英文化学元素周期表

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2024年5月11日发(作者：)

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The Periodic Table of Elements

by Anthony Carpi, Ph.D.

In 1869, the Russian chemist Dmitri Mendeleev first proposed that the chemical

elements exhibited a "periodicity of properties." Mendeleev had tried to organize

the chemical elements according to their atomic weights, assuming that the properties

of the elements would gradually change as atomic weight increased. 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.

The modern periodic table of elements is based on Mendeleev's observations; however,

instead of being organized by atomic weight, the modern table is arranged by atomic

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.

For example, oxygen (O), fluorine (F), and neon (Ne) (z = 8, 9 and 10, respectively)

all are stable nonmetals that are gases at room temperature. Sodium (Na, z = 11),

however, is a silver metal that is solid at room temperature, much like the element

lithium (z = 3). Thus sodium begins a new row in the periodic table and is placed

directly beneath lithium, highlighting their chemical similarities.

Rows in the periodic table are called periods. As one moves from left to right in

a given period, the chemical properties of the elements slowly change. Columns in

the periodic table are called groups. 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.

The Periodic Table of Elements

Electron Configuration and the Table

The "periodic" nature of chemical properties that Mendeleev had discovered is related

to the electron configuration of the atoms of the elements. In other words, the way

in which an atom's electrons are arranged around its nucleus affects the properties

of the atom.

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Bohr's theory of the atom tells us that electrons are not located randomly around

an atom's nucleus, but they occur in specific electron shells (see our Atomic Theory

II module for more information). Each shell has a limited capacity for electrons.

As lower shells are filled, additional electrons reside in more-distant shells.

The capacity of the first electron shell is two electrons and for the second shell

the capacity is eight. Thus, in our example discussed above, oxygen, with eight protons

and 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 eleven electrons. 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 third electron shell in 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 two elements. But why is sodium similar

to lithium? Let's look at the electron configurations of these elements.

Group IA VIA VIIA VIIIA

Lithium Oxygen

Fluorine Neon

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