Introduction to atomic numbers

The atomic number is the number of protons in an atom, also called proton number. The atomic number is defined as the sum of the number of protons found in the nucleus of an atom of any element. It is the same as the charge number of the nucleus. Those atoms whose atomic number is the same but their number of neutrons are different, are called isotopes. Although there are many models which determine the structure of an atom, Rutherford’s atomic model and quantum mechanical model is considered to be the most accurate model while determining the structure of an atom. 

The First 20 Elements of the Periodic Table

The elements in the periodic table are arranged into increasing order of their atomic numbers. The list of the first 20 elements of the periodic table is shown below with their atomic number, name of element, and their symbol.

Rutherford atomic model

The Rutherford model is also known by other names, such as the nuclear model and planetary model of an atom. It was proposed by the physicist Ernest Ruderforn in 1911. The Rutherford model tells about the structure of an atom. The Rutherford model defines the structure of an atom as follows ‘An atom is a  tiny, dense, positively charged nucleus. And the major portion of the mass of an atom is situated in the nucleus. The electrons circulate the nucleus just like the planets revolve around the sun in fixed orbits or paths ’.

Rutherford’s alpha scattering experiment

This experiment was conducted by Ernest Rutherford to determine the structure of the atom. The experiment was done by bombarding alpha particles on a very thin sheet of gold foil. The trajectory of the path of light was studied to get the results. 

Observations of the Rutherford alpha ray scattering experiment

Through the experiment, Rutherford concluded some points that are mentioned below:

1. The major part of the alpha rays passed through the goal foil, which means there is much space in the atom. 
2. The rays deflected from the sheet at a very low angle led to the conclusion that positive charge is not uniformly distributed. Only some part of the atom contains a positive charge.
3. A few of the rays completely deflected back with an angle of nearly 180 degrees. It depicts that the volume occupied by the positive charge is very less than the total volume of the atom. 

So these were the observations. Now the points that Ruderford depicts from these observations are mentioned below. This model tried its best to describe the structure of an atom.  

1. The positive charge present in the atom is situated in a very less area than Rutherford, called the nucleus of the atom. 
2. The model proposed that the negative charge revolving around the nucleus is the electrons. It also depicted that electrons revolve around the nucleus in a circular path at very high speed.
3. The electrons are negative charges which are responsible for the electromagnetic force inside the atom. Due to this electromagnetic force, the atom is held together as both positive and negative charges balance out each other. 

Limitations of the Rutherford atomic model

1. According to Rutherford, the electrons orbit in a fixed path outside the nucleus. According to the Maxwell model, which was proposed before, the charged particles should emit electromagnetic radiations, and hence electrons should also emit radiations. Caring of energy leads to shrinking the size, which will ultimately lead to the collapse of the nucleus. According to some calculations of the Rutherford model, an electron revolving around the nucleus would collapse in the nucleus in less than 8-10 seconds. Therefore, Rutherford’s model couldn’t explain the stability of an atom. 
2. Another thing that Rutherford couldn’t explain in the model is the arrangement of electrons inside the atom which eventually made his theory incomplete.

Quantum mechanical model of atom

The quantum mechanical model of an atom is the result that was obtained from the Schrodinger wave equation.

The time-dependent Schrodinger wave equation is represented as follows: 

H = E

Here H is the hamiltonian of the atom

is the wave function of the atom

E is the energy of the atom

According to the Schrodinger wave equation, the probability of finding an electron in an atom is proportional to |ψ|2 where ψ  is corresponding to the wave function of an electron.

Features of the quantum mechanical model of the atom

1. The energy of an electron is quantized which means that energy is fixed and cannot vary. There is a specific value of energy that every electron holds. 
2. The quantized energy of an electron is the ultimate result of the Schrodinger wave equation.
3. According to Heisenberg’s uncertainty principle, the exact location and momentum of an electron are impossible to determine. That’s why they came up with the probability for its location. The probability is the highest chance of finding the electron at a particular point.
4. An atomic orbital is the wave function of an atom which is represented by (ψ). A single electron may have many wave functions, then there will be many atomic orbitals for an electron. Every wave function has a specific shape and energy associated with it. Inside the quantum mechanical model of an atom, the orbitals are named s, p, d, and f.

1. | ψ |2 is also called probability density and as it is square, therefore it is always positive.

Conclusion

The number of electrons or protons in an atom is known as its atomic number. Every element has a different atomic number associated with it.  Some elements have different numbers of electrons and protons. In that case, the atomic number is the number of protons.  It starts from 1 and goes till the number of elements present in the periodic table. To determine the structure of an atom, there were many theories described by some great scientists. Some of them are the Thompson model, Bohr model, Rutherford model, and quantum mechanical model. The Thompson model was the oldest one. The Rutherford model describes the atom as it has a positive charge in the centre and negative charges are revolving around it. Many corrections were made in the quantum mechanical model which describes the position of an electron inside the atom.