GCSE EDEXCEL Chemistry SC3: The Atomic Structure

• John Dalton published his own three-part atomic theory in 1803: all substances are made of atoms, atoms are small particles that cannot be created, divided, or destroyed, and atoms of the same element are exactly alike, while atoms of different elements are different.
• J.J Thomson used a cathode-ray tube to conduct an experiment in 1897, identifying an error in Dalton’s atomic theory: atoms can be divided into smaller parts.
• Thomson proposed a model of an atom called the “plum-pudding” model, in which negative electrons are scattered throughout soft blobs of positively charged material.
• Ernest Rutherford shot a beam of positively charged particles into a sheet of gold foil in 1909, and found that some of the particles were deflected to the sides and a few bounced straight back.
• The relative atomic mass of chlorine in the sample is
• The mass of atoms is so small that we compare their masses to each other.
• The mass number of an atom is the number of protons and neutrons in the nucleus.
• The mass number of neutrons can be calculated by subtracting the atomic number from the atomic mass.
• Atoms of a given element have the same number of protons in the nucleus and this number is unique to that element.
• The relative atomic mass of an element can be calculated from the relative masses and abundances of its isotopes.
• A carbon atom having a mass number 12, i.e. (12C) is taken as standard for this comparison and its relative atomic mass is 12.
• The relative atomic mass is calculated using the abundance of different isotopes and because it is an average it can lead to the relative atomic mass not being a whole number.
• Isotopes are different atoms of the same element containing the same number of protons but different numbers of neutrons in their nuclei.
• Rutherford developed a new model which said that most of the atom’s mass is found in a region in the center called the nucleus.
• The atomic number is the number of protons in an atom.
• A sample of chlorine gas is a mixture of 2 isotopes, chlorine-35 and chlorine-37.
• The average mass, or R.A.M. of chlorine can be calculated using the following equation:

  R.A.M. =

  ​​ (mass of isotope-A x % of isotope-A) + (mass of isotope-B x % of isotope-B) ÷100.
• The existence of isotopes results in relative atomic masses of some elements not being whole numbers because isotopes have the same number of protons but different numbers of neutrons, they are still atoms of the same element, but they have different atomic masses.
• Most of the mass of an atom is concentrated in the nucleus.
• In Rutherford’s model the atom is mostly empty space, and the electrons travel in random paths around the nucleus.
• Isotopes are atoms with different numbers of neutrons but same number of protons
• The atomic number is the number of protons
• Atoms are made up of three subatomic particles; protons, neutrons and electrons
• Protons are positively charged and found in the nucleus
• Nuclear reactions involve changes to the nuclei of atoms
• Protons are positively charged (1+) and found in the nucleus
• Neutrons are neutral (0-) and also found in the nucleus
• Electrons are negatively charged (-1) and orbit the nucleus at a distance from it
• Neutrons have no charge and are also found in the nucleus
• Electrons are negatively charged and orbit the nucleus
• Isotopes have different numbers of neutrons but the same number of protons so they all have the same atomic number.
• The number of protons is equal to the atomic number, which determines what element an atom belongs to.
• Atoms with more than one electron can be represented by their electronic configuration, which shows how many electrons there are in each energy level or shell around the nucleus.