Atomic Structure and the Periodic Table

Atomic structure

  • Atoms are made up of sub-atomic – protons, neutrons and electrons
  • protons have a positive charge, neutrons has neutral charge (same mass as protons) and electrons are negatively charged (same as no. of protons
Particle Relative charge Mass (atomic mass)
Proton +1 1
Neutron 0 1
Electron -1 1/18371​
  • Proton number: number of protons in an atom [atomic number]
  • Nucleon number: total number of protons and neutrons in the nucleus of an atom [mass number]

Definition of terms

  • Atom is the unit particle of an element
  • Element is a substance that consists of atoms all with the same proton number.
  • Proton no. is number of protons in an atom (and no. of electrons in an atom). in the periodic table, proton number increases with group number
  • Nucleon no. is number of protons + neutrons in an atom
  • Isotopes are atoms of same element with different number of neutrons eg chlorine 35 and 37


Electronic Configuration

  • Electrons are arranged in electron shells.
  • Electron shell structure: 2, 8, 18.
  • Atoms want to have full outer shells (full set of valency electrons), this is why they react.
  • Noble gases have full outer shells so they have no need to react.


Structure of the Periodic Table

  • The number of protons in each element increases by 1 across each row
  • Period number is the number of occupied shells in an element

Group number is the number of electrons in the outermost shell


atoms of the same element which have the same protons number, but a different nucleon number. They have the same chemical properties due to same number of outermost shell electrons.
  • E.g. Carbon 12 and Carbon 14.
  • Two types: Non-radioactive isotopes and radioactive-isotopes (unstable atoms that break down and produce radiation)
  • Medical use: cancer treatment (radiotherapy) – rays kill cancer cells using cobalt-6
  • Industrial use: to check for leaks – radioisotopes (tracers) added to oil/gas. At leaks radiation is detected using a Geiger counter.



Terms to be familiar with

  • Element: Pure substance consisting of one type of atom
  • Mixture: two or more elements mixed together but not chemically combined
  • Compound: substance in which two or more different elements are chemically combined
  • Alloy: Mixture of two or more elements in which at least one is a metal, eg. brass (copper and zinc)


Elements, Compounds and Mixtures

An element is a substance that cannot be broken down into simpler substances through any chemical or physical means. Elements can exist as atoms or molecules. Each molecule of an element can consist of two or more atoms that are chemically combined.

A compound is a substance that contains two or more elements which are chemically combined in a fixed ratio. It can consist of either molecules or ions. The properties of a compound differ from its constituent elements.

A mixture consists of two or more substances that are mixed together. These substances can be elements or compounds. The ratio of these substances in a mixture is not fixed. The components in a mixture can easily be separated through physical methods.


Metals Non-metals
Good conductors of heat & electricity Poor conductors of heat & electricity (except graphite)
High m.p. and b.p. Lower m.p. and b.p. than metals
High density Low density
Forms basic oxides Forms acidic oxides
Forms cations in reactions Forms anions in reactions
Malleable and ductile Not malleable or ductile

Ions and Ionic Bonds

Ionic bonding: Electrostatic force of attraction between a lattice of alternating positive and negative ions

  • Chemical bonds are formed by transfer of electrons from one atom to another
  • Metals lose electrons to form cations; non-metals gain electrons to form anions
  • Positive cations & negative anions attract to each other
  • Strong electrostatic force of attraction between positive cations and negative anions is called ionic bonding


Ionic bonding between Group I metal and Group VII non-metal

Property Reason
Form giant lattice Cations and anions attract
High m.p. and b.p. Strong bonds between ions
Don’t conduct electricity when solid Ions can’t move
Conduct electricity when molten/aqueous Ions can move
Usually soluble in water Not required

Molecules and Covalent Bonds

Covalent bonding: When atoms share electrons to obtain a full outer shell electron configuration; only between non-metals.

Single Bond Double Bond Triple Bond
2ē shared(1 from each atom) 4ēs shared(2 from each atom) 6ēs shared(3 from each atom)


HCl CO2 N2
Covalent bonding Ionic bonding
Mostly volatile Mostly non-volatile
Insoluble in water Soluble in water
Poor electrical conductors Good electrical conductors

Covalent bonds, due to the sharing of electrons between atoms, have weaker attractive forces than ionic bonds. Thus they have lower melting and boiling points.


Diamond Graphite Silicon Dioxide
1 carbon atom bonded to 4 carbon atoms [tetrahedral structure] 1 carbon atom bonded to 3 carbon atoms [hexagonal layers] Each silicon is bonded to 4 oxygen atoms, and each oxygen is bonded to 2 silicon atoms [tetrahedral structure]
High m.p. and b.p High m.p. and b.p High m.p. and b.p
no free electrons Conducts electricity (free electrons) no free electrons
Used for cutting as it is strongest known substance Used in pencil lead and as a lubricant Used in production of glass

Metallic Bonding

Metallic bonding: An electrostatic force of attraction between a lattice of positive metal ions and a sea of mobile electrons

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