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Science

Different substances and their properties

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Ionic substances form giant ionic latticeslattice: A lattice is a regular grid-like arrangement of atoms in a material. containing oppositely charged ions. They have high melting and boiling points, and conductconduct: To allow electricity, heat or other energy forms to pass through. electricity when melted or dissolved in water.

Simple molecular substances consist of moleculesmolecules: a collection of two or more atoms held together by chemical bonds. The fundamental unit of compounds in which the atoms are joined by strong covalent bondscovalent bonds: A covalent bond between atoms forms when atoms share electrons to achieve a full outer shell of electrons.. Their molecules are held together by weak forces, so these substances have low melting and boiling points. They do not conduct electricity.

Giant covalent structures contain many atoms joined together by covalent bonds to form a giant lattice. They have high melting and boiling points. Graphite and diamond have different properties because they have different structures. Graphite conducts heat and electricity well because it also has free electrons.

Nanoparticles are 1-100 nm in size, typically the size of small molecules - far too small to see with a microscope. They have remarkable properties that are different to the same substance in bulk form.

Metals form giant structures containing free electrons [electrons: Sub-atomic particles, with a negative charge and a negligible mass relative to protons and neutrons. ], making them good conductors of heat and electricity.

Ionic compounds

Ionic bonds form when a metal reacts with a non-metal. Metals form positive ions; non-metals form negative ions. Ionic bonds are the electrostatic [electrostatic: An electrostatic force is generated by differences in electric charge (ie positive and negative) between two particles. It can also refer to electricity at rest. ] forces of attraction between oppositely charged ions.

 Ionic lattice of sodium chloride, showing positively charged sodium ions bonded to negatively charged chloride ions

Positively charged Na ions and negatively charged Cl ions

The oppositely charged ions are arranged in a regular way to form giant ionic latticeslattice: A lattice is a regular grid-like arrangement of atoms in a material.. Ionic compoundscompounds: Substances formed by the chemical union (involving bond formation) of two or more elements. often form crystals as a result. The illustration shows part of a sodium chloride (NaCl) ionic lattice.

Properties of ionic compounds

  • High melting and boiling points - Ionic bonds are very strong - a lot of energy is needed to break them. So ionic compounds have high melting and boiling points.
  • Conductive when liquid - Ions are charged particles, but ionic compounds can only conductconduct: To allow electricity, heat or other energy forms to pass through. electricity if their ions are free to move. Ionic compounds do not conduct electricity when they are solid - only when dissolved in water or melted.

Properties of ionic compounds

Ionic compoundProperties
Sodium chloride, NaCl

High melting point: 800ºC

Non-conductive in its solid state, but when dissolved in water or moltenmolten: Molten means reduced to liquid form by heating. It is mainly used to describe rock, glass or metal. NaCl will conduct electricity.

Magnesium oxide, MgO

Higher melting point than sodium chloride: around 2,800ºC. This is because its Mg2+ and O2- ions have a greater number of charges, so they form stronger ionic bonds than the Na+ and Cl- ions in sodium chloride.

Because magnesium oxide stays solid at such high temperatures, it remains non-conductive. It is used for high-temperature electrical insulation.

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