Democritus was a Greek philosopher and is known as one of the first people to bring up the theory of the atom. His theory was that atoms were invisible particles and, alongside voids, made up matter.
He also said that they were indestructible, homogeneous ("of the same kind; alike") and unchangeable. Democritus' atomic model stated all atoms had different sizes, shapes, mass, positions and arrangements, with a void existing between them and that the atom was solid.
Democritus was heavily inspired by the teachings of two Greek philosophers that came before him, Leucippus and Anaxagoras. He developed his ideas on reasoning rather than science. For example,
" Democritus knew that if you took a stone and cut it in half, each half had the same properties as the original stone. He reasoned that if you continued to cut the stone into smaller and smaller pieces, at some point you would reach a piece so tiny that it could no longer be divided. "
John Dalton was an English chemist, meteorologist and physicist who established
the Atomic Theory of Matter using experimental evidence.
The Atomic Theory of Matter states that:
All elements are composed of atoms. - it is impossible to divide or destroy an atom.
Atoms in different element differ from one another
All atoms of the same element are alike (e.g one atom of carbon is like another atom of carbon)
Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element can never change into atoms of another element as a result of a chemical reaction.
Dalton theorised that atoms were tiny spheres so he drew circles with different patterns in them to represent and distinguish them. He constructed a 'Table of Atomic Weights' and using the information from this table, he placed them in a chronological order determined by the weight of the elements, comparing them to hydrogen, the lightest element, which is the first number on the table and has the atomic number of 1.
Atoms of different elements bond to form a compound. These atoms must be in definite integer ratios. E.g. water is a compound made up of a hydrogen to oxygen ratio of 2:1 Three atoms of hydrogen and 2 atoms of oxygen cannot combine to make water.
Billiard Ball Model
J.J. Thomson is well known for creating the 'Plum Pudding' model of the atom. His proposal was a sphere filled with positively charged fluid - the 'pudding' of the atom. The 'plums' of the pudding were negatively charged electrons and Thomson proposed that they were held together by the positive fluid due to its electrical forces.
Thomson's discovery of the electron completely transformed the latter-day view of the atom. His studies urged that the atom was not an 'indivisible' particle - as John Dalton had put forward - but a jigsaw puzzle made of smaller pieces.
Lord Ernest Rutherford
Lord Ernest Rutherford suggested that atoms were composed of a small dense core that he named the nucleus, consisting of positive charges. He added that negatively charged electrons were dispersed around the centre and were held in orbit by the positively charged nucleus because of electrical forces.
Rutherford conducted numerous experiments such as the Gold Foil Experiment where he fired 'alpha particles' ("a type of particle that is ejected from radioactive nuclei. Alpha particles consist of two protons and two neutrons and thus are equivalent to helium nuclei") at solid objects such as gold foil. Although most of the particles passed through the gold foil, a small number passed through at an angle (as if they had bumped against something) and some bounced straight back. This suggested that matter had holes in it which allowed the alpha particles to pass through while a small number rebounded because they hit a solid object.
that the alpha particles would pass right through the gap in the atom. His atom was parallel to a tiny solar system with the positively charged nucleus always at the center and the electrons orbiting the nucleus.
Rutherford suggested that the nucleus was so dense that the alpha particles would bounce off of it, but the electrons were so minuscule, and expanded at large interspaces,
Niels Bohr expanded on Rutherford's ideas, putting forward his model of the atom named the 'Energy Level' model which also consisted of a dense, positive nucleus, however, he turned his observations more to describing the electron. He specified that it moved in fixed, circular orbits (or energy levels) around the nucleus called electron shells. He added that the shells were anchored distances from the nucleus and this was congruent with all atoms. He stated that electrons increased in size and energy the further away they are from the nucleus, also identifying that an electron can leap from a lower to a higher trajectory by absorbing energy, this can be done in the forms of heat, light or electricity. When they cool, these electrons lose energy, giving them the emission spectra ("a spectrum of the electromagnetic radiation emitted by a source") for all various elements.
His model showed that electrons change orbits in a sudden quantum leap. The energy difference between the initial and final orbit is emitted by the atom in bundles of electromagnetic radiation called photons. He found, through experimentation, that the closer an electron is to the nucleus, the less energy it needs so he numbered the energy levels -the higher the energy-level number, the farther away the electron is from the nucleus — and the higher the energy.
Bohr also established that the different energy levels can hold varying numbers of electrons: energy level 1 may hold up to 2 electrons, energy level 2 may hold up to 8 electrons, and so on.
Schrödinger identified that electrons don't move in a set path . He theorised that electrons move in waves, and they have no exact location.
He introduced the 'Quantum Mechanical' model of the atom which was based off Bohr's model but used mathematical equations to predict the likelihood of finding an electron in a certain position. This model can be represented as a nucleus encompassed by an electron cloud. Where the cloud is most dense, the probability of finding the electron is greatest and, contrariwise, the electron is less likely to be in a less dense area of the cloud. Consequently, this model initiated the concept of sub-energy levels.
James Chadwick's atomic theory led to the discovery of neutrons. Chadwick attacked beryllium atoms with alpha particles and found that an unfamiliar radiation was produced. Chadwick elucidated this radiation as being made up of particles with a neutral electrical charge and the approximate mass of a proton which were later named neutrons.
"The quantum mechanical model is based on quantum theory, which says matter also has properties associated with waves. According to quantum theory, it’s impossible to know the exact position and momentum of an electron at the same time. This is known as the Uncertainty Principle. The quantum mechanical model of the atom uses complex shapes of orbitals(sometimes called electron clouds), volumes of space in which there is likely to be an electron. So, this model is based on probability rather than certainty."
Peter Higgs, alongside other scientists, discovered the "Higgs Boson" which is a part of the 17 particles in the Standard Model (a theory of fundamental particles and how they interrelate. The elementary particles are either the building blocks of matter, called fermions, or the intermediaries of interactions, called bosons). The Higgs particle is a boson and helps give mass to all elementary particles that have mass, such as electrons and protons.
In the Standard Model, the Higgs particle is a boson with spin zero, no electric charge and no colour charge. It is also very unstable as it decays into other particles almost instantly. The Higgs field is a scalar field, with two neutral and two electrically charged components that form a complex doublet of the weak isospin SU(2) symmetry.
The Higgs Boson is so highly viewed - even gaining the nickname 'The God Particle' - because it assists in giving mass to all elementary particles that have mass, such as electrons and protons.