COPY: Mastering the Elements

The Periodic Table: A Summary

The periodic table is a graphic formulation of the periodic law, which states that the properties of the chemical elements exhibit an approximate periodic dependence on their atomic numbers.
The table is divided into four rectangular blocks, with rows called periods and columns called groups. Elements from the same group of the periodic table show similar chemical characteristics.
The trends of nonmetallic character increasing from left to right and from down to up across a group, and metallic character increasing in the opposite direction, are due to electron configurations of atoms.
The first periodic table was formulated by Dmitri Mendeleev in 1869, who used the periodic law to predict properties of some of the missing elements.
With the discovery of the atomic number and pioneering work in quantum mechanics, a recognisably modern form of the table was reached.
Today, all the first 118 elements are known, completing the first seven rows of the table, but chemical characterisation is still needed for the heaviest elements to confirm that their properties match their positions.
The periodic table exclusively lists electrically neutral atoms that have an equal number of positively charged protons and negatively charged electrons and puts isotopes (atoms with the same number of protons but different numbers of neutrons) at the same place.
Atoms can be subdivided into different types based on the number of protons they have, called the atomic number.
Elements are placed in the periodic table by their electron configurations, which exhibit periodic recurrences that explain the trends of properties across the periodic table.
An electron can be thought of as inhabiting an atomic orbital, which characterises the probability it can be found in any particular region of the atom.
The order of subshell filling is given by the Aufbau principle, which specifies the sequence of filling according to the increasing n + ℓ rule, with the exception of s-orbitals being raised in energy to approach that of the next n + ℓ group.
The precise energy ordering of 3d and 4s changes along the row, and also changes depending on how many electrons are removed from an atom.The Modern Periodic Table: Structure, Electron Configurations, and Variations
The periodic table is a tabular arrangement of the chemical elements, ordered by their atomic number, electron configurations, and chemical properties.
The table is divided into seven rows (periods) and 18 columns (groups), with each element occupying a unique position.
The electron configurations of the elements determine their positions in the table, with elements in the same row having the same number of electron shells and elements in the same column having similar valence electron configurations.
The first row of the table consists of two s-block elements (hydrogen and helium), followed by six p-block elements (lithium through neon).
The second row consists of two s-block elements (beryllium and magnesium), followed by ten d-block elements (scandium through zinc), and six p-block elements (gallium through argon).
The third to sixth rows follow a similar pattern, with the addition of f-block elements (lanthanides and actinides) in the sixth and seventh rows.
The f-block elements are sometimes shifted one element to the right, so that lanthanum and actinium become d-block elements in group 3, and Ce–Lu and Th–Lr form the f-block.
The electron configurations of the elements determine their chemical properties, with elements in the same group having similar valence electron configurations and similar chemical behavior.
The periodic table is presented in two forms: the 32-column or long form, which includes the f-block elements in the main body, and the 18-column or medium-long form, which cuts out the f-block elements and places them below the main body.
The groups are numbered numerically from 1 to 18 from left to right, and can also be named by their first element (e.g. the "scandium group" for group 3).
The placement of hydrogen and helium in the periodic table remains an open issue under discussion, with conflicting views on whether chemical or electronic properties should primarily decide their placement.
The periodic table exhibits periodic trends, with elements in the same row exhibiting recurring similarities and trends in chemical behavior.
The physical size of atoms was unknown until the early 20th century, with the first calculated estimate of the atomic radius of hydrogen published by physicist Artur Haas in 1910.Properties of Elements in the Periodic Table
The Bohr radius, a measure of atomic size, was first calculated by Arnold Sommerfeld and refined by John Slater and Edmund Stoner. It is about 10 times the size of the accepted value.
Atomic radii generally decrease from left to right along the main-group elements, but increase going down a column. Anomalies arise for the 1s, 2p, 3d, and 4f subshells, which have no inner analogues and experience strong repulsion from their inner analogues.
In transition elements, the size of the atom is determined by the outer electrons. The increasing nuclear charge across the series and the increased number of inner electrons for shielding somewhat compensate each other, so the decrease in radius is smaller.
Thallium and lead atoms are about the same size as indium and tin atoms respectively, but from bismuth to radon, the 6p atoms are larger than the analogous 5p atoms due to relativistic effects.
The first ionisation energy of an atom is the energy required to remove an electron from it. It generally increases left to right and down to up, but there are exceptions such as oxygen.
In the transition series, the outer electrons are preferentially lost even though the inner orbitals are filling.
The opposite property to ionisation energy is the electron affinity, which is the energy released when adding an electron to the atom. It generally increases down to up and left to right, except for the noble gases.
The valence of an element can be defined as the number of hydrogen atoms that can combine with it to form a simple binary hydride, or as twice the number of oxygen atoms that can combine with it to form a simple binary oxide. The valences of the main-group elements are directly related to the group number.
Common oxidation states of the heavier p-block elements tend to vary by steps of 2, while common oxidation states of transition metals tend to vary by steps of 1.
Electronegativity is the tendency of an atom towards gaining or losing electrons. It tends to fall going up to down, and rise going left to right.
An element's electronegativity varies with the identity and number of the atoms it is bonded to, as well as how many electrons it has already lost.
Simple substances of the more electronegative atoms tend to share electrons with each other, forming small molecules or giant structures stretching indefinitely. The noble gases simply stay as single atoms.triad, as did sulfur, selenium, and tellurium. Döbereiner also observed that the atomic weight of the middle element in each triad was approximately the average of the atomic weights of the other two elements.

In 1862, French geologist Alexandre-Emile Béguyer de Chancourtois published a list of the elements arranged by increasing atomic weight on a cylinder. He noticed that elements with similar properties lined up vertically, and called this the "telluric helix".

In 1869, Russian chemist Dmitri Mendeleev published his version of the periodic table, arranging the elements by increasing atomic weight and placing elements with similar properties in the same column. He left gaps for undiscovered elements and predicted their properties based on the properties of the surrounding elements.

In the early 20th century, British physicist Henry Moseley discovered that the elements' atomic numbers, not their atomic weights, were the key to their properties. This led to the modern understanding of the periodic table, with elements arranged by increasing atomic number and properties recurring periodically.

Modern periodic table

The modern periodic table has 118 elements, arranged in rows (periods) and columns (groups). The periods represent the number of electron shells in an element's atom, while the groups represent the number of valence electrons.

The elements are classified into groups based on their properties. The alkali metals (group 1) are highly reactive metals that easily lose their valence electron to form a +1 ion. The alkaline earth metals (group 2) are also reactive metals, but less so than the alkali metals. The transition metals (groups 3-12) are less reactive metals that often form colorful compounds.

The nonmetals (groups 14-17) are elements that tend to gain electrons to form negative ions. The halogens (group 17) are highly reactive nonmetals that form salts with metals. The noble gases (group 18) are inert gases that rarely react with other elements.

There are also other sets of elements that behave similarly, such as the lanthanides and actinides, which are both sets of metals with similar properties.

The periodic table is used to predict the properties of unknown elements and compounds, and is essential to modern chemistry.