2.1 Atomic Structure & the Periodic Table

• Elements, Compounds & Mixtures:

  • Elements are substances made of atoms that all contain the same number of protons and cannot be split into anything simpler
  • There are 118 elements found in the Periodic Table
  • Compounds are pure substances made up of two or more elements chemically combined
  • Compounds cannot be separated into their elements by physical means
  • Mixtures are a combination of two or more substances (elements and/or compounds) that are not chemically combined
  • Mixtures can be separated by physical methods such as filtration or evaporation
• Atomic Structure:

  • All substances are made of tiny particles of matter called atoms, which are the building blocks of all matter
  • Each atom is made of subatomic particles called protons, neutrons, and electrons
  • Protons and neutrons are located at the center of the atom, which is called the nucleus
  • Electrons move very fast around the nucleus in orbital paths called shells
  • The mass of the electron is negligible, hence the mass of an atom is contained within the nucleus where the protons and neutrons are located
• Protons, Neutrons & Electrons:

  • Atoms are so tiny that their masses are compared using a unit called the relative atomic mass
  • One relative atomic mass unit is equal to 1/12 the mass of a carbon-12 atom
  • The relative atomic mass has no units and elements are measured relative to the mass of a carbon-12 atom
  • The relative mass and charge of the subatomic particles are shown in the table below
• Defining Proton Number:

  • The atomic number (or proton number) is the number of protons in the nucleus of an atom
  • The symbol for atomic number is Z
  • It is also the number of electrons present in a neutral atom and determines the position of the element on the Periodic Table
• Defining Mass Number:

  • The Nucleon number (or mass number) is the total number of protons and neutrons in the nucleus of an atom
  • The symbol for nucleon number is A
  • The nucleon number minus the proton number gives you the number of neutrons of an atom
  • Protons and neutrons collectively can be called nucleons
  • The atomic number and mass number of an element can be shown using atomic notation
• Atomic number and mass number of an element can be shown using atomic notation
• The Periodic Table shows the elements together with their atomic (proton) number at the top and relative atomic mass at the bottom
• There is a difference between relative atomic mass and mass number, but for your exam, you can use the relative atomic mass as the mass number (with the exception of chlorine)
• Diagram showing atomic notation
• Both the atomic number and the relative atomic number (which you can use as the mass number) are given on the Periodic Table
• Mass = Massive, as the mass number is always the bigger of the two numbers, the other smaller one is thus the atomic/proton number
• Beware that some Periodic Tables show the numbers the other way round with the atomic number at the bottom
• Finding the protons
• The atomic number of an atom and ion determines which element it is
• All atoms and ions of the same element have the same number of protons (atomic number) in the nucleus
• Number of protons equals the atomic (proton) number
• The number of protons of an unknown element can be calculated by using its mass number and number of neutrons
• Finding the electrons
• An atom is neutral and therefore has the same number of protons and electrons
• Finding the neutrons
• The mass and atomic numbers can be used to find the number of neutrons in ions and atoms
• Number of neutrons = mass number - number of protons
• Determine the number of protons, electrons, and neutrons in an atom of element X with atomic number 29 and mass number 63
• Worked Example
• Number of protons of element X is the same as the atomic number
• Number of protons = 29
• The neutral atom of element X therefore also has 29 electrons
• The atomic number of an element X atom is 29 and its mass number is 63
• Number of neutrons = 63 - 29
• Number of neutrons = 34
• Electronic configuration can be explained using numbers instead of drawing electron shell diagrams
• The number of electrons in each electron shell can be written down, separated by commas
• This notation is called the electronic configuration (or electronic structure)
• For example, Carbon has 6 electrons, 2 in the 1st shell and 4 in the 2nd shell
• The electronic configuration of Carbon is 2,4
• Electronic configurations can also be written for ions
• For example, a Sodium atom has 11 electrons, a Sodium ion has lost one electron, therefore has 10 electrons; 2 in the first shell and 8 in the 2nd shell
• The electronic configuration of Sodium is 2,8
• The Electronic Configuration of the First Twenty Elements
• Although the third shell can hold up to 18 electrons, the filling of the shells follows a more complicated pattern after potassium and calcium