The Building Blocks of the Chemical World How our understanding of atoms evolved
Using the following information, create a timeline for your classroom showing how our understanding of atoms has evolved over time.
1913: Niels Bohr suggests that electrons are found in shells around the nucleus. 1909: Ernest Rutherford discovers the proton.
400 bc: Democritus suggests that all things are made of particles.
1931: Ernest Walton and John Cockcroft discover that the nuclei of atoms can be split to form new elements.
1932: James Chadwick proves that neutrons exist. 1911: Ernest Rutherford discovers the nucleus. 1805: John Dalton suggests that atoms of the same element are alike. 1897: J. J. Thomson discovers the electron.
Today: Research on the structure of the atom is taking place at CERN, the European Organization for Nuclear Research, in Switzerland.
Atomic and Mass Numbers
Atoms of each element have a specific number of protons, neutrons and electrons. The atomic number is the number of protons in the atom of that element.
The number of protons is equal to the number of electrons in neutral atoms. They are neutral because the number of positive charges (protons) and the number of negative charges (electrons) are equal.
The mass number is the total number of protons and neutrons in an atom.
For example, Fig. 10.2.2 tells us the following information about an atom of lithium: zz zz
Lithium has 3 protons.
Lithium has 3 electrons (the positive and the negative charges are equal, so if lithium has 3 protons it must have 3 electrons).
zz zz
The total number of protons and neutrons in an atom of lithium is 7.
Lithium has 4 neutrons (to calculate the number of neutrons in an atom you need to subtract the atomic number (for lithium, this is 3) from the mass number (for lithium, this is 7).
Making connections Use the periodic table to complete the table below.
Atomic number Mass number Number of
protons 6 13 56 26
13 24 40
12 20 14 Fe 177
Number of neutrons
Number of electrons
Name of element
Symbol of element
Atomic number
7 3
Li Mass number Fig. 10.2.2
The atomic number and mass number of lithium. Note that the atomic number is always the smaller number.
