SSS 2: THE PERIODIC TABLE
The periodic table is a table which shows the arrangement of all the known chemical elements in order of increasing atomic number (number of protons in the nucleus)
The elements with similar properties are placed in the same group (column).
The table is called "periodic" because elements with similar properties appear at regular intervals or periods.
Mendeleev's Periodic Table
Dmitri Mendeleev in 1869, a Russian scientist created the first widely recognized periodic table.
Mendeleev's contributions
1. Mendeleev arranged elements in order of increasing atomic mass
2. He leaves gaps for undiscovered elements and successfully predicts their properties e.g Gallium, Germanium etc.
3. He grouped elements with similar properties together, even if the atomic mass order was slightly violated
The Modern Periodic Law
Henry Mosley in 1913 discovered that the fundamental property for classification is the atomic number, not atomic mass.
This resolved the inconsistencies in Mendeleev's table.
The Modern Periodic Law states that the properties of elements are a periodic function of their atomic numbers.
Structure of the Modern Periodic Table.
(I) Groups (Columns)
(II) Periods (Rows)
Groups
There are eight vertical groups numbered from zero to seven (0-7). Elements in the same group have the same number of electrons in the outermost shell of their atoms i.e. they have the same number of valence electrons.
Besides the eight main groups, there are also the transition groups of elements. They lie between Groups 2 and 3 in the periodic table
Periods
The horizontal rows of elements or periods are numbered from 1 to 7. Elements in the same period have the same number of electron shells. Among the elements of periods 6 and 7 are the elements of Lanthanide and Actinide series known as INNER TRANSITION ELEMENTS
The elements in the periodic table maybe divided into blocks according to the orbital their valence electrons are found.
These are:
1. s-block: The s-block elements have s-electronic configuration in their outermost energy level. They are made up of Group 1 elements (alkali metals) and group 2 elements (alkali earth metals).
Electronic configurations of Group 1 elements
2. p-block: The p-block elements have both s and p electrons in their electronic configurations. Groups 3 to 7 and 0 form the p-block
3. d-block: The transition elements form the d-block. They contain d-electrons in addition to s and p. They are located between Groups 2 and 3.
4. f-block: The Lanthanides and Actinides contain f-electrons in addition to the s, p and d electrons.
Families of Elements
A family of elements consists of elements having similar characteristics. The elements of a family may belong to the same group or spread across several groups.
GROUP TREND: Group trend is defined as the gradual change of properties within a group.
GROUP I (ALKALI METALS)
The Group I elements are located in the first column of the periodic table.
The elements are:
· Lithium (Li)
· Sodium (Na)
· Potassium (K)
· Rubidium (Rb)
· Cesium (Cs)
· Francium (Fr)
Properties of group I elements
1. They are univalent elements
2. They are good reducing agents. They ionize by donating their single valence electron
3. They are good conductors of heat and electricity
4. They react vigorously with cold water to liberate hydrogen gas and form alkali. Hence they are called ALKALI METALS.
2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)
5. Oxides of group I elements dissolve in water to give very strong alkali
K₂O (s) + 2H₂O(l) → 2KOH(aq)
GROUP II (ALKALI EARTH METALS)
The Group II elements are located in the second column of the periodic table. They are:
· Beryllium (Be)
· Magnesium (Mg)
· Calcium (Ca)
· Strontium (Sr)
· Barium (Ba)
· Radium (Ra) (it is radioactive)
Properties of group II elements
1. They are divalent elements
2. They are good reducing agents. They ionize by donating their two valence electrons
3. They are good conductors of heat and electricity. They are hard metals, malleable and ductile.
4. Their oxides are insoluble in water except for calcium oxide which dissolves in water to form an alkali.
CaO (s) + H₂O (l) → Ca(OH)₂ (aq)
Quicklime. + Water → Slaked lime
NOTE:
Beryllium does not react with cold water or steam.
Magnesium reacts with steam only.
Calcium react slowly with cold water to liberate hydrogen gas.
GROUP III
The Group III elements are located in the third column of the periodic table. They are:
· Boron (B)
· Aluminium (Al)
· Gallium (Ga)
· Indium (In)
· Thallium (Tl)
Aluminium is the most familiar element in the group.
Properties of group III elements
1. They are trivalent elements
2. They are good reducing agents. They ionize by donating their three valence electrons
3. Only Aluminium can react with steam to liberate hydrogen gas
4. Oxide and hydroxide of aluminium are amphoteric in nature i.e. they have both acidic and basic properties.
Al₂O3(s)+ 3H₂SO4 (aq) → Al₂(SO4)3(aq) + 3H₂O (l)
Al(OH)3(s) + NaOH (aq) → NaAl(OH)4(aq)
GROUP IV
The Group IV elements are located in the fourth column of the periodic table. They are:
· Carbon (C)
· Silicon (Si)
· Germanium (Ge)
· Tin (Sn)
· Lead (Pb)
Properties of group IV elements
1. They are tetravalent i.e. each of their atoms has four valence electrons
2. They form covalent compounds
3. They exhibit two oxidation states: +2 and +4
4. Electropositivity increases down the group. Carbon is a non-metal; silicon and germanium are metalloids while tin and lead are metals.
5. Carbon and silver form more stable +4 compounds while Tin and lead form more stable +2 state compounds.
: +2 oxidation state:
CO<SiO<GeO<SnO<PbO
Stability increases →
: +4 oxidation state:
CO₂>SiO₂>GeO₂> SnO₂>PbO₂
Stability decreases →
6. Carbon does not react with water in any form but silicon and tin react with steam at red heat to form the +4 state oxides and hydrogen.
Si(s) + 2H₂O(g) →SiO₂(s) + 2H₂(g)
GROUP V
The Group V elements are located in the fifth column of the periodic table. They are:
· Nitrogen (N)
· Phosphorus (P)
· Arsenic (As)
· Antimony (Sb)
· Bismuth (Bi)
Properties of group V elements
1. They are non-metals
2. They exhibit oxidation state of -3 and -5.
3. They are oxidizing agents. Hence they are electron acceptors
4. They also show group trend. Nitrogen and Phosphorus are non-metals; Arsenic and antimony are metalloids while Bismuth is a metal.
5. They form oxides that dissolve in water to form acids except nitrogen (I)oxide.
GROUP VI
The Group VI elements are located in the sixth column of the periodic table. They are:
· Oxygen (O)
· Sulphur (S)
· Selenium (Se)
· Tellurium (Te)
· Polonium (Po)
Properties of group VI elements
1. They are non-metals
2. They exist as solid at room temperature except for Oxygen
3. They are oxidizing agents. Hence they are electron acceptors
4. Oxygen is slightly soluble in water while sulphur is insoluble. Both oxygen and sulphur react directly with hydrogen to yield water and hydrogen sulphide respectively.
2H₂(g) + O2(g) →2H₂O(l)
H₂(g) + S (s) →H₂O(g)
GROUP VII (HALOGENS)
The Group VII elements are located in the seventh column of the periodic table. They are called halogens (salt-makers) because they exist mainly as salts rather than free elements. They are the most reactive non-metals. Flourine is the most reactive non-metal in the group.
They include:
· Flourine (F)
· Chlorine (Cl)
· Bromine (Br)
· Iodine (I)
· Astatine (At)
Properties of group VII elements
1. They are all non-metals
2. They are coloured
3. They exist as diatomic molecules
4. All halogens are good oxidizing agents. Hence, they are electron acceptors
5. They ionize to form univalent anions. They react with metallic ions to form electrovalent compounds
6. They exhibit group trend. Flourine and chlorine are gases. Bromine is a liquid. Iodine and astatine are solids at room temperature.
Increasing order of reactivity of the halogens: I₂ < Br₂ < CI₂ < F₂
GROUP VIII (0)
The Group 0 elements are located in the eighth column of the periodic table. They are also known as rare, noble or inert gases.
They are:
· Helium (He)
· Neon (Ne)
· Argon (Ar)
· Krypton (Kr)
· Xenon (Xe)
· Radon (Rn) (It is Radioactive)
Properties of group VIII elements
1. They are non-metals
2. They are non-reactive
3. They exist freely as monoatomic molecules in the atmosphere.
4. Their melting and boiling points increase down the group while their ionization energies decrease down the group.
TRANSITION ELEMENTS
They are found in-between group 2 and 3 of the periodic table. The first transition series consists of elements: Scandium (Sc), Titanium (Ti), Vanadium (V), Chromium (Cr), Manganese (Mn), Iron (Fe), Cobalt (Co), Nickel (Ni), Copper (Cu) and Zinc (Zn).
Transition elements are all metals with typical metallic properties.
The properties are:
1. They have variable oxidation states
2. They have high tensile strength
3. They have high melting and boiling points
4. They form coloured ions: this is due to the presence of unpaired electrons in their partially filled d orbitals.
5. They form complex ions
6. They are paramagnetic in mixture
7. They have catalytic ability e.g. Manganese and nickel.
>>>>>(Read more on Lanthanides and Actinides below the note)
PERIODICITY
Periodicity is the variation of the properties of elements in a regular pattern both down the group and across the periods.
The atomic physical properties of elements that shows periodic variation are:
(I) Atomic radius
(II) Ionic radius
(III) Electron affinity
(IV) Electronegativity
(V) Ionization energy
Atomic radius
Atomic radius is the difference from the nucleus of an atom to its outermost electron shell.
It is calculated as half the distance between the nuclei of two bonded atoms.
Example: The distance between the nucleus of iodine atoms in an iodine molecules is 0.266nm. Calculate the atomic radius.
Solution
Atomic radius= ½ x distance between two nuclei
= ½ x 0.266nm
= 0.133nm
Ionic radius
Ionic radius is the radius of an atom after it it gains or loses an electron.
The ionic radii of metals are usually less than their atomic radii.
Electron Affinity
Electron affinity is the energy change that occurs when an electron is added to a neutral gaseous atom to form a negative ion (anion).
OR
Electron affinity can be defined as the energy change which accompanies the addition of one mole of electrons to one mole of gaseous atom of an element to form negatively charged ion.
Electronegativity
Electronegativity is the power of an atom of an element to attract electrons to become negatively charged.
The greater the difference in Electronegativity between bonded atoms, the higher the polarity of the bond.
Electropositivity
Electropositivity is the power of an atom of an element to lose an electron to become positively charged.
Ionization energy
Ionization energy is the energy required to remove a valence electron from the atom of an element to form an ion.
The ionization energy of an atom is affected by:
1. The distance of the outermost electron from the nucleus
2. The size of the positive nuclear charge
3. The screening (shielding) effect of the inner electrons
Screening effect
The screening effect, or shielding effect, is the reduction of the attractive force of the nucleus on the outermost electrons due to the presence of inner-shell electrons.
Inner electrons repel the outer electrons, acting like a shield and reducing the net positive charge experienced by the valence electrons, known as the effective nuclear charge.
First ionization energy
Is the minimum energy required to remove one mole of electron from a gaseous atom to form one mole of a gaseous ion with a positive charge.
Trends in atomic properties in the periodic table
Variation in other physical properties
1. Melting and boiling points
2. Electrical and thermal conductivity
LANTHANIDES (RARE EARTH ELEMENTS)
The lanthanides are a series of 15 metallic chemical elements with atomic numbers 57 through 71 on the periodic table (Lanthanium to Lutetium). They are also known as rare earth elements, although this term often includes scandium and yttrium.
They are called "lanthanides" because they appear after the element Lanthanum (La,57) and have very similar chemical properties.
Key Characteristics and Properties
1. Electron Configuration: This is the most defining feature. As you move across the series from Cerium (Ce, #58) to Lutetium (Lu, #71), electrons are added to the inner 4f orbital. The outer electron shell (5s and 5p) remains largely the same.
· Consequence: This "shielding" of the f-electrons is why all lanthanides have incredibly similar chemical properties. It's very difficult to separate them from one another in their pure forms.
2. Lanthanide Contraction:
· As the atomic number increases, the increasing positive charge in the nucleus pulls the electron shells inward more strongly.
· The 4f orbitals are poor at shielding this increased nuclear charge.
· Result: The atomic and ionic radii decrease significantly across the series. This contraction has important consequences, such as making the elements following the lanthanides (Hafnium through Gold) denser and having properties very similar to their lighter counterparts (e.g., Zr and Hf, Nb and Ta).
Physical Properties:
1. They are typically silvery-white, soft, malleable metals with high melting and boiling points.
2. They are paramagnetic (weakly attracted to a magnet), but some, like Gadolinium, are strongly ferromagnetic.
3. They are highly reactive, especially at elevated temperatures. They tarnish easily in air and react with water to liberate hydrogen gas.
4. They most commonly exhibit a +3 oxidation state in their compounds, which is their most stable state.
Importance of Lanthanides
Lanthanides are critical to modern technology. Despite the name "rare earth," they are not particularly rare in the Earth's crust (Cerium is about as abundant as copper), but they are rarely found in concentrated, economically exploitable deposits.
1. Permanent Magnets: Neodymium (Nd) and Samarium (Sm) are vital for powerful, compact magnets used in:
· Electric vehicle motors
· Wind turbine generators
· Hard disk drives
· Headphones and speakers
· Smartphones
2. Phosphors and Lighting:
· Europium (Eu), Terbium (Tb), and Yttrium (Y) are used to create red, green, and the matrix for blue light in color TV, computer, and phone screens (OLED/LCD).
· Used in fluorescent and LED lighting.
3. Catalysts:
· Cerium (Ce) is a key component in automotive catalytic converters.
· Lanthanum (La) is used in fluid catalytic cracking in petroleum refining.
4. Metallurgy:
· Added in small amounts to alloys to improve strength, workability, and resistance to oxidation and corrosion. Mischmetal (a mix of primarily Ce, La, and Nd) is used for this purpose.
5. Glass and Ceramics:
· Cerium (Ce) is used for polishing glass.
· Neodymium (Nd) is used to make glass that changes color with different lighting.
· Praseodymium (Pr) is used to create yellow stains for ceramics.
6. Medical Applications:
· Gadolinium (Gd) is used as a contrast agent in Magnetic Resonance Imaging (MRI).
· Holmium and Ytterbium lasers are used in surgical procedures.
Challenges and Geopolitics
1. Mining and Refining: The process of separating lanthanides from each other is chemically complex, expensive, and can generate significant radioactive waste (as they are often found with radioactive elements like Thorium and Uranium).
2. Supply Chain Dominance: For decades, China has dominated the global supply chain, controlling the majority of mining, separation, and processing. This has led to concerns about supply security for other nations, driving efforts to find new sources and develop recycling technologies.
In summary, the lanthanides are a fascinating and critically important group of elements. Their unique electronic structure gives them a suite of magnetic, phosphorescent, and catalytic properties that are indispensable for the high-tech, green, and medical industries of the 21st century.
ACTINIDES AND THE ARTIFICIAL ELEMENTS
Of course. Building on the lanthanides, let's explore the actinides and the fascinating world of artificial elements.
The actinides are the series of 15 metallic elements with atomic numbers 89 through 103, placed below the lanthanides on the periodic table. They are named after the first element in the series, Actinium (Ac, #89).
Like the lanthanides, their defining feature is the filling of an inner f-orbital—in this case, the 5f orbital.
Key Characteristics of the Actinides
1. Radioactivity: This is the most critical difference from the lanthanides. All actinides are radioactive. They are the Radioactive Heavyweights. Elements up to Uranium (#92) are primordial (found in nature), while those beyond Neptunium (#93) are synthetic (man-made).
2. Electron Configuration: Electrons fill the 5f orbital across the series. However, the 5f and 6d orbitals are very close in energy, leading to more complex chemistry and less similarity across the series compared to the lanthanides.
3. Multiple Oxidation States: While the lanthanides strongly prefer the +3 state, actinides can exhibit a wide variety of oxidation states, commonly from +3 to +6. This is particularly true for the earlier actinides (e.g., Uranium can be +3, +4, +5, +6).
4. Application: Nuclear Energy: The most significant application of the actinides is in nuclear technology, driven by their property of fission.
· Fission: The nucleus of a heavy atom (like Uranium-235 or Plutonium-239) can split into smaller nuclei when struck by a neutron, releasing a tremendous amount of energy and more neutrons.
The Artificial (Transuranium) Elements
Artificial elements are those that do not occur naturally on Earth and must be synthesized in laboratories. This category includes all elements beyond Uranium (Z > 92), which are called transuranium elements.
How Are They Made?
Creating new elements is a monumental task. The primary methods are:
1. Neutron Capture: In nuclear reactors, a nucleus captures neutrons, which then undergo beta decay, converting a neutron to a proton and increasing the atomic number. This is how elements like Neptunium (93) and Plutonium (94) were first produced.
2. Particle Acceleration: The primary method for creating heavier elements (Z > 100). A beam of light nuclei (e.g., Carbon-12, Calcium-48) is accelerated to high speeds and smashed into a target of a heavy element (e.g., Lead, Californium).
· If the nuclei fuse, they create a new, heavier nucleus. This new atom exists for only a fraction of a second before decaying.
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