2.1 The nuclear atom

2.1 The nuclear atom

Thomson atomic model says that An atom is similar to a watermelon in which electrons are like watermelon seeds and the flesh is positive sphere. Thus electrons are present in the positive sphere of atom.

The over all atom is neutral because positive and negative charges are equal in number.

Rutherford’s nuclear model :

He proposed the following Atomic model:

Atom is mostly emplty or hollow except the dense center which is nucleus.

Nucleus has protons( positively charged particles) and Neutrons(Neutral particles).

Electrons ( Negatively charged particles) revolve around the nucleus in fixed circular orbits.

Niels Bohr discovery

He improved Rutherford’s nuclear model by suggesting that the electrons revolve in the discrete energy orbits and they do not loose energy while they are in the orbit. Once they move from one orbit to another they emit energy.

Niels Bohr who proposed the Bohr model of atomNiels Bohr

Niels Bohr 2nformula

Electrons are found by 2nformula in different shells where n=shell

So n=1 , 2 electron,

n=2, 8 electrons,

n=3, 18 electrons,

n=4, 32 electrons

He also suggested that outermost shell(orbit) cannot accommodate more than 8 electrons.

Energy of electron E= hv where h= plank’s constant  and v is frequency of electron.

C=vλ  where c= speed of light and λ  is wavelength . Thus frequency and wavelength are inversely proportional to each other.

What is an Atom?

An Atom is the smallest particle of an element which can take part in a chemical reaction to form a new substance.

Atomic size or atomic radius : Atom is very tiny which cannot be seen by naked eyes. However the size can be measured in nanometer.

1nm=1/ 109 m

Atomic radii of hydrogen atom = 1 × 10–10 m.

Sub atomic particles:  As per IB guidanceRelative masses and charges of the subatomic particles should be known, actual values are given in section 4 of the data booklet. The mass of the electron can be considered negligible. 

Protons-Positively charged(+1) particle- Inside the nucleus 

Neutrons – Neutral particle-Inside the nucleus

Electron- Negatively charged(-1) particle-revolve around the nucleus

The mass (nucleon) number, A, is the sum of the protons and the neutrons. (= 12 for Carbon)

The atomic (proton) number, Z, of an element is the number of protons in the nucleus of the atom. In the neutral atom it is equal to the number of electrons. ( 6 for Carbon)


 Atoms of same element with different number of neutrons or mass number.

Example: Isotopes of Hydrogen H1  , H2  , H3

Isotopes of Chlorine Cl35 and Cl37

Calculations involving non-integer relative atomic masses and abundance of isotopes from given data, including mass spectra.

If the % abundance of Isotopes of Chlorine Cl35 and Cl37 are 75% and 25% respectively. 

The average relative atomic mass= (35×75) + (37×25)/100= 35.5

Use of the nuclear symbol notation to deduce the number of protons, neutrons and electrons in atoms and ions. 

11Na23   is the nuclear symbol of Sodium where Z= 11 and A= 23. 

You have learned the subtopic 2.1 the nuclear atom by now and can solve questions. You can learn get chemistry IA ideas here.

You can learn about Rutherford’s model by using simulation.

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