, because this agreed with the observed chemical properties of these elements.


Moseley likewise emulated Mendeleev"s accomplishment of discovering new elements top top paper, finding four atomic numbers v no matching elements.He guess the presence of aspects with atomic numbers 43, 61, 72, and 75.These aspects were undoubtedly discovered; we now call them technetium, promethium, hafnium and rhenium.

Today the chemical elements are still arranged in stimulate of boosting atomic number (Z) as we look indigenous left come right throughout the routine table.We speak to the horizontal rows periods. For example, below is duration 4:


We speak to the upright rows groups.

for example, right here is group 2:


We likewise now recognize that an element"s chemistry is figured out by the way its electrons space arranged - the electron configuration.

Electrons in atoms deserve to be pictured as occupying layers or shells neighboring the atomic nucleus.This is displayed in the diagram of a lithium atom on the left next of this page.We photo electrons as tiny planets whizzing about a sun-like nucleus, i beg your pardon is whereby the protons and neutrons room located.This is dubbed a Bohr depiction of an atom.This is in reality an approximation, yet it"s a an excellent starting suggest for understanding the chemical properties the the elements.

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

These external electrons are called valence electrons.

Valence Electrons

It is the valence electron which reason chemical reactivity.

All the the facets in group 1 have actually one valence electron; team 2, 2 valence electrons; team 13, three valence electrons; group 14, four valence electrons; team 15, 5 valence electrons;Group 16, six valence electrons; group 17, 7 valence electrons; and also Group 18, eight valence electrons, other than for helium, which has two.

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


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


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

All ions room either positively or negative charged.


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

The chlorine atom, i beg your pardon starts with seven valence electrons, benefit a solitary electron and also becomes a negatively charged chlorine ion: Cl-.This ion has actually the same electron configuration together the noble gas argon, and also therefore this ion is chemically steady too.

The positive charged salt ion and negatively charged chlorine ion entice one an additional electrostatically, developing a steady chemical compound, sodium chloride.These electrostatic binding are referred to as ionic bonds.

For the shift metals the situation is a small more complicated than those described above, due to the fact that electrons from the lower shells in shift metal atom can come to be valence electrons.This is why, for example, we can obtain different types of copper ions, Cu+ and Cu2+, and also iron ions, Fe2+ and Fe3+.

The number of valence electron in atoms is the communication of the consistent patterns it was observed by Mendeleev in 1869, patterns which at some point have offered us our modern periodic table.

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Ionization energies that the elements - the power required to eliminate one electron.These measurements were not possible in Mendeleev"s time.They show the periodic (regular pattern) habits of the elements.