This essay will explain how we came to todays understanding of the atom and the major work that has been done over time to aid us in understanding of its structure. Atom is the smallest building block of an element, which can’t be further broken by using any chemical process.The word atom comes from the Greek word atomos meaning “indivisible”. An atom contains a nucleus containing protons and neutrons and is surrounded by electrons arranged in an orbit at a fixed distance in shells. A neutron, as the names suggests are neutral meaning it has no charge, whereas protons and electrons have charges and have the same size of charges. Electrons have a negative charge and protons have a positive charge. The total number of electrons in an atom is always the same as the number of protons in the nucleus. This means atoms have no overall electrical charge. An example of an atom is a carbon atom- a carbon atom has an atomic number or proton number of 6. Which means it has 6 electrons, 2 in its first shell and 4 in its second shell. It also has an atomic mass number of 12 so 6 protons and 6 neutrons in its nucleus. Electron shells always have the maximum of 2 electrons in its first shell and 8 in the second and 8 in the third.Atoms are arranged in the periodic table in order of increasing atomic number. The same types of atoms together are named elements. There are over 100 different elements in the periodic table. The horizontal rows in the periodic table are called periods and the vertical columns are called groups. The elements in a group have similar properties to each other.The atoms of each element are represented by a chemical symbol. The first letter is always an uppercase whereas the second is lowercase so magnesium has the chemical symbol of Mg; oxygen has the chemical symbol of O and sodium the chemical symbol of Na.The development of the atomic theory started a very long time ago and till this day we continue to discover new theories. Starting with john Dalton in the 1800s, his known for his experiments working out atomic weights, and inventing symbols for atoms. His important conclusions are all matter is made of atoms and cannot be broken down into any anything simpler. All the atoms of a particular element are identical and different from other atoms. Atoms are rearranged in a chemical reaction and compounds are formed when two or more different kinds of atoms join together. Dalton’s theory was developed and changed as new evidence was discovered. In 1897 JJ Thomson discovered the electron this gave rise to the fact that an atom contained smaller pieces whereas Dalton previously thought atoms could not be broken down into anything simpler. In the earlier models, the atom was imagined as a sphere of positive charge with negatively charged electrons dotted around inside like plums in a pudding, an experiment was carried out in 1905 to support the ‘plum pudding’ model however the results were unexpected.In 1911 Ernest Rutherford used experimental evidence to show that that an atom must contain a central nucleus. This was further evidence that an atom contained smaller pieces. Rutherford designed an experiment to test the plum pudding model. A beam of alpha particles was aimed at very thin gold foil and their passage through the foil detected. The scientists expected the alpha particles to pass straight through the foil but something else also happened.Some of the alpha particles emerged from the foil at different angles and some even came straight back. The scientists realised that the positively charged alpha particles were being repelled and deflected by a tiny concentration of positive charge in the atom. As a result of this experiment, the plum pudding model was replaced by the nuclear model of the atom.Niels Bohr further developed Rutherford’s atom model. He used evidence from experiments to support the idea that electrons occupy particular orbits or shells around the nucleus of an atom.The development of the theory of atomic structure continues to change as new evidence is found and scientific explanation is provisional but become more convincing as predictions based on it are confirmed at a later date.