Chemistry Revision

These cards cover:

  • Atomic Structure
  • Bonding
  • Materials

Atomic Structure

Atoms have a small central nucleus, around which are three electrons.

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 protons.

The number of protons in an atom is called its atomic number (proton number).

Electrons occupy the lowest energy level.

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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 popsitively charged ions.
  • Atoms that gain electrons become negatively charged ions.

Ions have the atomic structure of a noble gas (Group 0).

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Ions in Sodium Chloride

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Ions in Magnesium oxide

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Ions in calcium chloride

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Covalent bonds in ammonia

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Covalent bonds in hydrogen chloride

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Covalent bonds in chlorine

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Covalent bonds in methane

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Covalent bonds in oxygen

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Metals and ionic compounds

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 to move through the whole structure.

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.

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Ionic compounds have regular structures (giant ionic lattices) nin which there are strong forces in all directions between oppositely charged ions. These compounds have high melting points and high boiling points.

When melted or dissolved in water, ionic compounds conduct electricity because the ions are free to move and carry the current.

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Simple molecules

Substances that consist of simple molecules are gases, liquids or solids that have relatively low melting points and boiling points.

Substances that consist of simple molecules only have weak forces between the molecules (intermolecular forces). It is these intermolecular forces that are overcome, not the covalent bonds, when the substance melts or boils.

Substances that consist of simple molecules do not conduct electricity because the molecules do not have an overall electric charge.

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Sharing electrons

Atoms that share electrons can also form giant structures or macromolecules. Diamond and graphite (forms of carbon) and silicon dioxide (silica) are examples of giant covalent structures (lattices) of atoms. All the atoms in these structures are linked to other atoms by strong covalent bonds and so they have very high melting points.

In diamond, each carbon atom forms four covalent bonds with other carbon atoms in a giant covalent structure, so diamond is very hard.

In graphite, each carbon atom bonds to three others, forming layers. The layers are free to slide over each other and so graphite is soft and slippery.

Higher - In graphite, one electron from each carbon atom is delocalised. These delocalised electrons allow graphite to conduct heat and electricity.

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