element

Examples of element in the following topics:

• Transuranium Elements

  • In chemistry, transuranium elements, also known as transuranic elements, are the chemical elements with atomic numbers greater than 92, which is the atomic number of uranium.
  • None of these elements is stable and each of them decays radioactively into other elements.
  • Transuranium elements that can be found on Earth now are artificially-generated, synthetic elements made via nuclear reactors or particle accelerators.
  • Heavy transuranic elements are difficult and expensive to produce.
  • Blue - Elements that contain at least one stable isotope.

• The Periodic Table

  • The periodic table only lists chemical elements, and includes each isotope of each element within one cell.
  • In the typical periodic table, each element is listed by its element symbol and atomic number.
  • Many periodic tables include the full name of element as well and color-code the elements based on their phase at room temperature (solid, liquid, or gas).
  • Elements within the same period or group have similar properties.
  • Chemical properties of each element are determined by the element's electronic configuration, and particularly by its outermost valence electrons.

• Elements and Compounds

  • A chemical element is a pure substance that consists of one type of atom.
  • The periodic table of elements is ordered by ascending atomic number.
  • Pure samples of isolated elements are uncommon in nature.
  • Still, most of these elements are found in mixtures.
  • Most elements on Earth bond with other elements to form chemical compounds, such as sodium (Na) and Chloride (Cl), which combine to form table salt (NaCl).

• Average Atomic Mass

  • The atomic number of an element defines the element's identity and signifies the number of protons in the nucleus of one atom.
  • For example, the element hydrogen (the lightest element) will always have one proton in its nucleus.
  • All elements exist as a collection of isotopes.
  • The average atomic mass of an element can be found on the periodic table, typically under the elemental symbol.
  • Whenever we do mass calculations involving elements or compounds (combinations of elements), we always use average atomic masses.

• Variation of Physical Properties Across a Period

  • The physical properties of elements vary across a period, mostly as a function of bonding.
  • The periodic table of elements has a total of 118 entries.
  • Elements in the same period have the same number of electron shells; moving across a period (so progressing from group to group), elements gain electrons and protons and become less metallic.
  • The physical properties of the chlorides of elements in Groups 1 and 2 are very different compared to the chlorides of the elements in Groups 4, 5, and 6.
  • This again demonstrates the type of bonding that these compounds exhibit: the left-most elements form more ionic bonds, and the further-right elements tend to form more covalent bonds.

• Periodic Trends in Metallic Properties

  • Keep in mind that group names can give clues about the elements' metallic properties.
  • When two elements are joined in a chemical bond, the element that attracts the shared electrons more strongly has more electronegativity.
  • Elements with low electronegativity tend to have more metallic properties.
  • So, the metallic properties of elements tends to decrease across a period and increase down a group.
  • These elements all possess low electronegativities and readily form positive ions.

• Writing Lewis Symbols for Atoms

  • The Lewis symbol for an atom depicts its valence electrons as dots around the symbol for the element.
  • Since we have established that the number of valence electrons determines the chemical reactivity of an element, the table orders the elements by number of valence electrons.
  • Each of these elements has one valence electron.
  • Lewis symbols for the elements depict the number of valence electrons as dots.
  • The heavier elements will follow the same trends depending on their group.

• Variation of Physical Properties Within a Group

  • In chemistry, a group is a vertical column in the periodic table of the chemical elements.
  • There are 18 groups in the standard periodic table, including the d-block elements but excluding the f-block elements.
  • Before a discussion of the melting points of various elements, it should be noted that some elements exist in different forms.
  • On the left is sodium, a very metallic element (ductile, malleable, conducts electricity).
  • On the right is sulfur, a very non-metallic element.

• The Law of Multiple Proportions

  • The law of multiple proportions states that elements combine in small whole number ratios to form compounds.
  • The law, which was based on Dalton's observations of the reactions of atmospheric gases, states that when elements form compounds, the proportions of the elements in those chemical compounds can be expressed in small whole number ratios.
  • For example, the reaction of the elements carbon and oxygen can yield both carbon monoxide (CO) and carbon dioxide (CO2).
  • Dalton's law of multiple proportions is part of the basis for modern atomic theory, along with Joseph Proust's law of definite composition (which states that compounds are formed by defined mass ratios of reacting elements) and the law of conservation of mass that was proposed by Antoine Lavoisier.

• The Law of Definite Composition

  • The law of definite composition states that chemical compounds are composed of a fixed ratio of elements as determined by mass.
  • French chemist Joseph Proust proposed the law of definite composition or proportions based on his experiments conducted between 1798 and 1804 on the elemental composition of water and copper carbonate.
  • It stated that chemical compounds are formed of constant and defined ratios of elements, as determined by mass.
  • Berthollet supported the concept that elements could mix in any ratio.
  • Dalton's law of multiple proportions expanded on the law of definite composition to postulate that, in situations in which elements can combine to form multiple combinations, the ratio of the elements in those compounds can be expressed as small whole numbers.