Small molecules

• A covalent bond is a shared pair of electrons.
• Covalent bonding forms molecules.
• Substances with small molecules have low melting and boiling points, and do not conduct electricity.
• A covalent bond is formed when two atoms share electrons to achieve a full outer shell of electrons.
• The smallest part of an element that can exist is a subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons.
• Covalent bonding occurs in most non-metal elements.
• An element that is a poor conductor of electricity and heat, and which forms acidic oxides is a non-metal element.
• A substance made of one type of atom only is a substance formed by the chemical union of two or more elements.
• A covalent bond is formed between non-metals when electrons are shared in the outer shells of the atoms.
• Usually each atom contributes one electron to the shared pair of electrons.
• Molecules of the same element or compound always contain the same number of atoms of each element.
• The atoms in a molecule are always joined together by a covalent bond.
• Substances that are made up of ions do not form molecules.
• A small molecule contains only a few atoms, so atoms and small molecules have a similar range of sizes.
• Individual atoms and molecules are too small to see even with the most powerful light microscope.
• Some electron microscopes can produce images of atoms and simple molecules.
• A water molecule, H 2 O, is about 0.3 nm (3 × 10 -10 m) across.
• The chemical formula of a substance with small molecules shows the number of atoms of each element in one molecule of the substance.
• A methane molecule is made up of one atom of carbon and four atoms of hydrogen.
• A chlorine molecule is made up of two chlorine atoms.
• The slideshow shows how a covalent bond forms between a hydrogen atom and a chlorine atom, making hydrogen chloride.
• A hydrogen atom with one electron and a chlorine atom with 17 electrons is depicted in the structures of a hydrogen atom and a chlorine atom.
• The hydrogen atom has bonded with the chlorine atom, meaning there is now a shared pair of electrons.
• Most covalently bonded substances consist of small molecules.
• A molecule is a group of two or more atoms joined together by covalent bonds.