The three subatomic particles in an atom are 

• Neutrons (which are neutral in charge).
• Electrons (which are negatively charged).
• Protons (which are positively charged).

An atom is always neutral, as it contains an equal number of electrons and protons. But, during a chemical reaction, an atom can gain electrons or lose electrons. 

When an atom loses its electrons to form a chemical bond, the number of protons outnumber the number of electrons, and hence, the atom becomes positively charged. This positively charged atom is called a cation. 

Similarly, an atom can gain electrons to form a chemical bond, which causes the number of electrons to outnumber the number of protons, and hence, the atom becomes negatively charged. This negatively charged atom is called an anion. 

To form a chemical compound, usually, a cationic atom reacts with an anionic atom. An atom always tries to acquire stable noble gas electronic configuration. To develop a noble gas configuration, an atom loses or gains electrons. And thus, they form a chemical bond or participate in a chemical reaction.

Electropositivity and Electronegativity:

• The tendency to lose or gain electrons to acquire noble gas configuration is valency. To form a chemical bond, an atom must satisfy its valency. The tendency of an atom to lose an electron is known as electropositivity. This type of behaviour is common in metals, and hence, it is also known as metallic character. The lesser the number of electrons required to lose by an atom, the greater will be its electropositivity as it will readily lose the electron to complete the octet. This is why alkali metals are the most electropositive elements in the whole periodic table.
• On the other hand, the tendency of an atom to gain an electron is known as electronegativity or electron affinity. Atoms gain electrons to complete their octet. The lesser the number of electrons required to complete the octet, the greater an atom’s tendency to attract electrons towards itself. Thus, greater is electronegativity. This is why halogens are the most electronegative element in the whole periodic table.

Generally, electropositive elements form an ionic bond with electronegative elements. An electropositive element gives electrons to electronegative elements, and this complete transfer of electrons creates an ionic bond.

For example, Sodium is an electropositive element that loses one electron to the chlorine atom, an electronegative element.

Trends of electropositivity and electronegativity in the periodic table:

• The modern periodic table comprises rows and columns in it. The vertical columns are called groups, while the horizontal rows are called periods.
• As we move down the group, the size of elements increases. The increase in size is because a new shell is being added, which makes the last shell electron farther from the nucleus, and hence, the nucleus is unable to pull the electrons, which causes easy removal of electrons. Hence down the group, electropositivity increases. 
• In the case of electronegativity, it works the opposite of electropositivity. Generally, due to the increase in the size of an atom, the last shell also becomes farther from the nucleus. Hence, it becomes difficult for the nucleus to attract the incoming electron. Therefore, down the group, the electronegativity decreases.
• In the case of periods, the size of elements decreases on moving from left to right along the row. This is because the number of shells does not increase along the period, and the electron is added to the same subshell. The attraction power of the nucleus increases as more protons are present in it. As the negative charge of electrons is at a fixed distance from the positive charge of protons in the nucleus, the outermost shell electrons are strongly pulled toward the center of the atom. This decrease in size results in a lesser energy requirement to attract electrons but a greater need for energy to remove or lose an electron. Hence, electropositivity decreases along with the group as the size of the atom becomes small, and it becomes difficult to remove the electron. 
• But in the case of electronegativity, it increases as we move from left to right along the period because the size reduces and the approach of the nucleus to the incoming electron also increases. This results in the easy gain of an electron.

Hence, most metals such as alkali metals and alkaline earth metals are electropositive, while non-metals are electronegative.

Further about elements, so alkali metals are the most electropositive because they have to lose only one electron. The loss of only one electron requires much less energy. Hence, their tendency to lose an electron is maximum. Another factor contributing to their high electropositivity is that they have a large size, and the outermost shell electron is way too far from the influence of the nucleus. This results in less attractive forces and easy electron ejection.

Halogens are the most electronegative elements in nature because they have to gain only one electron. The gain of only one electron requires less energy, and hence, their tendency to gain an electron (which is also known as electron affinity) is maximum. One another contributing factor is their size. Due to their small size, the influence of the nucleus on the incoming electron is strong and readily attracts the electron. 

Conclusion:

It can be concluded that the main factor responsible for an atom becoming electropositive or electronegative is its valency. The tendency of an element to lose electrons is known as electropositivity or metallic character. At the same time, the tendency of an atom to gain electrons is known as electronegativity or electron affinity. The size of an element affects the extent of electropositivity and electronegativity. Greater the size will be electropositivity but lesser electronegativity. Whereas the more deficient the size, the lesser the electropositivity, but greater the electronegativity.

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