Atoms, Bonds and Material Families

Learning outcomes

The learner should be able to relate atomic structure and bonding to broad material behaviour; distinguish elements, isotopes, ions, compounds, mixtures and alloys; compare metallic, ionic and covalent bonding; explain why polymers, ceramics, metals and composites respond differently; and connect bonding to NDT-relevant properties.

2.1 Atomic building blocks

An atom contains a nucleus of protons and neutrons surrounded by electrons. The proton count is the atomic number and identifies the element. Isotopes share the same proton count but have different neutron counts. An ion forms when an atom or group gains or loses electrons and therefore carries net charge.

The mass number is the total number of protons and neutrons in one isotope. The tabulated relative atomic mass reflects the natural isotope mixture rather than simply rounding one mass number. Some nuclei are unstable and transform while emitting ionising radiation. Industrial radiography may use sealed radioactive sources, but source selection, security, exposure control and emergency response belong to an authorised radiation-safety system, not ordinary materials identification.

Electrons occupy permitted energy states. The outer electrons have the greatest influence on chemical reaction and bonding. NDT personnel do not need to solve quantum wave functions, but should understand the consequence: the way electrons are held or shared controls conductivity, magnetism, chemical stability and much of a material's mechanical behaviour.

An element contains one type of atom. A compound contains elements joined in defined chemical proportions. A mixture combines substances without a fixed chemical ratio. An alloy is a metallic material deliberately containing more than one element; alloying atoms may substitute for host atoms or occupy spaces between them.

2.2 Bonding and properties

In metallic bonding, positive ion cores share a mobile population of electrons. This helps explain electrical and thermal conductivity and permits atomic planes to move without every bond failing at once. Metals are therefore commonly workable and tough, although microstructure can also make them hard or brittle.

In ionic bonding, electron transfer creates oppositely charged ions. Ionic solids are often hard and heat resistant, but slip can bring like charges together and cause cleavage. Many ceramics are therefore strong in compression and comparatively brittle in tension.

In covalent bonding, atoms share electrons in directional bonds. Diamond is extremely hard; many polymers use strong covalent bonds along molecular chains but weaker secondary forces between chains. Chain arrangement, cross-linking, temperature and loading rate strongly affect polymer response.

Secondary attractions such as van der Waals forces and hydrogen bonding are weaker than primary bonds but can dominate soft materials, coatings and interfaces.

2.3 Material families and NDT relevance

• Metals and alloys generally conduct electricity and ultrasound, but grain size, phase and texture can scatter or redirect sound. Ferromagnetism is limited to particular materials and conditions.
• Ceramics and glasses are often hard, corrosion resistant and brittle. Porosity and scattering can complicate ultrasonic examination.
• Polymers can be viscoelastic, strongly temperature dependent and attenuative. Solvents used in surface methods can attack some polymers.
• Composites combine constituents with distinct roles. Directional reinforcement and interfaces create anisotropic mechanical and ultrasonic behaviour.

Material properties are not determined by chemistry alone. Processing controls phases, grain shape, porosity, residual stress and interfaces. Two components with nominally the same composition can behave differently after different thermal or mechanical histories.

2.4 Useful quantities

Density is mass per unit volume:

rho = m / V

where rho is density, m is mass and V is volume. Acoustic impedance for a simple wave mode is:

Z = rho c

where c is wave velocity. A change in impedance influences reflection at an interface. Density alone does not determine velocity, and a high density does not automatically mean a high impedance without the elastic properties embedded in c.

Thermal expansion over a moderate range can be approximated by:

Delta L = alpha L0 Delta T

where alpha is the linear expansion coefficient. Temperature changes therefore affect dimensional measurements, fits, residual stress and sometimes NDT calibration.

Practical checkpoint - Compare four samples

For one metal, polymer, ceramic and fibre composite, record expected conductivity, magnetic response, surface porosity, ultrasonic attenuation, chemical compatibility and likely fracture mode. Then nominate one promising NDT method and one likely limitation for each. Confirm actual properties from controlled data before field use.

Common errors and misconceptions

Several common errors recur in this topic: assuming all metals are magnetic; treating an isotope as a different element; predicting performance from composition while ignoring microstructure; using a penetrant or cleaner on a polymer without compatibility evidence; and treating a composite as a weak version of an isotropic metal.

Chapter summary

Atomic identity comes from proton count, while electrons govern bonding and chemical behaviour. Metallic, ionic and covalent bonds lead to different property patterns. Material family and processing together influence conductivity, magnetism, sound transmission, solvent compatibility and fracture.