[Chemistry Class Notes] on Structure of Atom Pdf for Exam

It should be noted that the structure of an atom has protons, neutrons and electrons present in it. These basic components can be consumed from the mass and charge of the atoms. The nucleus comprises proton and neutron, with the electron orbiting around that.

Introduction to Structure of an Atom

Atoms

Atoms are the building blocks of matter. It is the littlest unit of matter that’s composed of three subatomic particles: the proton, the neutron, and therefore the electron.

Cathode Ray Experiment

• J. J. Thomson discovered the existence of electrons.

• A beam tube was used by the scientist for the experiment which may be a vacuum-sealed tube with a cathode and anode at one end that created a beam of electrons moving towards the end of the tube.

• The characteristics of beams (electrons) don’t depend on the fabric of electrodes and therefore the nature of the gas present within the cathode ray tube.

• The experiment showed that the atom wasn’t an easy , indivisible particle and contained a minimum of one elementary particle – the electron.

Electrons

• Electrons are the charged subatomic particles of an atom.

• The mass of an electron is considered to be negligible, and its charge is -1.

• The symbol for an electron is e¯.

• Electrons are extremely small.

• They are found outside the nucleus.

Thomson’s Model of an Atom

• According to Thomson,(i) An atom consists of a charged sphere, and therefore the electrons are embedded in it. (ii) In magnitude, the negative and positive charges are equal. So, the atom as an entire is electrically neutral.

• The first model of an atom to be suggested and brought into consideration.

• He proposed a model of the atom almost like that of a Christmas pudding/watermelon.

• The red edible part of the watermelon is compared with the charge within the atom.

• The black seeds in the watermelon are compared with the electrons which are embedded on it.

Radioactivity

• Radioactivity is the term for the method by which an unstable nucleus of an atom loses energy by giving out particles.

• It does so by giving out particles like alpha and beta particles.

• This process is spontaneous.

• An atom is unstable if the nucleus is disbalanced which means a difference in the protons and neutrons.

Rutherford Model

Rutherford’s experiment and observations

• In this experiment, fast-moving alpha (α)-particles were made to fall on a skinny foil . His observations were:

• A major fraction of the α-particles bombarded towards the gold sheet skilled it with no deflection, and hence most of the space in an atom is empty.

• The charge during an atom is concentrated in a very small volume.

• Very few of the α-particles were deflected back, that’s only a couple of α-particles had nearly 180o angle of deflection. So the volume occupied by the charged particles in an atom is extremely small as compared to the entire volume of an atom.

Rutherford’s Model of An Atom

Rutherford came to a conclusion that the model of the atom from the α-particle scattering experiment as:

(i)  There’s a charged centre in an atom called the nucleus. Nearly all the mass of an atom resides within the nucleus.

(ii) The electrons revolve around the nucleus in well-defined orbits.

(iii) The dimensions of the nucleus is extremely small as compared to the dimensions of the atom.

Drawbacks of Rutherford’s Model

• He explained that the electrons in an atom revolve round the nucleus in well-defined orbits. Particles in a circular orbit would experience acceleration.

• Thus, the revolving electron would lose energy and eventually fall under the nucleus.

• But this cannot happen because the atom would be unstable and matter wouldn’t exist within the form we all know.

Be More Curious!!!

• Robert A performed the experiment  of The Millikan’s Oil Drop Experiment. In the experiment, Millikan allowed charged tiny oil droplets to undergo a hole into an electrical field.

• By varying the strength of the electric field, the charge over an oil droplet was calculated, which always came as an integral value of ‘e.’

• The conclusion comes that the charge is said to be quantized. 

Neil Bohr Model

Properties of Electrons, Protons, and Neutrons

Bohr’s Model of an atom

Bohr came up with these postulates to overcome the objections raised against Rutherford’s model:

• Electrons revolve around the nucleus in stable orbits without emission of energy. Each orbit features a definite energy and is named an energy shell or energy state.

• An orbit or energy state is designated as K, L, M, N shells. When the electron is within the lowest energy state, it’s said to be within the state.

• An electron emits or absorbs energy when it jumps from one orbit or energy state to a different.

• When it jumps from a higher energy level to lower energy level, it emits energy while it absorbs energy when it jumps from lower energy level to higher energy level.

Orbits

Orbits are energy shells surrounding the nucleus during which electrons revolve.

Electron Distribution in Different Orbits

The distribution was suggested by Bohr and Bury;

• The maximum number of electrons present during a shell is given by the formula 2n2, where ‘n’ is the orbit number or energy state index, 1,2,3,….

• The maximum number of electrons in s
  everal shells are as follows: the primary orbit will have 2*12=2, the second orbit will have 2*2Msup > 2 = 8, the third orbit will have 2*32=18, fourth orbit 2*42 = 32 and so on.

• The shells are always filled during a stepwise manner from the lower to higher energy levels. Electrons aren’t filled within the next shell unless previous shells are filled.

Valency

• The electrons present within the outermost shell of an atom are referred to as the valence electrons.

• The combining capacity of the atoms or their tendency to react and form molecules with atoms of an equivalent or different elements is understood as valency of the atom.

• Atoms of elements, having a totally filled outermost shell show little chemical activity.

• Their combining capacity or valency is zero.

• For example, we know that the number of electrons in the outermost shell of hydrogen is 1, and in magnesium, it is 2.

• Therefore the valency of hydrogen is 1 because it can easily lose 1 electron and become stable.

• On the opposite hand, that of magnesium is 2 because it can lose 2 electrons easily and also attain stability.

Atomic Number

The number of protons found within the nucleus of an atom is termed because the number. It is denoted by the letter ‘Z’.

Mass Number and Representation of an Atom

Protons and neutrons are present in the nucleus, so the mass number is the total of these protons and neutrons.

Isotopes and Isobars

Isotopes are defined because the atoms of an equivalent element, having an equivalent number (number of protons) but different mass numbers (number of protons+neutrons).

For example: within the case of Hydrogen we have:

Atoms of various elements with different atomic numbers, which have an equivalent nucleon number , are referred to as isobars.

For example, Calcium and Argon: both have an equivalent nucleon number – 40

20Ca40 and 18Ar40.

Calculation of Mass Number for Isotopic Elements

When a component has an isotope, the nucleon numbers are often calculated by the various proportions it exists in.

For example take 98% Carbon-12u and 2% Carbon-13u.

This doesn’t mean that any Carbon atoms exist with the nucleon number of 12.02u. If you’re taking a particular amount of Carbon, it’ll contain both isotopes of Carbon, and therefore the average mass is 12.02 u.