Historical development of periodic table
Classification of the elements began with Lavoisier, whose classification was based on metals and non-metals. It was the Russian scientist, Dmitri Mendeleev who, between 1869-71, developed the periodic law. The law states that the properties of the elements varied periodically with their relative atomic masses.
While the modern periodic law states that the physical and chemical properties of the elements are periodic function of atomic numbers
Features of the periodic table
The modern periodic table is divided into eight vertical columns known as groups and seven horizontal columns known as periods. The features are as follows:
Groups: the groups are numbered from 0 to 7 and constitute elements of similar chemical properties. Elements in the same group have the same number of electrons in the outermost shell of their atoms.
Periods: the periods consist of those elements that are arranged in the horizontal row in the periodic table and are numbered 1-7. Elements in the same period have the same number of electron shells, which correspond to the period.
Diagram of a modern periodic table
Electronic configuration of atom as the basic of the periodic table
Electrons are arranged in shells or energy levels, situated at various distances from the nucleus. Starting from the shell nearest to the nucleus, the shells are named as K,L,M,N…….. etc.
|Shell||Energy level (n)||Maximum number of electrons (2n2)|
Note: Since the periodic table classifies elements by chemical behavior and the chemical behavior is determined by electronic configuration, the periodic table may also be said to classify elements by electronic configuration.
Diagram of periodicity of the melting and boiling points with atomic number for the first twenty elements
Periodicity of fundamental properties
The key to the periodicity of an elements lies in the electronic configuration of their atoms; and such physical properties include:
1.Melting and boiling point: the melting and boiling points of metallic elements increase from left to right while those of non-metallic elements decrease. Within a group, the melting and boiling points of metallic elements decrease down the group while those of non-metallic elements increase.
2. Ionization energy: ionization energy generally decreases steadily down a group because the outer electrons become progressively separated from the nucleus and so are less tightly held. In passing from left to right across a period, the ionization energy increases i.e formation of a cation becomes more difficult owing to the increase in nuclear charge.
Diagram of periodicity of the ionization with atomic number for the first twenty elements.
3. Electrical and thermal conductivities: these properties of elements decrease across the period and increase down the group. Thus, metals are good electrical and thermal conductors, non metals are poor electrical and thermal conductors.
Families of elements
Elements in the same group may be said to belong to one family. They show similar properties because their atoms have the same number of valence electrons. At the same time, certain properties of the elements in the same group show a gradual change with increasing atomic numbers e.g physical properties. Therefore the gradual change of property within a group is known as group trend. Which are as follows:
Group I: group 1 elements include lithium, sodium, potassium, rubidium, caesium and francium. They are univalent elements because they have one valence electron.
Group II: group II elements include beryllium, magnesium, calcium, strontium, barium, and radium. They are divalent elements because they have two valence electrons and they ionize by donating their two valence electrons.
Group III: boron, aluminium, gallium, indium and thallium belong to group III. They are trivalent because each atom has three valence electrons. They are donors of three electrons. They are donors of three electrons. Hence, they are reduced in nature and form electrovalent compounds.
Group IV: are elements that include carbon, silicon, germanium, tin and lead. They are tetravalent and they form covalent compounds.
Group V: nitrogen, phosphorus, arsenic, antimony and bismuth belong to this group. They are non-metals and show two valencies of 3 and 5.
Group VI: elements in group VI include oxygen, sulphur, selenium, tellurium and polonium. They are non-metals, electron acceptors and oxidizing in nature.
Group VII: group VII elements are known as the halogens. They include fluorine, chlorine, bromine, iodine and astatine. They are most reactive nonmetals and exist mainly as salts rather than as free elements in nature.
Group O: the elements in group O are known as rare gases or Noble gases. They have no bonding electrons in the outermost shell. They include helium, neon, argon, Krypton, xenon and radon (which is radioactive), because of their non-reactivity, they exist freely as monatomic molecules in the atmosphere.
Note: transition elements are elements found in between groups II and III of the periodic table. They are characterised by inner building (i.e the last electron is added to the second shell from the outermost shell). While inner transition elements are elements found at the bottom of the periodic table, but should properly be seen to follow group II.
Elements like lithium and magnesium, beryllium and aluminium, boron and silicon, there is a diagonal resemblance. This is to be expected, since in moving from left to right across the periodic table, whilst on descending a group, they increase. Therefore elements diagonally below one another have similar properties.
1.In the periodic table, what is the property that decreases along the period and increases down the group? (Jamb 1997)
A. Atomic number
B. Electron affinity
C. Ionization potential
D. Atomic radius
2. Which of the following statements is correct about the periodic table? (Jamb 2000)
A. Elements in the same period have the same number of valence electrons
B. The valence electrons of the elements in the same period increase progressively across the period
C. Elements in the same group have the same number of electron shells
D. The non-metallic properties of the elements tend to decrease across each period.
3. Elements in the same period in the periodic table have the same? (Jamb 2010)
A. Chemical properties
B. Physical properties
C. Number of shells
D. Atomic number
4. In the periodic table, electrical and thermal conductivities are properties of elements that ? (Jamb 2008)
A. Decrease across the period and increase down the group
B. Increase across the period and decrease down the group
C. Decrease across the period and down the group
D. Increase across the period and down the group
5. Defined the periodic law of elements?
Answer: the law states that the properties of the elements are periodic function of atomic numbers
6. Defined ionization energy?
Answer: ionization energy is the energy required for complete removal of a valence electron in its ground state from an isolated atom in the gaseous phase at a specified temperature to form an ion.
7. What are transition elements?
Answer: these are elements found in between groups II and III of the periodic table.
8. List the two features of periodic table
Answer: groups and periods