This is the systematic arrangement of elements in order of their increasing atomic number in the Periodic Table.
Dmitri Mendeleev, a Russian chemist was the first person to formulate the periodic table. He noticed that, when elements were arranged in order of increasing atomic mass, certain properties recurred periodically.
He used this observation to create a table with elements grouped by their shared properties.
Mendeleev’s contributions include:
He arranged the known elements in increasing order of their atomic masses and observed patterns in their properties.
Grouping elements with similar properties into horizontal rows called periods.
Leaving gaps in the table for undiscovered elements, predicting their properties based on their positions.
He proposed the periodic law “the properties of elements are a periodic function of their atomic masses.”
Weaknesses of Mendeleev’s Periodic Table:
It was based on atomic masses, which led to some inconsistencies.
Isotopes could not be accommodated in his periodic table.
Mendeleev’s periodic table provided a framework for understanding the relationships between elements and their properties.
His work laid the foundation for the modern periodic table and the periodic law.
The modern period table
In the modern periodic table, elements are arranged in increasing order of atomic number, which reflects the number of protons in the nucleus.
It provides a more accurate representation of the relationships between elements and their properties.
The modern periodic law states that “the chemical properties of elements are a periodic function of their atomic numbers”. This means that the chemical properties of elements are determined by their atomic numbers which are equal to the number of electrons those elements contain.
Periodic table is a table in which elements are arranged in order of increasing of atomic number. Periodicity is the repetition of the properties of elements after a certain intervals when elements are arranged in order of increasing atomic numbers.
The modern periodic table is divided into groups and periods. Groups are the vertical columns of elements in the periodic table.
They are usually written by Roman numerals.
There are eight (8) groups in the modern periodic table.
Some characteristics of elements in the same group:
Elements in the same group have the same number of electrons in their outermost shell (valence shell)
Elements in the same group have similar chemical properties because they have the same valence electrons.
Periods are the horizontal rows of elements in the periodic table.
They are indicated by English numerals.
There are 7 periods of elements in the modern periodic table.
Some characteristics of elements in the same period:
Elements within the same period have the same number of shells occupied by electrons.
Each period ends with a noble gas.
Groups→
I
II
III
IV
V
VI
VII
VIII
VIII/0
Period 1
H
He
Period 2
Li
Be
B
C
N
O
F
Ne
Period 3
Na
Mg
Al
Si
P
S
Cl
Ar
Period 4
K
Ca
The part of the Periodic Table showing the first 20 elements.
Some groups have special names and so their elements.
Group I elements are called Alkali metals.
Group II elements are called Alkaline Earth metals.
Group VII elements are called Halogens.
Group VIII /0 are called Noble gases (or inert elements).
Generally, group I-III are metals while group V-VIII are non-metals.
ATOMIC STRUCTURE AND PERIODIC TABLE.
Group number of elements is the same as number of electrons in their outermost shells. Therefore elements in the same group have the same number of electrons in their outer shell.
Group I
K 2:8:8:1 group I
Na 2:8:1 group 1
Group II
Ca 2:8:8:2 They have 2 electrons in their outer shell. They are in group II.
Mg 2:8:2
The number of shells an element has represents the Period in which that element occurs in the periodic table.
Element
Electronic configuration
Period
Li
2:1
Period 2
K
2:8:8:1
Period 4
Na
2:8:1
Period 3
GENERAL PERIODIC TRENDS
Atomic size
The size of an atom usually is measured by using its radius.
Atomic radius is the distance from the nucleus of an atom to the outermost shell.
Atomic size or radius decreases from left to right of the period.
On moving down a group in the periodic Table, the atomic size increases.
Ionization energy
Ionization energy is the minimum amount of energy required to remove completely electron from an atom.
The ionization energy increases from left to right across the periods.
The ionization energy decreases from top to the bottom of the periodic table.
Metallic character (Electropositivity).
This is the tendency of an atom to lose electrons and form positive ions in a chemical reaction.
It is a measure how easily an atom can donate its valence electrons to other atoms. Metal atoms have this tendency of donating electrons.
Electropositivity decreases from left to right in the periods and down the group the electropositivity increases.
Electronegativity (non-metallic character)
This is the tendency of an atom to attract the electrons towards itself in a chemical bond.
Moving from left to right across a period, the electronegativity of the elements generally increases. The element with highest electronegativity is Fluorine.
The electronegativity decreases down the group.
Electron affinity
E.A is the amount of energy released or spent when an electron is added to the atom to form a negative ion. It is a measure of the attraction an atom has for an additional electron.
The electron affinity increases from left to right across the period in the Periodic Table. Electron affinity decreases down the group.
Melting point and boiling points
Generally, melting and boiling points increase from left to right across a period and increase down the group.
Note that the MP& BP are varied due to molecular complexity, crystal structure and presence of specific bonds.
SPECIFIC TRENDS IN THE GROUPS
Group I – Alkali metals
They are called alkali metals because they produce hydroxide which are soluble in water.
They are very soft and can be easily cut with a knife.
They have low density.
They are good conductors of heat and electricity.
They have low melting and boiling points compared to other metals.
Chemical properties
They are the most reactive metals and usually stored under the oil to prevent them coming into contact with water or vapour. Some of chemical properties are:
React with oxygen or air with a characteristic flame colour forming white solid oxide
Metal + oxygen → metal oxide
These metal oxides dissolve in water to form alkaline solutions
Metal oxide + water → metal hydroxide.
React vigorously with water to give the alkaline solution and hydrogen gas
Metal + water →metal hydroxide + hydrogen
Reacts vigorously with halogens to form metal halides.
Eg. Sodium + chlorine →sodium chloride.
Group II-Alkaline Earth Metals
They are called alkaline earth metals because they are the only metals found beneath the earth which are alkaline.
Physical properties
They are harder than those in group I
They are silvery grey in colour pure and clean. They tarnish quickly when exposed to air.
They are good conductors of heat and electricity.
Chemical properties
Alkaline earth metals react similar to alkali metals except that they are less reactive than group I or alkali metals. Most alkaline earth metals are commonly used in the school chemistry laboratories example magnesium.
React with oxygen to form solid white metal oxide.
Metal + oxygen → metal oxide
React with water to give metal hydroxide and hydrogen gas.