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Key mechanical design properties are:

• Stiffness. Stiffness is the ability of an object to resist deformation in response to an applied force.
• Strength.Strengthis the ability of a material to resist deformation.
• Hardness. Hardness is the ability to withstand surface indentation and scratching.
• 延性. Ductility is the ability of a material to deform under tensile load (% elongation).
• Toughness. Toughness is the ability of a material to absorb energy (or withstand shock) and plastically deform without fracturing.

Pure metal is a material (usually solid) comprising one metallic element (e.g., iron, aluminum, copper, chromium, titanium, gold, nickel). Analloyis a mixture of two or more materials, at least one of which is a metal.一个lloyscan have a microstructure consisting of solid solutions, where secondary atoms are introduced as substitutionals or interstitials in a crystal lattice.Steelsare iron–carbon alloys that may contain appreciable concentrations of other alloying elements.

This question has no answer. There is no material perfect for all purposes. In most cases, engineers must take into account all their mechanical and thermal properties. Some materials must withstand high temperatures, some not. There are some advanced materials like titanium alloys, but these materials are so expensive that can be used only in reasonable cases.

The atoms are about 10^–10meters(or 10^–8centimeters) in size, which corresponds to the angstrom. The ångström is not a part of the SI system of units, but it can be considered part of the metric system in general.The volume of an atomis about15 orders of magnitudelargerthan the volume of a nucleus. Foruranium atom,Van der Waals radiusis about186 pm = 1.86 ×10⁻¹⁰m.

It must be noted, atoms lack a well-defined outer boundary. The atomic radius of a chemical element is a measure of the distance out to which the electron cloud extends from the nucleus. However, this assumes the atom to exhibit a spherical shape, which is only obeyed for atoms in vacuum or free space. Therefore, there are various non-equivalent definitions of atomic radius.

• Van der Waals radius.In principle, Vana der Waals radius is half the minimum distance between the nuclei of two atoms of the element that are not bound to the same molecule.
• Ionic radius.一个n ionic radius is one-half the distance between the nuclei of two ions in an ionic bond.
• Covalent radius.Covalent radius is the nominal radius of the atoms of an element when covalently bound to other atoms.
• Metallic radius.一个metallic radius is one-half the distance between the nuclei of two adjacent atoms in a crystalline structure, when joined to other atoms by metallic bonds.

Why are atoms radioactive?

原子核由质子和中子组成,which attract each other throughthe nuclear force, while protons repel each other viatheelectric forcedue to their positive charge. These two forces compete, leading to various stability of nuclei. There are only certain combinations of neutrons and protons, which formsstable nuclei.

Neutrons stabilize the nucleus, because they attract each other and protons , which helps offset the electrical repulsion between protons. As a result, as the number of protons increases,an increasing ratio of neutrons to protons is neededto form a stable nucleus. If there are too many or too few neutrons for a given number of protons, the resulting nucleus is not stable and it undergoes radioactive decay.Unstable isotopesdecay through various radioactive decay pathways, most commonly alpha decay, beta decay, or electron capture. Many other rare types of decay, such as spontaneous fission or neutron emission are known. It should be noted that all of these decay pathways may be accompanied bythe subsequent emission of gamma radiation. Pure alpha or beta decays are very rare.

What are the 4 types of radiation?

• Ionizing radiation has different ionization mechanisms, and may be grouped as:
  • Directly ionizing. Charged particles (atomic nuclei, electrons, positrons, protons, muons, etc.) can ionize atoms directly by fundamental interaction through the Coulomb force if it carries sufficient kinetic energy.
    • 一个lpha radiation. Alpha radiation consist ofalpha particlesat high energy/speed. The production of alpha particles is termedalpha decay.
    • Beta radiation. Beta radiation consist offree electronsorpositronsat relativistic speeds. The production of beta particles is termedβ衰变.
  • Indirectly ionizing. Indirect ionizing radiation is electrically neutral particles and therefore does not interact strongly with matter.
    • Photon radiation(Gamma raysorX-rays). Photon radiation consist of high energyphotons. According to the currently valid definition,X-rays are emitted by electronsoutside the nucleus, whilegamma rays are emitted by the nucleus. The production of gamma rays is termedgamma decay.
    • Neutron radiation. Neutron radiation consist offree neutronsat any energies/speeds. This type of radiation can be produced bynuclear reactorsor in flight,neutrons contribute 40 – 80%of theequivalent dose.