Atomic Theory Bhavik Patel Chemistry AP Period: 3-4 Gangluff Atomic Theory • John Dalton and His Atomic Theory • In 1803, John Dalton (1766–1844) forcefully revived the idea of atoms. Dalton linked the existence of elements, which cannot be decomposed chemically, to the idea of atoms, which are indivisible. John Dalton’s Theory • Compounds, which can be broken down into two or more new substances, must contain two or more different kinds of atoms. Dalton went further to say that each kind of atom must have its own properties, including mass. This idea allowed his theory to account quantitatively for the masses of different elements that combine chemically to form compounds. Dalton’s Theory • 1. All matter is composed of tiny indivisible particles called atoms. • 2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element. Dalton’s Theory • 3. Atoms of different elements can combine with one another in simple whole number ratios to form compounds. • e.g. H2O CO2 HCl • 4. Chemical reactions occur when atoms are separated, joined or rearranged. Atoms of one element are not changed into atoms of another by a chemical reaction. Electrons • Electrons are subatomic particles found rotating around the nucleus of every atom. The number of electrons outside the nucleus is always the same as the number of protons in the nucleus. This makes the atom electrically neutral. Arrangement • The electrons which move around the nucleus of an atom have different quantities of energy. Those restricted to being closest to the nucleus have the least energy while those with the ability to be furthest away from the nucleus have the most energy. Electron Arrangement • The electrons can therefore be regarded as occupying different spherical volumes of different sizes, these levels usually being referred to as shells. It is worth noting that within each shell the electrons are not orbiting the nucleus at a fixed distance but can travel anywhere within the spherical shape of that shell. Protons • Protons are stable subatomic particles found in the nucleus of every atom. • The number of protons in the nucleus is the proton number of that atom, which tells which element the atom belongs to. • The proton number determines the relative position of the element on the periodic table. Proton: Discovery • Two scientists, W. Wien in 1898, and J.J.Thomson in 1910, concluded that a positive particle existed and had a mass equal to that of a hydrogen atom. • In 1919, E.Rutherford demonstrated that particles were emitted when nitrogen was bombarded with alpha particles. In 1920 he proposed that these hydrogen nuclei were fundamental particles and he named them PROTONS. Neutrons • Neutrons are subatomic particles found in the nucleus of every atom. The number of neutrons in the nucleus does not follow a regular pattern. The number of neutrons can vary even for atoms of the same element.(Isotope) • Neutrons have no charge. Neutrons • Discovery: Neutrons were the last of the three main subatomic particles to be discovered, being first recorded in 1932. Concept Map Atomic Structure Atom Electrons Nucleus Protons Neutrons Imagine This! • If an atom were the size of a football stadium, the nucleus would be the size of a marble! • If people were the size of atoms, all the people living on the earth today would fit on the head of a pin!