Periodic Table: Structure and History

• Introduction to the Periodic Table covers the historical background and evolution of the modern periodic table.

Johann Dobereiner (Model of triads)

• In 1829, some elements were classified into groups of three named triads.
• Elements in a triad shared similar chemical and orderly physical properties.
• The atomic mass of the central element approximated the arithmetic mean of the other two elements' atomic masses.
• Properties of the middle element were in between those of the end members.
• Dobereiner's Triads include:
  • Lithium, Sodium, Potassium
  • Chlorine, Bromine, Iodine
  • Calcium, Strontium, Barium
  • Sulfur, Selenium, Tellurium
  • Phosphorus, Arsenic, Antimony
• In Döbereiner's Triads, the mean of atomic masses of 1st and 3rd elements are:
  • Li, K: 23
  • Cl, I: 81.25
  • Ca, Ba: 88.5
  • S, Te: 79.5
  • P, Sb: 75.5
• The triad concept had drawbacks:
  • It could not be applied to all known elements, only a limited number.
  • However, one point became clear that properties of the elements have some relationship with their Atomic masses/Atomic number.

John Newlands (Law of Octaves)

• Newlands' Law of Octaves states elements arranged by increasing atomic masses exhibited similar properties every eighth element.

• The eighth succeeding element was a repetition of the first, like the eighth note of a musical scale.

• Elements exhibited similar properties, like H, F, and Cl or Li, Na, and K.

• The Law of Octaves failed for heavier elements, for example, the dissimilar elements manganese, nitrogen, and phosphorus were grouped together.

• Similarly, iron, oxygen, and sulphur were inappropriately grouped.

Lother Meyer's Curve

• Emphasizes physical characteristics of the elements.
• The classification was presented in the form of a curve.
• Atomic volumes of known elements were calculated using the formula: Atomic volume = Atomic mass / Density.
• When plotting atomic volumes against atomic masses, a curve with sharp peaks and broad minima was obtained.
• Alkali metals, with the largest atomic volumes, occupy the maxima of the curve.
• Alkaline earth metals (Mg, Ca, Sr, Ba) are positioned at roughly the mid-points on the descending portions of the curve.
• Halogens (F, Cl, Br, I) are positioned on the ascending portions of the curve.
• Transition elements occupy minima of the curve.
• Based on observations, Lother Meyer proposed that the physical properties of elements are a periodic function of their atomic masses.

Dmitri Mendeleev

• Proposed Periodic Law states the properties of the elements are a periodic function of their atomic masses.
• A tabular arrangement of elements highlighting regular property repetition is called a periodic table.
• Mendeleev arranged elements between hydrogen and uranium, providing 90 spaces in order of increasing atomic masses.
• When Mendeleev presented the periodic table only 63 elements were known.
• The table had gaps for unknown elements and their predicted properties
• When Mendeleev presented the periodic table, gallium and germanium were unknown but were named eka-Aluminium and eka-Silicon.
• Properties were also predicted:
  • Property: Eka-aluminium; Gallium
  • Atomic mass: 68; 70
  • Density (g/cm³): 5.9; 5.94
  • Property: Eka-silicon; Germanium
  • Density (g/cm³): 5.5; 5.36
  • Formula of oxide: E₂O₃; Ga₂O₃, EO₂; GeO₂
  • Formula of chloride: ECl₃; GaCl₃, ECl₄; GeCl₄

Modern Periodic Table

• Moseley suggested the atomic numbers of elements should be the basis for classification, not atomic masses.

• Modified Periodic Law: Chemical element properties are a periodic function of their atomic numbers.

• Periods are horizontal rows

• Members have the same # of principal energy levels

• Groups or Families are vertical columns

• Members have the same number of valance electrons

• Members of the same group have similar chemical properties

• Period is another word for rows

• Elements in the same row have the same number of electron shells

• Period 1 (1st row) elements have one shell (H & He)

• Period 2 elements have two shells

• Column is another word for group

• Elements in a group have the number of valence electrons (electrons in the outer shell)

• They share similar characteristics with the other elements in their family.

• Group 1: 1 valence electron

• Group 2: 2 valence electrons

• Group 13: 3 valence electrons

• Group 14: 4 valence electrons

• Group 15: 5 valence electrons

• Group 16: 6 valence electrons

• Group 17: 7 valence electrons

• Group 18: 8 valence electrons except He who has 2

• The elements of the periodic table can be divided into three main categories:

  • Non-metals
  • Metals
  • Metalloids

S-Block (Group 1 - Alkali Metals)

• Has a valance electron of 1 (+1 ion)
• Most active metals
• Are not found in nature by themselves
• Form stable compounds
• Can be obtained by electrolysis of fused salt.
• Group 1A elements and electron configuration:
  • lithium = 1s²2s¹ or [He]2s¹
  • sodium = 1s²2s²2p⁶3s¹ or [Ne]3s¹
  • potassium = 1s²2s²2p⁶3s²3p⁶4s¹ or [Ar]4s¹
  • rubidium = 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶5s¹ or [Kr]5s¹
  • cesium = 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶5s²4d¹⁰5p⁶6s¹ or [Xe]6s¹

S-Block (Group 2 - Alkaline Earth Metals)

• Has 2 valance electrons (+2 ion)
• Second most active metal
• Some cannot be found in nature by themselves.
• Group 2A elements and electron configuration:
  • beryllium = 1s²2s² or [He]2s²
  • magnesium = 1s²2s²2p⁶3s² or [Ne]3s²
  • calcium = 1s²2s²2p⁶3s²3p⁶4s² or [Ar]4s²
  • strontium = 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶5s² or [Kr]5s²
  • barium = 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶5s²4d¹⁰5p⁶6s² or [Xe]6s²

d-block (Transition Elements) (group 3 to 12)

• Exhibit multiple positive oxidation numbers.
• Valence electrons exist in various energy levels, permitting the loss of more than one electron.
• Form colored ions in solutions.
• 3d series comprises Sc (21) [Ar] 3d¹4s² to Zn (30) [Ar] 3d¹⁰4s².
• 4d series comprises Y (39) [Kr] 4d¹5s² to Cd (48) [Kr] 4d¹⁰5s².
• 5d series comprises La (57) [Xe] 5d¹6s² to Hg (80) [Xe] 4f¹⁴5d¹⁰6s².
• 6d series comprises Ac (89) [Rn] 6d¹7s² to Unb (112) [Rn] 5f¹⁴6d¹⁰7s².
• The p-block order is:
  • ns² np¹ - (IIIrd group)
  • ns² np² - (IVth group)
  • ns² np³ - (Vth group)
  • ns² np⁴ - (VIth group)
  • ns² np⁵ - (VIIth group)
  • ns² np⁶ - (Zero group)
• The p-block breakdown is:
  • n = 2: B, C, N, O, F, Ne
  • n = 3: Al, Si, P, S, Cl, Ar
  • n = 4: Ga, Ge, As, Se, Br, Kr
  • n = 5: In, Sn, Sb, Te, I, Xe
  • n = 6: Tl, Pb, Bi, Po, At, Rn
  • n = 7: Uut, Uuq, Uup, Uuh, Uus, Uuo

Lanthanide Series

• Includes shiny reactive metals.
• They are mostly found in nature.
• Comprises elements with atomic numbers 57-70.

Actinides Series

• Includes radioactive, unstable elements.

• Most are man-made and not stable in nature.

• Include elements with atomic numbers 89-102.

• The 4f elements consist of: Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; and are elements 58-71.

• The 5f elements consist of: Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr and are elements 90-103.

• Horizontal rows in the periodic table are called period

• Vertical columns in the periodic table are called group

• Elements in a group have similar chemical properties.

• Element which replaced eka-aluminium and eka-silicon proposed by Mendeleev: Gallium and Germanium

• The Elements (group) for the following name are:

  • Alkali metals - Group 1: Li, Na, K, Rb, Cs, Fr.
  • Alkaline earth metals - The metals of group 2: Be, Mg, Ca, Sr, Ba, Ra.
  • Pnictogens - The elements of group 15: N, P, As, Sb, Bi.
  • Chalcogens - The elements of group 16: O, S, Se, Te, Po.
  • Halogens - The elements of group 17: F, Cl, Br, I, At.
  • Noble gases -The elements of group 18: He, Ne, Ar, Kr, Xe, Rn.
  • Lanthanides - Elements 57-71: La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu.
  • Actinides - Elements 89-103: Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No,
  • Rare-earth metals - Sc, Y, plus the lanthanides.
  • Transition elements - Elements in groups 3 to 11 or 3 to 12

• Russian chemist Dmitri Mendeleev created the first periodic table in 1869.

• Astatine (At) is the rarest naturally occurring element on Earth.

• Tungsten is the strongest natural metal in terms of tensile strength.

• Gallium is a metal with a melting point below body temperature, low toxicity, and reactivity.

• Hydrogen (H) is the lightest element on the periodic table.

• The periodic table has horizontal rows called periods and vertical columns called groups.

• The periodic table organizes all known elements. Elements in a group have similar chemical and physical properties because they have the same number of outer electrons.

• The Modern periodic table is organized by increasing atomic number, while the table created by Russian chemist Dmitri Mendeleev was organized by increasing atomic weight. Elements and symbol: The first element on the periodic table is hydrogen (H) and the last is oganesson (Og).

• Prevalence: Hydrogen is the most common element in the universe, making up about 75% of the observable cosmos.

• Artificially created elements: Technetium (Tc) was the first element to be created artificially, in 1937.

• All the elements on the periodic table have symbols that are missing the letter J.

• The modern periodic table uses color coding to help distinguish the properties of the atoms.

• Atomic size decreases across a period, even though the atomic number increases.

• The melting and boiling points of the halogens increase as you move down group 7, and the color gets darker.

Explore the organization of the periodic table, including groups, periods, and the contributions of scientists like Dobereiner and Newlands. Learn about the properties of elements within the same group and the historical models used to classify them.