O Level Notes : Chemistry - Atomic Structure

The atom consists of a ‘nucleus’, and there are orbits around it called ‘shells’. You can understand the shape better if you think of it as a miniature solar system: the sun is the nucleus and the orbits on which the planets rotate are the shells. The nucleus of an atom consists of sub atomic particles called ‘neutrons’ and ‘protons’. The protons are positively charged and the neutrons have no charge, which means they are neutral. So in general, the nucleus is positively charged. The shells have subatomic particles called ‘electrons’ on them, which rotate around the nucleus, staying on their orbits (you can think of them as the planets). These electrons are negatively charged.

The charge on an overall atom is zero, which means that there are an equal number of protons and electrons present in an atom. The number of neutrons does not affect the charge on the atom, so there is no fixed trend as to how many neutrons each atom contains.

The following table shows some of the properties of the three subatomic particles:

Atomic Number:

The atomic number is the number by which each type of element is known. In all the atoms of a single element, the atomic number will be the same. Also, this atomic number will not be shared by an atom of any other type of element. This means that every element has a unique atomic number.

This atomic number is basically the number of protons in each element. For example, each atom of the element Hydrogen contains 1 proton, so Hydrogen has the atomic number ‘1’. This atomic number is shared by no other element, and neither does any element have only 1 proton. So basically, the number of protons of an atom decides what element it belongs to. All these numbers are given in the periodic table of elements. The atomic number is also called the proton number of an element. Its symbol is Z.

Nucleon Number:

The nucleon number of an element is the total number of protons and neutrons in its atom, i.e. the number of subatomic particles in its nucleus. As we saw earlier that both the particles neutron and proton have the relative mass of ‘1’, while the electron has negligible mass, therefore the sole mass of the atom depends on how many subatomic particles are present in the nucleus. We ignore the electrons when taking the mass of an atom. Therefore, the nucleon number is also the mass number of the element. Its symbol is A. This number for every element is also given in the periodic table.

If we subtract the nucleon number (A) from the proton number (Z), we can get the number of neutrons in each atom.

Isotopes:

Isotopes are atoms of the same element with different number of neutrons.

As I mentioned earlier, the ‘type’ of element depends on the number of protons it has. Therefore if the number of neutrons varies in the atoms of a single element, it does not mean that the atoms with the different nucleon numbers belong to different elements.

For example, let’s take Hydrogen. It has 1 proton and no neutron (nucleon number = 1), but it has 2 naturally occurring isotopes. These are ‘deuterium’, with 1 proton and 1 neutron, and tritium, with 1 proton and 2 neutrons. As you can see, the proton number does not change, so we can say that deuterium and tritium are actually the element hydrogen, but as their nucleon number is different, we call them ‘isotopes of hydrogen’. They are called this because their chemical properties (which depend on proton number) are the same as hydrogen, but their physical properties are slightly different.

Arrangement of Electrons in an Atom (The Electronic Configuration):

As we have already seen, the electrons revolve around the nucleus on orbits called shells. But how are these electrons arranged on these shells? They are divided between these shells in fixed quantities, that is, the first shell (which is always filled first) can hold a maximum of 2 electrons. The second, third and subsequent shells can generally hold up to 8 electrons. At this level, u need not worry about shells further than the third, but learn that all shells after the first tend to complete their ‘octet’, i.e. they each hold up to 8 electrons. E.g., let’s take Calcium (proton number 20). It has 20 electrons, so its ‘configuration’ (division of electrons) will be:

First shell: 2

Second shell: 8

Third shell: 8

Fourth shell: 2

This configuration can be written as: 2,8,8,2.

Fig 5.1. The electronic configuration of Calcium (2,8,8,2).

In the figure above, only the shells and the electrons are shown. The nucleus (not shown) lies in the middle.

You must learn the electronic configuration of the first 20 elements.

These configurations are very important, as the chemical properties of an element depend upon the number of valence electrons in its atoms. The valence shell is the outermost shell of an atom, and the electrons in this shell are known as the valence electrons.

Another importance of these configurations is that they are responsible for the arrangement of the elements in the periodic table. You will learn more about this in the chapter ‘The Periodic Table’, but just for a general idea you can learn that the elements in the periodic table are arranged according to their increasing proton number, i.e. 1, 2, 3, and so on. Also, the number of shells decides which period does an element belong to, while the number of valence electrons decide which group does it fall in.