英文化学元素周期表

The capacity of the firstelectron shellis twoelectronsand for the second shell the capacity is eight. Thus, in our example discussed above, oxygen, with eightprotonsand eight electrons, carries two electrons in its first shell and six in its second shell. Fluorine, with nine electrons, carries two in its first shell and seven in the second. Neon, with ten electrons, carries two in the first and eight in the second. Because the number of electrons in the second shell increases, we can begin to imagine why the chemical properties gradually change as we move from oxygen to fluorine to neon.
Sodium has elevenelectrons. Two fit in its first shell, but remember that the second shell can only carry eight electrons. Sodium's eleventh electron cannot fit into either its first or its second shell. This electron takes up residence in yet another orbit, a thirdelectron shellin sodium. The reason that there is a dramatic shift in chemical properties when moving from neon to sodium is because there is a dramatic shift in electron configuration between the twoelements. But why is sodium similar to lithium? Let's look at the electron configurations of these elements.
The Periodic Table of Elements
Electron Configuration and the Table
The "periodic" nature of chemical properties that Mendeleev had discovered is related to theelectronconfiguration of theatomsof theelements. In other words, the way in which an atom's electrons are arranged around itsnucleusaffects the properties of the atom.
Lithium
Sodium
Anatom'svalence shell
"covers" innerelectronshells
Thevalence shellelectronsin anatomdetermine the way it will interact with neighboring atoms, and therefore determine its chemical properties. Since both sodium and lithium have onevalence electron, they share similar chemical properties.
Rows in the periodic table are calledperiods. As one moves from left to right in a given period, the chemical properties of theelementsslowly change. Columns in the periodic table are calledgroups. Elements in a given group in the periodic table share many similar chemical and physical properties. The link below will open a copy of the periodic table of elements in a new window.
Bohr'stheoryof theatomtells us thatelectronsare not located randomly around an atom'snucleus, but they occur in specific electron shells (see ourAtomic Theory IImodule for more information). Each shell has a limited capacity for electrons. As lower shells are filled, additional electrons reside in more-distant shells.
For example, oxygen (O), fluorine (F), and neon (Ne) (z = 8, 9 and 10, respectively) all are stable nonmetals thae. Sodium (Na, z = 11), however, is a silver metal that is solid at room temperature, much like theelementlithium (z = 3). Thus sodium begins a new row in the periodic table and is placed directly beneath lithium, highlighting their chemical similarities.
Anelement's chemical properties are determined by the way in which itsatomsinteract with other atoms. If we picture the outer (valence)electron shellof an atom as a sphere encompassing everything inside, then it is only thevalence shellthat can interact with other atoms - much the same way as it is only the paint on the exterior of your house that "interacts" with, and gets wet by, rain water.
The Periodic Table of Elements
by Anthony Carpi, Ph.D.
In 1869, the Russian chemistDmitri Mendeleevfirst proposed that the chemicalelementsexhibited a "periodicity of properties." Mendeleev had tried to organize the chemical elements according to their atomicweights, assuming that the properties of the elements would gradually change asatomic weightincreased. What he found, however, was that the chemical and physical properties of the elements increased gradually and then suddenly changed at distinct steps, or periods. To account for these repeating trends, Mendeleev grouped the elements in a table that had both rows and columns.
Electron Configuration Shorthand:
Forelementsin groups labeled A in the periodic table (IA, IIA, etc.), the number ofvalence electronscorresponds to the group number. Thus Li, Na, and other elements in group IA have onevalenceelectron. Be, Mg, and other group-IIA elements have two valence electrons. B, Al and other group-IIIA elements have three valence electrons, and so on. The row, or period, number that an element resides in on the table is equal to the number of total shells that contain electrons in theatom. H and He in the first period normally have electrons in only the firstshell; Li, Be, B, and other period-two elements have two shells occupied, and so on. To write the electron configuration of elements, scientists often use a shorthand in which the element's symbol is followed by the element'selectron shells, written as a right-hand parentheses symbol ")". The number of electrons in each shell is then written after the ) symbol. A few examples are shown below.
Group IA
VIA
VIIA
VIIIA
Lithium
Oxygen
Fluorine
Neon
Sodium
Electron Configurations for Selected Elements
As you can see in the illustration, while sodium has threeelectron shellsand lithium two, the characteristic they share in common is that they both have only oneelectronin their outermost electron shell. These outer-shell electrons (calledvalenceelectrons) are important in determining the chemical properties of theelements.
The modern periodic table ofelementsis based on Mendeleev's observations; however, instead of being organized by atomicweight, the modern table is arranged byatomic number(z). As one moves from left to right in a row of the periodic table, the properties of the elements gradually change. At the end of each row, a drastic shift occurs in chemical properties. The next element in order of atomic number is more similar (chemically speaking) to the first element in the row above it; thus a new row begins on the table.