Education material for 14 to 16 year-olds. The article looks at the electronic structure of both Alkali metals and Halogens, relating it to their positioning in the periodic table, and how this leads to ionic bonding between the atoms of both groups of elements. GCSE standard science.
When chemical reactions occur, there is a change to the electronic structure of the atoms involved.
- Some atoms gain electrons to form negatively charged ions
- Some atoms lose electrons to form positively charged ions
- Whilst other atoms share electrons
Once these chemical reactions have taken place the atoms and ions are usually more stable, making them less reactive.
We are going to look at the process involved in the ionic bonding of Alkali Metals to Halogens.
To do this we need to understand electronic structure and how this affects the stability of atoms.
We know that atoms in the noble gases, such as those of helium, neon and argon have a tendency not to react at all.
Why should this be?
Scientists have concluded that this is due to the electronic structures in individual atoms. So let us look at what is known about the structure of atoms.
All the elements in the periodic table are made up from three different particles
- Protons, which are positively charged particle and are situated in the nucleus
- Electrons, which are negatively charged particles, and orbit the nucleus at specific energy levels arranged in layers and known as shells
- And neutrons, non-charged neutral particles, which are also situated in the nucleus
Atoms always contain the same number of protons as electrons, therefore there is no net charge, and the overall charge on the atom is zero.
It is the number of these particles in the atom that give the elements in the periodic table their different characteristics. And it is also the electronic structure of these particles in an element that give individual groups within the periodic table similar characteristics.
One major characteristic of the noble gases is their stability, their tendency not to react. These elements have the most stable electronic structures.
Atoms are at their most stable when they have the same electronic structure as the noble gases. This occurs when:
- The first shell contains 2 electrons orbiting the nucleus (eg. Helium, which only contains two electrons and is the first of the noble gases).
- The second shell contains 8 electrons orbiting the nucleus (eg. Neon, which contains two electrons in its first shell and eight electrons in its second shell.).
- The third shell contains either 8 or 18 electrons orbiting the nucleus (eg Argon, which has the electronic structure 2,8,8)
Now, if we look at Group I of the periodic table, the alkali metals, we see that their electronic structures are also similar to each other
- Lithium has two electrons in its first shell and one electron in its second shell
- Sodium has two electrons in its first shell, eight in its second and one in its third
- Potassium has an electronic structure of 2,8,8,1
The alkali metals have one electron in their outer shell, which gets further away from the nucleus the further we move down the group in the periodic table, and the alkali metals also get more reactive as we move down the group.
These elements have a tendency to lose this outer electron in chemical reactions, creating an electronic structure that is similar to that of the noble gases. However, this loss of an electron from the atom leaves the atom with a net positive charge. Atoms that gain or lose electrons, which cause net charges, are called ions. And Alkali metals have a tendency to form positive ions with a net charge of 1+.
Hence Sodium: Na Na+ + an electron.
If we know look at group VII in the periodic table. These elements are known as the halogens (salt formers). Reactions involving halogens can also be related to their electronic structures. Halogens have 7 electrons in their outer-shell therefore, to adopt the electronic structure of a noble gas halogens have to gain an electron. When a halogen atom gains an electron it takes on a net negative charge to form a negative ion. Unlike the Alkali metals, halogens get less reactive as their relative atomic mass increases.
Hence Chlorine: Cl + an electron Cl-.
Ionic Bonding involves the transfer of electrons, with one atom donating electrons whilst the other atom adopts electrons. In the case of Alkali metals, each atom in the alkali metal donates the single electron in its outer shell, whereas in halogens there is one space in the outer to adopt an electron.
For example, in the formation of sodium chloride, the sodium atom donates an electron to become a sodium ion (Na+) and the Chlorine atom adopts this electron to become a chlorine ion (Cl-), opposite charges attract, and it is this attraction between the oppositely charged ions that forms the Ionic bond.