KEY IDEA: All chemical elements are systematized in the periodic table of elements.
- The table displays the known elements in such a manner that the relationships between their physical and chemical properties become evident.
2017 Periodic Table.PDF (Credit: Todd Helmenstine)
Historical Note
Since ancient times, certain substances were found to be more fundamental than others; these substances could not be broken down into simpler substances by any ordinary physical or chemical process. This defines an element. Generally, the discovery of new elements depended on the development of new technologies, which in turn gave rise to even newer technologies.
Some elements (gold, silver, copper, platinum) have been known since antiquity; they are easily found in nature in their "free" elemental state. Elements iron, tin, antimony, mercury, and lead are easily produced in an intense fire by smelting certain rocks (common ores). Non-metals carbon (charcoal) and sulfur (from volcano eruptions) were also well-known in prehistoric times.
Thru the 16th century, alchemists separated arsenic, zinc, bismuth, and phosphorus. 17th century chemists devised new techniques to purify and characterize many more elements. The battery provided an important technique, called electrolysis, by which elements were isolated. Electroplating is a technique developed for purifying metals.
Flame spectroscopy led to discoveries during 1870-1890. Rare elements (atomic numbers 57–71) and the inert, or “noble” gases were identified. Isolation of fluorine remained a great challenge of the late 19th century. The first of the radioactive elements, polonium and radium, was discovered in 1898.
Thru the 16th century, alchemists separated arsenic, zinc, bismuth, and phosphorus. 17th century chemists devised new techniques to purify and characterize many more elements. The battery provided an important technique, called electrolysis, by which elements were isolated. Electroplating is a technique developed for purifying metals.
Flame spectroscopy led to discoveries during 1870-1890. Rare elements (atomic numbers 57–71) and the inert, or “noble” gases were identified. Isolation of fluorine remained a great challenge of the late 19th century. The first of the radioactive elements, polonium and radium, was discovered in 1898.
Periodic Table
As more elements were discovered, scientists recognized repeating patterns in their properties and began to devise a way to organize them.
All chemical elements all have measurable properties, a basis by which to classify:
Dmitri Mendelyeev credited with the first successful periodic table based on mass. Elements with similar properties were arranged in families:
All chemical elements all have measurable properties, a basis by which to classify:
- relative mass: determined by the ratios of weights when compounds are decomposed
- physical & chemical properties:
- Alkali metals: soft, silvery, metals; react with water, form 2:1 compounds with oxygen (such as Na2O)
- Alkaline-earth metals: form 1:2 compounds with chlorine (CaCl2) and 1:1 compounds with oxygen (such as CaO, or lime).
- Halogens - form 1:1 compounds with hydrogen and alkali metals, and 2:1 compounds with alkaline earth metals.
Dmitri Mendelyeev credited with the first successful periodic table based on mass. Elements with similar properties were arranged in families:
Mendeleev's periodic table made predictions about elements that had not yet been isolated. Soon, new elements were discovered and expanded the periodic table. The modern periodic table has changed form as science progressed, but the basic structure remains the same.
WATCH TED-Ed "The Genius of Mendeleev's Periodic Table"
LINK Mystery of Matter: The Periodic Table
WATCH TED-Ed "The Genius of Mendeleev's Periodic Table"
LINK Mystery of Matter: The Periodic Table
WATCH TED-Ed. "Solving the Puzzle of the Periodic Table"
WATCH Who REALLY Invented the Periodic Table? (25 min)
periodic law
In 1913 (following the development of atomic theory), Henry Moseley put the finishing touch on the periodic table when he correctly assigned atomic numbers to the elements. This is historically important in quantitatively justifying the conception of the nuclear model of the atom.
Before Moseley's discovery, atomic numbers were thought of as a semi-sequential number (based on mass), but not known to be associated with any measurable physical quantity. In 1913, he made a pioneering use of X-ray spectroscopy in physics and discovered a systematic mathematical relationship in the spectra of various elements. Moseley demonstrated that the atomic numbers of elements are not an arbitrary number, but rather the amount of positive charge in the atom's nucleus. Mosely's mentor, Ernest Rutherford discovered the subatomic particle responsible for this charge: the proton. Mosely and Rutherford elevated the merit of the Mendeleev's periodic table.
The modern periodic table represents centuries of collective work, which continues today.
LINK: Theo Gray's Photographic Periodic Table
The modern periodic table represents centuries of collective work, which continues today.
LINK: Theo Gray's Photographic Periodic Table
Intro to the periodic table
- PERIOD - horizontal rows (numbered 1 - 7)
- FAMILY - vertical groups, share similar properties, similar valence (numbered 1-18)
There are 3 general "types" of elements, distinguished by chemical and physical properties:
Metals lie to bottom-left on the periodic table; non-metals lie up-right. Metalloids lie along the frontier between the two types of elements:
- metal
- non-metal
- semi-metal, or metalloid
Metals lie to bottom-left on the periodic table; non-metals lie up-right. Metalloids lie along the frontier between the two types of elements:
ADDITIONAL RESOURCES:
LINK What are the parts of the periodic table? AboutChemistry.com
LINK The Periodic Table in Pictures & Words Think Zone
LINK What are the parts of the periodic table? AboutChemistry.com
LINK The Periodic Table in Pictures & Words Think Zone
Periodicity
The periodic table supports an understanding of the modern atomic model. Several properties of the elements exhibit definite periodic trends, that is, atomic size, ionization energy, and electronegativity. The periodic properties of the elements suggest that elements with similar chemical properties have similar arrangements of electrons. Chemical reactions are due to the interaction of electrons.
The key to understanding how atoms combine is that certain “magic numbers” of electrons are much more stable than other numbers. The most stable electron arrangements contain exactly 2, 10, 18, 36, 54, or 86 electrons. Much of chemistry can be understood as a game in which every atom (or group of atoms) adopt a strategy to achieve one of these magic numbers of electrons.
The key to understanding how atoms combine is that certain “magic numbers” of electrons are much more stable than other numbers. The most stable electron arrangements contain exactly 2, 10, 18, 36, 54, or 86 electrons. Much of chemistry can be understood as a game in which every atom (or group of atoms) adopt a strategy to achieve one of these magic numbers of electrons.
Atomic radii tend to increase going down a family, and decrease from left-to-right within a period.
Within a family, atomic size increases with increasing atomic number; within a period, atomic size decreases with increasing atomic number.
Graph: Atomic radius vs. Atomic number
Along a (horizontal) period, atom size tends to decrease with increasing atomic number (the "nuclear charge"). The electrons experience a greater attraction towards the nucleus with increasing nuclear charge; thus, a smaller atom. Within a group, atom size increases with the addition of electron energy levels at larger distances from the nucleus:
Coulombic forces at play within an atom:
Valence electrons (the "outermost" electrons) feel a less effective nuclear charge, which is less than the nuclear charge, due to shielding (or "blocking") by inner "core" electrons. Shielding increases for larger atoms, due to more core electrons. The effective nuclear charge (Zeff) can be expressed mathematically as: Zeff = Z – S
where "Z" is the atomic number (nuclear charge), and "S" is the number of core electrons.
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The effective nuclear charge increases left to right across any period of the table since the number of shielding electrons remains the same.
IONIC RADIUS
Atoms lose and gain electrons to form a charged ion.
Atoms lose and gain electrons to form a charged ion.
- A loss of electrons forms a positively charged ion, a "cation."
- A gain of electrons forms a negatively charged ion, an "anion."
Cations are smaller than their neutral atom due to a decrease in electron-electron repulsion.
In contrast, anions are larger than their neutral atom due to an increase in electron-electron repulsion
In contrast, anions are larger than their neutral atom due to an increase in electron-electron repulsion
IONIZATION ENERGY
Ionization Energy is the energy required to remove an electron from an isolated atom (forming a cation).
Non-metals have higher ionization energies than metals.
Ionization Energy is the energy required to remove an electron from an isolated atom (forming a cation).
Non-metals have higher ionization energies than metals.
Graph: Atomic radius vs. Atomic number
ELECTRONEGATIVITY
Electronegativity is defined as the ability of an atom (element) to attract electrons in a chemical bond.
Electronegativity is a 4-pt scale: 4.0 = VERY STRONG attraction for electrons; 0 = weak attraction.
Electronegativity is defined as the ability of an atom (element) to attract electrons in a chemical bond.
Electronegativity is a 4-pt scale: 4.0 = VERY STRONG attraction for electrons; 0 = weak attraction.
ELECTRON AFFINITY
The electron affinity is the energy emitted in the process of adding an electron to an atom (forming an anion).
Elements with the highest electron affinities lie toward the upper right corner of the periodic table.
The electron affinity is the energy emitted in the process of adding an electron to an atom (forming an anion).
Elements with the highest electron affinities lie toward the upper right corner of the periodic table.