8.1
Periodic Classification of the Elements
The periodic table arranges atoms based on increasing atomic number so that elements with the same chemical properties recur periodically. When their…
The electrons occupying the outermost shell of an atom are valence electrons, while electrons occupying the inner principal energy levels are core electrons.
A sodium atom with a [Ne]3s1 electron configuration has three principal energy levels. The complete inner principal energy levels with [He]2s22p6 indicate that there are ten core electrons followed by the third outermost level containing the remaining one electron. Therefore sodium has one valence electron. Similarly, chlorine has ten core electrons and seven valence electrons.
Valence electrons are the farthest from the nucleus and are held most loosely. Hence, they are the easiest to lose or share and play an important role in chemical bonding.
Elements that have the same number of valence electrons exhibit similar chemical properties, as can be seen from the arrangement in the modern periodic table.
For main-group elements, the lettered group number equals the number of valence electrons and the row number equals the highest principal quantum number of that element.
Each element in a group has the same number of electrons available for bonding. Down the group, the principal quantum number increases by one, whereas the number of valence electrons remains the same.
The two far-left columns of the periodic table constitute the s-block. For these elements, the last electron enters an s-orbital. Group one elements except for hydrogen are called the alkali metals and are extremely reactive as they have only one valence electron. Group two elements are the alkaline earth metals with two electrons in the valence shell.
The six far-right columns form the p-block. The valence shell of these elements has completely occupied s-orbitals and the last electron enters the p-orbital. From group three A to group eight A, the number of electrons in p orbitals increases by one. The noble gases have eight valence electrons except for helium, which belongs to the s-block, and only has two electrons.
The d-block consists of the ten columns placed between the s and p-block. These elements are referred to as the transition metals. The last electron enters the d-orbital of the principal shell number one less than the row number. In the fourth row, three d-orbitals fill, in the fifth row, four d-orbitals fill, and so on.
Inner transition elements constitute the f-block and have the last electron entering an f-orbital. The principal quantum number of the f-orbitals that fill across each row is two less than the row number. In the sixth row, the four f-orbitals fill, and in the seventh row, the five f-orbitals fill. Inner transition elements are arranged in the lanthanide series and the actinide series.
The number of columns in each block indicates how many electrons can be filled in the sublevel of the block. Two columns in the s-block correlate to an s-orbital with two electrons, six columns in the p-block represent three p-orbitals with six electrons, while ten columns in the d-block correspond to five d-orbitals with two electrons each.
Lastly, the f-block comprises fourteen columns indicating the maximum capacity of seven f-orbitals.
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Q1: What are valence electrons and why do they matter in chemistry?
Valence electrons are the outermost electrons in an atom, held most loosely and farthest from the nucleus. They play the most important role in chemical bonding because they are the easiest to lose or share. Elements with the same number of valence electrons exhibit similar chemical properties, which is why the periodic table groups them together.
Q2: How does the periodic table organize elements by their electron configurations?
The periodic table arranges elements by increasing atomic number so that elements with similar outer electron configurations recur periodically. For main-group elements, the group number equals the number of valence electrons, and the row number equals the highest principal quantum number. This organization reflects the periodic recurrence of similar electron configurations in outer shells.
Q3: What distinguishes alkali metals from other main-group elements?
Alkali metals are Group 1 elements (except hydrogen) with only one valence electron in their outermost s-orbital. Because they have just one electron available for bonding, they are extremely reactive. Their single valence electron makes them eager to lose it and form positive ions in chemical reactions.
Q4: Why are noble gases so unreactive?
Noble gases have eight valence electrons in their outermost shell, except helium which has two. This complete valence shell makes them highly stable and unreactive because they have no tendency to gain, lose, or share electrons. Their filled outer electron configuration means they do not participate readily in chemical bonding.
Q5: What are transition metals and how do they differ from main-group elements?
Transition metals are d-block elements where the last electron enters a d-orbital. Unlike main-group elements, their valence electrons include both the outermost s electrons and the (n-1) d electrons. The d-block contains ten columns because five d-orbitals can hold a maximum of ten electrons total.
Q6: How are inner transition elements organized in the periodic table?
Inner transition elements form the f-block, where the last electron enters an f-orbital. They are arranged in two series: the lanthanide series and the actinide series. The principal quantum number of the f-orbitals that fill is two less than the row number, and the f-block contains fourteen columns representing seven f-orbitals.
Q7: How does the number of valence electrons change across a period and down a group?
Across a period, the number of valence electrons increases by one with each consecutive element. Down a group, the number of valence electrons remains the same while the principal quantum number increases by one. This pattern explains why elements in the same group share similar chemical properties despite being in different periods.