Atomic Structure

1. All the matter is made up of molecules. The molecules are combination of atoms of different elements or the same element. Thus, atom is the smallest constituent particle of matter.

2. Atom has an average diameter of 0.0000000001m or 1×10^-9 m

3. Dalton’s atomic theory

◦ John Dalton proposed the atomic theory in the year 1808.

◦ He proposed that matter consists of very small particles which he named atoms.

◦ An atom is the smallest indivisible particle, it is spherical in shape.

Drawbacks

◦ His theory does not propose anything about the positive and negative charges of an atom.

4. J.J. Thomson’s theory

◦ Thomson compared an atom to a watermelon.

◦ His theory proposed that the atom has positively charged part like the red part of the watermelon and in it are embedded, like the seeds, negatively charged particles which he called electrons.

◦ According to this theory as the positive and negative charges are equal, the atom as a whole does not have any resultant charge.

◦ Thomson’s greatest contribution was to prove by experimentation the existence of the negatively charged particles or electrons in an atom. For this discovery, he was awarded the Nobel Prize.

Drawbacks

◦ On further discovery, it was found that an atom is not like a watermelon.

5. Rutherford’s theory

Earnest Rutherford’s experiment and observations

▪ Set up: He bombarded a very thin layer of gold with positively charged alpha rays.

▪ He found that most of these rays which travel at a great velocity passed through the gold sheet without encountering any obstacles.

▪ A few are, however, turned back from the sheet. Rutherford considered this remarkable and miraculous as if a bullet had turned back after colliding with tissue paper.

Conclusions drawn from the experiment.

◦ The fact that most alpha particles pass through the gold sheet means that the atom consists mainly of empty space.

◦ The part from which the positively charged particles are turned back is positively charged but very small in size as compared to the empty space.

◦ For this theory, he was awarded the Nobel Prize for chemistry.

Rutherford’s theory proposes that

◦ The nucleus at the centre of the atom has the positive charge. Most of the mass of the atom is concentrated in the nucleus.

◦ The negatively charged electrons revolve around the nucleus in specific orbits.

◦ In comparison with the size of the atom, the nucleus is very very small

6. Particles that make up the atom are called Subatomic Particles. The three main sub-atomic particles are: electron, neutron and proton.

◦ Proton (p)

The proton is the positively charged particle and it’s located in the nucleus. Its positive charge is of the same magnitude as that of the electron’s negative charge.

◦ Neutron (n)

Neutron is inside the nucleus. The neutron does not have any charge. Excepting hydrogen (protium), the nuclei of all atoms contain neutrons.

◦ Electron (e)

This is a negatively charged particle. Electrons revolve around the nucleus of the atom in specific orbits.

7. Characteristics of the sub-atomic particles:

◦ Protons and Neutrons are the two types of particles in the nucleus of an atom. They are called nucleons.

◦ The total negative charge of all an electrons outside the nucleus is equal to the total positive charge in the nucleus. That makes the atom electrically neutral.

◦ The mass of an electron is negligible as compared to that of a proton or neutron. Hence, the mass of an atom depends on the number of protons and neutrons in the nucleus

8. Atomic number (Z)

◦ The number of electrons or protons in an atom is called the atomic number of that atom.

◦ It is represented by the letter Z.

◦ Thus, if we know the atomic number of an atom, we know the number of electrons or protons in it.

9. Mass number (A) is equal to the sum of the number of protons (p) and neutrons (n) in the nucleus.

Atomic mass or mass number = Number of Protons + Number of Neutrons

A = p+n

10. When writing the symbol of an element, its atomic number (as subscript) and atomic mass (as superscript) number are also written. For example, the symbols of hydrogen, carbon and oxygen are written as ₁H¹ (atomin number – 1, atomic mass -1), ₆C¹² (atomic number – 6, atomic mass – 12), ₈O¹⁶ (atomic number – 8, atomic mass – 16) respectively.

11. Isotopes

◦ Atoms of the same element can have different number of neutrons. Such atoms will have same atomic number but different mass numbers. These atoms are called isotopes.

◦ For example, Hydrogen has three isotopes: Hydrogen (₁H¹), Deuterium (₁H²), and Tritium (₁H³).

12. Isobars

◦ Atoms that have the same mass number but different atomic numbers.

◦ For example: Calcium – 40 and Argon – 40

13. Isotones

◦ Atoms that have the same number of neutons but different number of protons.

14. Valency

▪ It is defined as the combining capacity of an element.

▪ Atoms of different elements combine with each other to form molecules.

▪ Valency determines the number of atoms of an element that combines with atom or atoms of another type.

▪ It is a measure of how many hydrogen atoms an atom can combine with.

15. For example: Oxygen can combine with two hydrogen atoms and create water molecule, the valency of oxygen atom is two.

16. Classification of elements on the basis of its valency.

◦ The element having valency one is called monovalent. For example: Hydrogen and Sodium.

◦ The elements having valency two are called divalent. For example: Oxygen and Beryllium.

◦ The elements having valency three are called trivalent. For example: Nitrogen and Aluminium.

◦ Some elements exhibits more than one valency. For example: Iron combines with oxygen to form two types ferrous oxide (exhibits valency 2) and ferric oxide (exhibits valency 3).

17. When atoms of different elements combine with each other than molecules of compounds are formed.