Atoms, ions and compounds

• The atom consists of a nucleus made up of two types of subatomic particles - protons and neutrons and electrons, which occupy a region outside the nucleus. Electrons are arranged around the nucleus in shells
• Mass: atoms and their subatomic particles have tiny masses. Instead of working in grams, chemists compare the masses of subatomic particles using relative masses
• a proton has virtually the same mass as a neutron
• an electron has negligible mass, about 1/1836th the mass of a proton
• Charge: a proton has a negative charge, an electron has a negative charge, the charge on a proton is equal but opposite to the charge on an electron - the charges balance, a neutron is neutral - it has no charge
• Nearly all of a atom's mass is in the nucleus, atoms contain the same number of protons as electrons
• the total positive charge from protons is cancelled by the total negative charge from electrons
• The overall charge of an atom is zero - an atom is neutral.
• Neutrons can be thought of as providing the glue that holds the nucleus together despite the electrostatic repulsion between its positively charged protons
• Most atoms contain the same number, or slightly more neutrons than protons - as the nucleus gets larger, the more and more neutrons are needed
• The number of protons in an atom identifies the element
• As of 2014 the existence of 114 elements has been confirmed and others have been tentatively reported
• Every atom of the same element contains the same number of protons
• different elements contain atoms that have different numbers of protons
• The periodic table lists elements in order of the number of protons in the nucleus. Each element is shown with the number of protons as its atomic number (proton number)
• every atom of an element has the same number of protons
• Unlike protons, the number of neutrons in the atoms of an electron can be different, within a narrow range
• isotopes are atoms of the same element with different numbers of neutrons and different masses.
• most elements are made up of a mixture of isotopes
• mass number (nucleon number) = A

  number of protons + number of nuetrons
• Atomic number (proton number) = Z

  number of protons
• chemical reactions involve the electrons surrounding the nucleus
• Different isotopes of the same element have the same number of electrons, the number of neutrons has no effect on the reactions of an element, different isotopes of an element therefore react in the same way
• There may be small differences in physical properties - with higher mass isotopes of an element having higher melting point, boiling point and density - but chemical reactions are the same
• An ion is a charged atom - the number of electrons is different from the number of protons
• Positive ions, or cations are atoms with fewer electrons than protons. Cations have an overall positive charge
• Negative ions, or anions, are atoms with more electrons than protons. Anions have an overall negative charge
• Ions and atoms of an element have the same number of protons but a different number of electrons