Structure of Atom

1. The atom of an element is the smallest particle which can exist.
2. The structure of an element decides the chemical properties of the element.
3. An atom consists of nucleus at the centre and the number of electrons orbiting around the nucleus.
4. The nucleus of the atom consists of protons and neutrons which are tightly bound together and are called nucleons.
5. Protons are positively charged, electrons are negatively charged and the neutrons are chargeless.
6. The positive electric charge on each proton is equal to 1.602 x 1010^-19columb and negatively charged electrons have equal charge as on protons.
7. The number of protons are equal to the number of electrons, thus the electrical charge in a neutral atom is zero.
8. Any addition or subtraction of electrons from neutral atom makes it negatively and positively charged atom respectively.
9. Two nuclei of a particular element may have same number of protons but different number of neutrons.
10. Such nuclei are called isotopes of an element. Nuclei having same number of protons will have similar physical and chemical properties.
11. The number of protons in the nucleus is called the atomic number, Z of an element or atom.
12. The sum of the numbers of protons and neutrons is called the mass number, A of an atom. Thus an element is designated as a symbol of element)^A e.g. light hydrogen atom has proton, nil neutrons and I electron. Thus it’s atomic number Z = 1 and mass number, A = 1 + 0 = 1, hence it is designated as ₁H¹. Most of the mass of an atom is concentrated in the nucleus. The mass of particles protons, neutrons and electrons of an atom are as follows.

[Note that 1 atomic mass unit (amu) = 1.66 x 10^-27
kg]

Proton mass, m_p = 1.007277 amu = 1.673 x 10^-27

Neutron mass, m_n = 1.008665 amu = 1.674 x 10^-27
kg

Electron mass, m_e = 0.0005486 amu = 9.109 x 10^-31
kg

Hydrogen (₁H¹) has two isotopes as
follows :

(a) Heavy hydrogen (₁H²) having 1 proton, 1 neutron thus its atomic number, Z is 1 and mass number, A is (1 + 1 = 2). It is called deuterium (D).

(b) Tritium (T) has 1 proton and 2 neutrons is designated as ₁H³. Light hydrogen combines with oxygen to form light water (H.0) and heavy hydrogen reacts with oxygen to form heavy water (1,0). Tritium is radioactive and it exists in nature in traces only.

Natural uranium consists of three isotopes of uranium as ₉₂U²³⁸, ₉₂U²³⁵, and ₉₂U²³⁴. In natural uranium the availability of U²³⁸ is largest upto the extent of 99.28%, U²³⁵ is only 0.715% which is most unstable and the remainder 0.0006% is U²³⁴. Atomic number is 92 in cach of the above three cases.

Two other sub-particles are of importance known as positrons and neutrino.

Positrons are positively charged electrons having the symbol + ₁e⁰ , e⁺ or β⁺ while neutrino is a tiny neutral particle which is ejected along with β particles during nuclear fission. The symbol for electrons being -e⁰,e^–or
β^–.