Simple particle theory is developed in this unit to include atomic structure and bonding. The arrangement of electrons in atoms can be used to explain what
happens when elements react and how atoms join together to form different types of
Simple particle theory is developed in this unit to include atomic structure and
Atoms have a small central nucleus made up of protons and
neutrons around which there are electrons.
The relative electrical charges are as shown:
Name of particle Charge
In an atom, the number of electrons is equal to the number of
protons in the nucleus. Atoms have no overall electrical charge.
All atoms of a particular element have the same number of
protons. Atoms of different elements have different numbers of
The number of protons in an atom is called its atomic number
(proton number). Atoms are arranged in the modern periodic
table in order of their atomic number (proton number).
Electrons occupy particular energy levels. Each electron in an
atom is at a particular energy level (in a particular shell). The
electrons in an atom occupy the lowest available energy levels
(innermost available shells). (Though only energy levels are
referred to throughout this specification, candidates may answer
in terms of shells if they prefer.)
Elements in the same group in the periodic table have the same
number of electrons in the highest energy levels (outer electrons).
Compounds are substances in which atoms of two, or more,
elements are not just mixed together but chemically combined.
Chemical bonding involves either transferring or sharing electrons
in the highest occupied energy levels (shells) of atoms.
When atoms form chemical bonds by transferring electrons, they
form ions. Atoms that lose electrons become positively charged
ions. Atoms that gain electrons become negatively charged ions.
Ions have the electronic structure of a noble gas (Group 0).
The elements in Group 1 of the periodic table, the alkali metals,
have similar chemical properties. They all react with non-metal
elements to form ionic compounds in which the metal ion has a
single positive charge.
The elements in Group 7 of the periodic table, the halogens, have
similar chemical properties. They react with the alkali metals to
form ionic compounds in which the halide ions have a single
An ionic compound is a giant structure of ions. Ionic compounds
are held together by strong forces of attraction between
oppositely charged ions. These forces act in all directions in the
lattice and this is called ionic bonding.
When atoms share pairs of electrons, they form covalent bonds.
These bonds between atoms are strong. Some covalently bonded
substances consist of simple molecules such as H2, Cl2, O2, HCl,
H2O and CH4. Others have giant covalent structures
(macromolecules), such as diamond and silicon dioxide.
Metals consist of giant structures of atoms arranged in a regular
pattern. The electrons in the highest occupied energy levels (outer
shell) of metal atoms are delocalised and so free…