Structure Of Atom

The heaviest fundamental particle is neutron. The mass of electron is negligible. The mass of proton is slightly less than that of neutron.

The deflection of the alpha particle (positively charged particle) is due to the force of repulsion. This indicates that the atom also contains positive charge. We know that the like charges repel each other. Hence, the alpha particles get deflected from their path.

The simplest atom that does not contain neutron is hydrogen (protium). The mass number (atomic mass) of hydrogen is 1 and no. of proton is 1.

Hence, no. of neutron = Mass number – No. of proton

No. of neutron = 1-1 = 0

Thus, the hydrogen atom does not contain neutron.

The reason for the existence of isotopes is change in neutron number. For example:

Mass number (atomic mass)

Explanation: Mass number: The total number of protons and neutrons present in the nucleus of an atom is known as mass number. It is represented by the symbol A.

nucleus

Explanation: Rutherford proposed that the mass of an atom is concentrated in the nucleus. The volume of nucleus is very small as compared to the volume of an atom.

neutral

Explanation: The particle of an atom that has no charge is neutral. In an atom, the number of electrons outside the nucleus (circular orbits) is equal to the number of positively charged particles in the nucleus. Hence the atom is neutral.

neutron

Explanation: There are three fundamental particles which are protons, electrons and neutrons. Among them, the fundamental particle that is not present in a normal hydrogen atom is neutron.

12

Explanation: Given: Atomic mass = 23

Atomic number = 11

Number of neutrons = atomic mass – atomic number

Number of neutrons = 23 -11

Number of neutrons = 12

Hence, the atom has 12 neutrons.

Postulates of Dalton’s atomic theory are:

i. Elements are made up of very small indivisible particles called atoms.

ii. All atoms of a particular element are identical in shape.

iii. The properties of atoms of a particular element are different from the atoms of other elements.

iv. An atom can neither be created nor destroyed.

v. Atoms of one element cannot be converted into those of another element.

J.J. Thomson’s experiment on the discovery of electrons:

i. J.J Thomson conducted the experiment using discharge tube apparatus.

ii. A discharge tube is glass tube connected to a vacuum pump.

iii. Metal electrodes are fixed to the ends of the glass tube.

iv. He created a very low pressure inside the discharge tube and applied high voltage.

v. The rays released from cathode are called cathode rays.

vi. He named these particles as electrons (negatively charged particles).

vii. The electron is much lighter than the hydrogen atom.

The characteristics of anode rays are:

i. Anode rays contain positively charged particles and are also called positive rays.

ii. Anode rays travel in the direction of opposite to that of the cathode rays.

iii. The path of the anode rays deflected towards cathode.

iv. The deflection of anode rays in an electric field was very little when compared to the deflection of cathode rays.

Rutherford’s model of an atom:

i. He conducted an experiment in which he directed high speed positively charged particles called alpha particles towards a thin gold foil.

ii. He observed that most of the alpha particles passed through the foil without the deflection.

iii. A few them were deflected at very small angles.

iv. Few were turned back on their path.

Neils Bohr’s atomic model:

i. Electrons revolve around the nucleus in certain circular orbits (shells)

ii. The first shell nearest to the nucleus is called “K shell”. The subsequent shells as we move away from the nucleus are L, M, N,O respectively.

iii. The energy of an electron remains same as long as it revolves around its own shell.

iv. The shells also represent energy levels.

v. He proposed that each shell can accommodate a maximum of 2n² electrons (n= shell number)

The properties of cathode rays:

i. Cathode rays contain negatively charged particles and are also called negative rays.

ii. Cathode rays travel in the direction of opposite to that of the anode rays.

iii. The path of the cathode rays deflected towards anode.

iv. The cathode rays are named as electrons.

v. These electrons are lighter than hydrogen atom.

Rutherford proposed that atomic nucleus is positively charged.

i. On the basis of Rutherford model atom, he assumed that positively charged particles of an atom are held together in the centre of atom.

ii. It is called nucleus. Nucleus is surrounded by electrons.

iii. The volume is nucleus is small when compared to the volume of an atom.

iv. The nucleus is positively charged which is different in magnitude for different elements.

The conclusions drawn by alpha ray scattering experiment of Rutherford are:

i. Most of the space inside the atom is empty because most of the alpha-particles passed without getting deflected.

ii. Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.

iii. A very small fraction of alpha-particles were deflected by 180^°, this indicates that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.

In an atom, the number of electrons outside the nucleus is equal to the number of positively charged particles in the nucleus. Hence the atom is neutral (no charge).

The properties of three fundamental particles are:

i. Electrons: Electrons are negatively charged particles. They have a negligible mass. These are much lighter than hydrogen atom. Electrons revolve in a circular orbit around the nucleus.

ii. Protons: Protons are positively charged particles. It remains inside the nucleus. Their mass is slightly less than that of neutrons.

iii. Neutrons: Neutrons has no charge and a mass nearly equal to that of proton. These are present in the nucleus of all atoms, except hydrogen. These are the heaviest fundamental particle.

Atomic number: The number of protons present in the nucleus of an atom is known as atomic number. It represents the number of electrons in the orbits. It is denoted by Z.

Mass number: The total number of protons and neutrons present in the nucleus of an atom is known as mass number. It is represented by the symbol A.

Isotope: Atoms of an element with different mass numbers are called isotopes.

Number of protons + Number of neutrons = Mass number

For example: Hydrogen has three isotopes:

For potassium:

1. Mass number of potassium is 40.

2. Atomic number of potassium is 19.

3. Distribution of electrons in an atom of potassium is 2,8,8,1

1-f, 2-d, 3-a, 4-e

1. Protons are positively charged particles. It is represented by the symbol “p”. Their mass is slightly less than that of neutron. They remain inside the nucleus.

2. L shell contains 8 electrons. As we know that every shell has 2n² electrons. For L shell (n=2):

2n² = 2× 2× 2

8 electrons

3. The mass of an electron is considered to be negligible.

4. Neutrons are the atomic particles which have no charge. They are electrically neutral. Their mass is nearly equal to that of proton. These are present inside the all atoms except hydrogen.