STRUCTURE OF ATOM
Atoms are the basic units of matter and the defining structure of elements. The term "atom" comes from the Greek word for indivisible, because it was once thought that atoms were the smallest things in the universe and could not be divided.
Where it all started?
Atoms were created after the Big Bang 13.7 billion years ago. As the hot, dense new universe cooled, conditions became suitable for quarks and electrons to form. Quarks came together to form protons and neutrons, and these particles combined into nuclei.
Now there are pieces of molecules that are smaller than atoms, these tiny sub-atomic molecules lie within the atom, these are what quarks are.
Who discovered them?
John Dalton first found out the existence of the atom as a fundamental particle, Robert Brown discovered that atoms exhibit a particular random zig-zag motion, JJ Thompson proved the existence of the electron by measuring the mass of cathode rays, Ernest Rutherford discovered the nucleus through the gold foil experiment, following which James Chadwick discovered the neutron. These were some of the earliest speculations about the structure of atom.
What they had to say about atoms?
Since each one of them had their very own opinions on what atoms looked and behaved like, they came up with postulates and laws to prove themselves right, some of them are
Dalton’s Postulates of Atomic Theory:
1)All matter is made of very tiny particles called atoms.
2)Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
3)Atoms of a given element are identical in mass and chemical properties.
4)Atoms of different elements have different masses and chemical properties.
5)Atoms combine in the ratio of small whole numbers to form compounds.
6)The relative number and kinds of atoms are constant in a given compound.
J.J.Thomson, in 1898, proposed that an atom possesses a spherical shape radius) approximately 10−10m) in which the positive charge is uniformly distributed. According to Thomson, atom is like watermelon and electrons are embedded like seeds in watermelon.The positive charge is distributed like
fibrous material of water melon. An important feature of this model is that the mass of the atom is assumed to be uniformly distributed over the atom. It cannot explain electrical neutrality of the atom.
Rutherford's Model of an Atom
Rutherford proposed atomic model based on α - ray scattering experiment. Scattering of a narrow beam of α - particles as they passed through a thin gold foil and it is covered with fluorescent ZnS screen. When α -particles struck the screen, then flash of light was produced at that point.
Bohr’s Model of Structure of Atom:
Neils Bohr proposed the Bohr’s model of structure of atom, wherein he assumed the electrons to orbit around the nucleus in a fixed set of orbits, and could change orbits by absorbing or emitting a photon in the form of electromagnetic radiation. Following are the postulates of the Bohr model:-
1. The nucleus in the atom is orbited by electrons.
2. Electrons can only orbit stably, without radiating, in certain orbits at certain discrete sets of distances from the nucleus. These orbits have definite energies and are also called energy shells or levels. In these orbits, the electron’s acceleration does not result in radiation and energy loss as required by classical electromagnetics.
Some of the shortcomings of Bohr’s model is as follows:-
1. It failed to explain much of the larger atom’s spectra.
2. It fails to explain the Zeeman effect – the changes in spectral lines due to external magnetic fields
Charges of Atoms
You can see that each part of the atom is labeled with a "+", "-", or a "0." Those symbols refer to the charge of the particle. The electron always has a "-", or negative, charge. The proton always has a "+", or positive, charge. If the charge of an entire atom is "0", or neutral, there are equal numbers of positive and negative charges. Neutral atoms have equal numbers of electrons and protons. The third particle is the neutron.
Atomic mass:Atomic mass of an atom is the sum of the mass of protons and neutrons. The mass number is denoted by A.
Relative atomic mass:Relative atomic mass of the atom of an element is defined as the average mass of the atom, as compared to 1/12th the mass of one carbon-12 atom.
Molecular mass: The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. It is therefore the relative mass of molecule expressed in atomic mass units (u).
This was a basic introduction to how atoms came into existence, and how scientists discovered them and derived postulates, theories and vital facts like atomic mass and charge to explain their functioning