NEET Foundation Chemistry Notes For Chapter 3 Atoms And Molecules About Atom
An atom is the smallest unit of matter which has the chemical properties of the element. An atom has number of constituents which may vary according to the different elements. It means that atoms of different elements will be of different size.
The earlier thinkers considered atom to be the smallest reachable unit which cannot be broken down further. But, it is now a basic knowledge that atom can be broken into its constituents but these constituents do not have the chemical properties of the atom.
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Size of the Atom
The atom is the smallest unit of the element. It is now known that atoms of different elements vary not only in mass but also in size. But, there is still no consensus on the shape of the atom. But as a general rule the shape of an atom is considered to be spherical. That is why the radius of the atom is discussed. The size of atom is measured in nanometre.
The magnified image of the atoms on the surface of the elements can be seen with the new technique called scanning tunneling microscopy (STM).
As we know, atoms are made up of three particles: protons, neutrons and electrons. Protons and neutrons are heavier than electrons and reside in the center of the atom, which is called the nucleus. Electrons are extremely lightweight and exist in a cloud orbiting the nucleus. The electron cloud has a radius 10,000 times greater than the nucleus.
Protons and neutrons have approximately the same mass. However, one proton weighs more than 1,800 electrons. Atoms always have an equal number of protons and electrons, and the number of protons and neutrons is usually the same as well. Adding a proton to an atom makes a new element, while adding a neutron makes an isotope, or heavier version, of that atom.
Atomic Number and Atomic Mass
Protons exist in the nucleus of an atom and the number of protons of an atom constitutes the atomic number. Its symbol representation is done by ’z’. Similar atomic number is possessed by all atoms of an element. Elements are defined by the number of protons they possess. So the total number of protons present in the nucleus of an atom is called atomic number.
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Example: For hydrogen, z = 1, because in hydrogen atom, only one proton is present in the nucleus. Similarly, for carbon, z = 6.
On observation of properties of the subatomic particles of an atom, it can be concluded that the mass of an atom is due to neutrons and protons. They are found in nucleus of the atom hence called nucleons. So, the mass of an atom resides in its nucleus. The amount of the total number of protons and neutrons present in the nucleus of an atom is called atomic mass.
Example: Mass of carbon is 12 u because it has 6 protons and 6 neutrons, 6 u + 6 u = 12 u. Similarly, the mass of aluminium is 27 u (13 protons + 14 neutrons).
Atomic masses of a few elements
Valency
Electrons exist in different orbits and the electrons present in the outermost orbit are known as valence electrons. The outmost shells can contain maximum of 8 electrons as per Bohr and Bury. It was noticed in an atom with completely filled outermost shell show little chemical activity meaning their combining capacity or valency is zero.
The only exception to this rule is helium. It has two electrons in its outermost shell. Except this all the inert elements have 8 electrons in the outermost orbit.
So, valency is a tendency in an atom to achieve the state of stability by achieving 8 electrons in its outermost shell by receiving, leaving or sharing of electrons from other atoms. It simply means that if an atom has 1 electron in its last orbit it will have a tendency to lose this electron for achieving stability. So it will have valency of
1. On the other hand if an atom has 6 electrons in its last orbit it should be easier for this atom to take 2 electrons for the other atom instead of leaving 6 electrons. So, the valency will be 2.
Example: CO2 is made up of 1 carbon and 2 oxygen atoms. Valency of carbon is 4 while valency of oxygen is 2. So, to be stable 1 carbon atom will have to combine with 2 oxygen atoms to form 1 molecule of carbon dioxide.