Topic 2 - atomic structure

Protons have a charge of +1, a mass of +1 and are found in the nucleus. Neutrons have a charge of 1, a mass of -1 and are found in the nucleus . Electrons have a charge of 0, a mass of 1/1840 and are found in shells surrounding the nucleus.

Protons and neutrons are found in the nucleus, in the centre of the atom. As a result, most of the mass of an atom is concentrated in the nucleus. Electrons orbit the nucleus in shells; there are a maximum number of electrons that fit in each shell. Two electrons can fit in the first shell and a maximum of eight electrons in each subsequent shell. The electronic configuration gives the arrangement of the electrons in shells.

Each element has two unique numbers associated with it. These are known as the atomic number and mass number and they give us important information about the number of each sub-atomic particle present in the atom.

The atomic number of an atom is the number of protons in an atom. For an atom, this number also corresponds to the number of electrons. As a result, atoms have no electrical charge because the number of protons is equal to the number of electrons.

The mass number is the total number of protons and neutrons in an atom.

Remember that for an atom number of protons = number of electrons = atomic number and number of neutrons = mass number.

Isotopes are atoms of an element with the same atomic number but a different mass number, indicating a different number of neutrons.

For example, 12C contains 6 neutrons, whereas 13C contains 7 neutrons and 14C contains 8 neutrons. All three carbon isotopes contain 6 protons and 6 electrons.

Chlorine has two main isotopes: 35Cl and 37Cl. For every 100 chlorine atoms, 75 are chlorine-35 and 25 are chlorine-37. The mass number for chlorine given on the Periodic Table (and for every other element) reflects the abundance of all the known isotopes for the element and is more correctly known as the relative atomic mass (Ar). This is defined as: “The average (weighted mean) mass of an atom of an element relative to one-twelfth of the mass of an atom of carbon-12.”

Ar = ((35 × 75) + (37 × 25)) / 100 = 35.5

When metal atoms react with non-metal atoms, they chemically combine to form compounds. To do this, the metal atom transfers one or more electrons to the non-metal atom so that both end up with full outer shells of electrons. As the number of electrons is different to the number of protons, the atoms are now charged particles known as ions, which are attracted to each other by electrostatic attraction.

An ion is a charged particle formed when an atom gains or loses electrons (eg Cl–) and a molecular ion is a charged particle containing more than one atom (eg SO42–) Metals lose electrons to form positive ions (cations) whilst non-metals gain electrons to form negative ions (anions). The number of electrons gained or lost by an atom is related to the group in which the element is found.

For example, when sodium reacts with chlorine to form sodium chloride (common salt), the sodium atom loses one electron to form the Na+ cation, which has electronic configuration 2,8. The chlorine atom gains this electron to form the Cl– anion, with electronic configuration 2,8,8. The number of protons and neutrons in each ion remains unchanged from the original atom. When atoms react compounds are formed. A compound is two or more elements chemically combined.