Structure of an Atom

Atoms are made up of protons, electrons and neutrons. Electrons carry negative charge, protons have positive charge and neutrons are neutral. A natural question arises at this point: 

Question: How exactly are these entities arranged inside an atom?

We shall try to answer that question here, by going through the historical developments in the pursuit of this very same question.

The plum pudding model

This model was first put forth by JJ Thompson. Let’s look at what it said, then we’ll see if it was a feasible model.

Thomson proposed that:

 (i) An atom consists of a positively charged sphere and the electrons are embedded in it. 

(ii) The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

To picture this better, think of the atom as something like this:

Question: What did the plum pudding model successfully explain?

The model was able to explain the neutrality of the atom despite the presence of electrons and protons.

Question: What could the model not explain?

The model could not explain the results of Rutherford scattering Experiment, which is discussed below. 

Rutherford’s Scattering Experiment

To determine the structure of an atom, Rutherford devised a famous experiment, in which fast moving alpha (α)-particles were made to fall on a thin gold foil. It was expected that since alpha particles have very high energy, many small deflections will be seen, when they hit the sub atomic particles (electrons and protons).

Question: Why did Rutherford choose gold as the element for making the foil out of?

He selected a gold foil because he wanted as thin a layer as possible. This gold foil was about 1000 atoms thick.

But, the α-particle scattering experiment gave unexpected results:

1. Most of the fast moving α-particles passed straight through the gold foil. 

2.  Some of the α-particles were deflected by the foil by small angles.

3.  The most surprising result was, one out of every 12000 particles appeared to rebound. This is unexpected because alpha particles had huge momentum and the subatomic particles didn’t have the mass to turn them around.

Following is a pictorial depiction of what happened during the experiment.

Let us now try to frame a model of the atom based on Rutherford’s observation.

1. The first observation seems to suggest that most space inside the atom must be empty.

2. Since some of the positively charged alpha particles were deflected only by small angles, it means the positive charge must occupy a very small volume within the atom.

3. The rebound must have happened when alpha particles collided with the positive charge. Since this happened only in about once in 12000 particles, it reinforces the idea that all the positive charge is concentrated in a small region.

On the basis of his experiment, Rutherford put forward the nuclear model of an atom, which had the following features:

1. There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus. 

2.  The electrons revolve around the nucleus in circular paths. 

3.  The size of the nucleus is very small as compared to the size of the atom.

Question: What could the Rutherford model not explain?

Actually, any charge that goes in a circular path must undergo acceleration, due to centripetal force. It is known that an accelerating charge radiates energy. Hence, the electrons in the atom must constantly lose energy, and eventually fall into the nucleus. In other words, the atomic model proposed by Rutherford is unstable.

To explain the stability of atom, another scientist named Neils Bohr put forth another model, that builds up on Rutherford’s model, along with the following postulates:

1. Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.

2.  While revolving in discrete orbits the electrons do not radiate energy.

3. These shells/orbits are called energy levels.

Now that the structure of atom is clear, the next question that we must tackle is this:

Question: How are electrons distributed within the energy levels? Is there a pattern to it?

By observing the electronic configuration of several atoms, we can arrive at the following results:

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

2. The maximum number of electrons that can be accommodated in the outermost orbit is 8.

3. Electrons are not accommodated in a given shell, unless the inner shells are filled.

Properties of elements defined by their atomic structure

The explanation of some of the properties of an element comes from its atomic structure. For example, we had previously defined valency as “combining capacity of an element”. The atomic structure explains how the valency of an element may be found. Note that in the following discussion, we are only interested in the number of electrons in the outermost shell, even if we don’t mention it explicitly.

We know that maximum number of electrons in the outermost shell of an atom is 8. This is because 8 electrons in the outermost shell makes the element stable. Now, an atom has two options to get to eight electrons: it can either

1. Lose electrons(s) so that the outermost shell is empty, and the shell just inside it, with eight electrons, becomes the outermost shell. Or

2. Gain electrons so that the outermost shell is completely filled.

Question: What determines if an atom will choose option a, or b?

It’s quite simple – the atom chooses whichever is more convenient for it, in terms of energy. The convenient thing is always a smaller change in number of electrons. For example, an atom with 6 or 7 electrons will try to gain electrons to complete its octet, while an atom with one or two electrons will try to lose those electrons. This change in the number of electrons to attain a stable octet is what the valency of an element is.  

Some important terms

Atomic Number: The number of protons of an atom, which determines its atomic number. It is denoted by ‘Z’. It is a characteristic of each element and is unique for every element.

Mass Number: The total number of protons and neutrons in the nucleus of an atom is the mass number. It is denoted by ‘A’.

In scientific notation, an element is denoted as follows:

Isotopes: Elements which have the same atomic number, but different atomic masses. For example, protium, deuterium and tritium have atomic number as 1, but have atomic masses as one, two and three, respectively.

Isobars:  Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobars.

Summary 

• JJ Thomson proposed the plum pudding model of atom.

• Rutherford proposed the model with a tiny positively charged nucleus at the centre of the atom.

• Neils Bohr perfected Rutherford’s model by adding concept of energy levels.

• The stability of an atom depends on its ability to complete its octet.

• The number of electrons an atom loses or gains to complete its octet defines its valency.

• The atomic number of an element is the same as the number of protons in the nucleus of its atom.

• The mass number of an atom is equal to the number of nucleons in its nucleus. 

• Isotopes are atoms of the same element, which have different mass numbers.

•  Isobars are atoms having the same mass number but different atomic numbers.

•  Elements are defined by the number of protons they possess.