Protons, Neutrons & Electrons

Everything in chemistry, from why sodium reacts with water to why salt dissolves, comes down to the particles inside atoms. An atom is about a ten-millionth of a millimetre across, yet scientists have worked out what is inside it, how many of each particle it holds and how its electrons are arranged. In this lesson you will meet the proton, neutron and electron, use atomic number and mass number to count them in atoms and ions, arrange electrons in shells for the first twenty elements, and see how experiments changed the model of the atom from Dalton to Bohr. UTME questions on these counts are quick marks if you are systematic.

To count particles: protons = Z; neutrons = A − Z; electrons = Z in a neutral atom. For an ion, electrons = Z − charge. A 2+ ion has lost two electrons, so it has Z − 2 electrons; a 2− ion has gained two, so it has Z + 2. Rutherford's gold-foil experiment showed the structure. Most alpha particles passed straight through the foil, so the atom is mainly empty space. A few were deflected through large angles, so the positive charge and almost all the mass are concentrated in a tiny nucleus. In Bohr's model the electrons occupy shells at increasing distances from the nucleus. The first shell (K) holds up to 2 electrons, the second (L) up to 8, and for the first twenty elements the third (M) is filled to 8 before electrons enter the fourth (N). Calcium (Z = 20) is therefore 2,8,8,2. The electrons in the outermost shell, the valence electrons, decide how an element reacts. Atoms with the same number of valence electrons behave similarly, which is the basis of the groups of the periodic table. Atoms or ions with the same electron arrangement are said to be isoelectronic; Na⁺, Mg²⁺, F⁻ and O²⁻ all have the arrangement of neon, 2,8.