Atoms: Building Blocks of Matter

An atom is the smallest particle of an element. It has two parts: the nucleus and the outer nuclear part. The nucleus of the atom consists of protons and neutrons. Protons are positively charged particles. Neutrons have no charge.

In the outer nuclear part, electrons are present. Electrons are negatively charged particles. The atomic number represents the number of protons in the atom. As an atom is electrically neutral, it may contain as many electrons as it has protons. The total number of protons and electrons present in one atom of an element is known as its mass number.

• Atoms of a given atomic number can have different numbers of neutrons; these are called isotopes.
• The atoms of different elements that have the same mass number but different atomic numbers are called isobars. These have different numbers of protons but equal sums of the number of protons and neutrons.
• The atoms of different elements that have the same number of neutrons but different atomic numbers are called isomers.

Atomic radius is measured in nanometres. It is a typical distance from the centre of the nucleus to the boundary of the surrounding shells of electrons.

• `"1"/(10^9)` m = 1 nm
• 1 m = 10⁹ nm

The atomic size depends on the number of electron orbits in the atom. The greater the number of orbits, the larger the size. For example, an atom of K is bigger than an atom of Na. If two atoms have the same outermost orbit, then the atom having the larger number of electrons in the outermost orbit is smaller than the one having fewer electrons in the same outermost orbit. For example, an atom of Mg is smaller than an atom of Na.

The discovery of the electron and proton is credited to J.J. Thomson and E. Goldstein, respectively. J.J. Thomson proposed that electrons are embedded in a positive sphere

• Electron was represented as ‘e-‘ and proton as ‘p+’. The mass of a proton is taken as one unit and its charge as plus one, where the mass of an electron was considered to be negligible and its charge minus one
• It seemed that atom consisted of electrons and protons, which balanced their charges mutually

Structure of an Atom:

Dalton’s atomic theory suggested that the atom was indivisible and indestructible. But the discovery of two fundamental particles (electrons and protons) inside the atom led to the failure of this aspect of Dalton’s atomic theory. The arrangement of electrons and protons within an atom became crucial to understand. Many scientists proposed various atomic models to explain this. Thomson proposed the first atom structure model.

Field ion microscope image of iridium atoms (every spot in this image is an atom)

Atoms are very tiny. Modern instruments like the electron microscope, field ion microscope, and scanning tunnelling microscope produce enlarged images of the atom. Examine the image of an atom that a field ion microscope has produced.