model of atom

• Atoms make up all substances and are the smallest part of an element that can exist
• Chemical symbols represent an atom of an element e.g. Na represents an atom of sodium
• Compounds are formed from elements by chemical reactions and contain two or more elements chemically combined in fixed proportions
• Compounds can only be separated into elements by chemical reactions
• Mixtures consist of two or more elements or compounds not chemically combined together
• The chemical properties of each substance in the mixture are unchanged
• Mixtures can be separated by filtration, crystallisation, simple distillation, fractional distillation, and chromatography
• The model of the atom has evolved over time
• First, atoms were thought to be tiny spheres that could not be divided
• Discovery of the electron led to the plum pudding model, where the atom is a ball of positive charge with negative electrons embedded in it
• The alpha particle scattering experiment concluded that the mass of an atom was concentrated at the centre (nucleus) and that the nucleus was charged
• Neil Bohr suggested that electrons orbit the nucleus at specific distances
• Later experiments showed that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each having the same positive charge (protons)
• James Chadwick's work provided evidence for the existence of neutrons within the nucleus
• Relative electrical charges of subatomic particles
• Atomic number is the number of protons in an atom of an element
• All atoms of a particular element have the same number of protons
• Atoms of different elements have different numbers of protons
• An atom has an overall charge of 0, so the number of protons equals the number of electrons
• Size and mass of atoms
• Atoms are very small with a radius of about 0.1 nm
• The radius of the nucleus is less than 1/10,000 of the atom's radius but holds almost all of the mass
• Mass number is the sum of the protons and neutrons in an atom
• Isotopes are atoms of the same element with different numbers of neutrons
• Relative atomic mass is an average value that takes into account the abundance of isotopes of the element
• Electronic structure
• Electrons occupy the lowest available energy levels (shells closest to the central nucleus)
• The electronic structure of an atom tells you how many electrons are in each shell
• Example: Sodium has an electronic structure of 2,8,1