Classification of Elements & Periodicity Flashcards

Practice vocabulary flashcards covering the history of element classification from Dobereiner to the Modern Periodic Table.

Dobereiner Triad theory

Proposed in 1829 by Johann Wolf Dobereiner, it states that when elements are arranged in increasing order of atomic masses, the atomic mass of the middle element is roughly the average of the atomic masses of the remaining two elements.

Dobereiner's 1st Triad

Consists of $Li$ ($7$), $Na$ ($23$), and $K$ ($39$), where the atomic mass of $Na$ is calculated as $\frac{7+39}{2} = 23$.

Dobereiner's 2nd Triad

Consists of $Ca$ ($40$), $Sr$ ($87.5$), and $Ba$ ($137$), where the atomic mass of $Sr$ is calculated as $\frac{137+40}{2} = 88.5$.

Dobereiner's 3rd Triad

Consists of $Cl$ ($35.5$), $Br$ ($80$), and $I$ ($127$), where the atomic mass of $Br$ is calculated as $\frac{127+35.5}{2} = 81.25$.

Dechancourtois classification

An 1862 arrangement where elements were placed in increasing order of atomic mass in a cylindrical table, with similar elements appearing in a vertical line from the centre of the spiral.

Newland's law of Octaves

An 1864 proposal by John Alexander Reina Newland stating that when elements are arranged by increasing atomic mass, the 8th element has similar properties to the first, analogous to notes on a musical scale.

Periodicity

The reoccurrence of properties of elements at regular intervals.

Lother Meyer classification

An 1869 study that plotted a graph between atomic volume and atomic masses, showing that elements with similar physical properties occupy similar positions on the curve.

Alkali metals on Lother Meyer curves

The most electropositive elements ($Li$, $Na$, $K$, $Rb$, $Cs$) which occupy the peaks on the curve.

Halogens on Lother Meyer curves

Elements including $F$, $Cl$, $Br$, and $I$ which occupy the ascending portion of the curve.

Alkaline earth metals on Lother Meyer curves

Elements including $Be$, $Mg$, $Ca$, $Sr$, and $Ba$ which occupy the descending portions of the curve.

Mendeleev's Periodic Law

The principle stating that the physical and chemical properties of the elements are the periodic functions of their atomic weights.

Mendeleev's Periodic Table structure

Consists of $7$ horizontal rows called periods and $8$ vertical columns called groups, which are further divided into sub groups $A$ and $B$.

Eka

A term meaning "preceding," used by Mendeleev to name gaps for undiscovered elements like Eka-Boron ($Sc$), Eka-Aluminium ($Ga$), Eka-Silicon ($Ge$), and Eka-Manganese ($Tc$).

Transition Triads

Groups of three elements in Mendeleev's VIII group, such as $Fe$, $Co$, $Ni$; $Ru$, $Rh$, $Pd$; and $Os$, $Ir$, $Pt$.

Inverted pairs (Anamolous pairs)

Pairs of elements in Mendeleev's table arranged with decreasing order of atomic masses, such as $Ar$ ($40$) and $K$ ($39$), or $Co$ ($58.9$) and $Ni$ ($58.7$).

Moseley's X-ray frequency formula

$\sqrt{\nu} = a(Z-b)$, where $\nu$ is the frequency of X-rays, $Z$ is the atomic number, and $a$ and $b$ are constants.

Mosley's Periodic Law

The principle stating that the physical and chemical properties of the elements are the periodic functions of their atomic number.

Long form Periodic table

Also known as Bohr's table, it was prepared by Werner, Bury, Rang, and Bohr based on atomic numbers.

Bridge elements

The $2nd$ period elements ($Li$, $Be$, $B$, $C$, $N$, $O$, $F$) that act as a bridge between blocks.

Typical elements

The $3rd$ period elements ($Na$, $Mg$, $Al$, $Si$, $P$, $S$, $Cl$) because they actively participate in chemical reactions.

Representative elements

The $A$ group elements ($IA$, $IIA$, $IIIA$ to $VIIA$) that represent chemical bonding.

Lanthanides

Elements with atomic numbers $58$ ($Ce$) to $71$ ($Lu$) that follow Lanthanum ($La$).

Actinides

Elements with atomic numbers $90$ ($Th$) to $103$ ($Lr$) that follow Actinium ($Ac$).

Block classification

The division of elements into $s$, $p$, $d$, and $f$ blocks based on the nature of the orbital into which the differentiating electron enters.