Atoms, Ions and Compounds


Protons, Neutrons and electrons

The Nuclear atom

  • In an atom, the nucleus consists of protons and neutrons
  • Electrons orbit the nucleus on the electron shells.

Properties of protons, neutrons and elecrons

  • Mass- a proton and neutron have a relative mass of 1, electrons have a negligible mass of 1/1836
  • Charge- a proton have a relative charge of +1, while electrons have a charge of -1. Neutrons have no charge.

Atomic Structure

  • Nearly all of the atoms mass is in the nucleus
  • Atoms contain the same number of protons as electrons
  • The total charge of an atom is always 0 since the charges from protons and neutrons cancel each other out
  • most atoms have the same or slightly more neutrons than electrons.
  • As the nucleus gets larger, more and more neutrons are needed.

Atomic Number

  • Every atom of the same element has the same amount of protons. Different elements mean different numbers of protons.
  • The periodic table lists its elements in order of the protons in the nucleus. Each elements number of protons is shown by its atomic number.
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  • Isotopes are atoms of the same element wih a different number of neutrons and different masses.
  • most elements are made up of a mixture of isotopes

Representing isotopes

  • Isotopes can be represented in the form azH, where a is the mass number and z is the atomic number, aH where the atomic number can be derived by looking at the element, or H-a.

Isotopes and chemical reactions

  • Different isotopes of the same elemet havbe the same number of electrons
  • he number of neutrons has no effect on reactions of an lement 
  • different isotopes of the same element therefore react in the same way.

Atomic structure of ions

  • An ion is a charged ion. Elements become charged by losing or gaining electrons
  • Positive ions are called cations, while negative ions are called anions
  • ions are always shown with their overall relative charge
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  • carbon-12 is an isotope of carbon that has exactly 6 neutrons, 6 protons and 6 electrons.
  • a unit of relative mass is defined by 1/12 of the mass of Carbon-12
  • one atomic mass unit is approximately the mass of one proton or electron.

Relative isotopic mass

  • Relative isotopic mass is the mass of an isotope compared to 1/12 of the mass of carbon-12
  • Masses must be written to one decimal place
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Relative Atomic Mass

Relative atomic mass

  • Relative atomic mass is the weighted mean mass of an atom of an element compared with 1/12 of the mass of carbon-12
  • The weighted mean mass takes into account the percentage abundance of an isotoope as well as the relative isotopic mass of an isotope

Determination of relative atomic masses

  • the percentage abundance of a sample of an element can be found using a mass spectrometer.
  • 1. a sample is placed in the mass spectrometer
  • 2. the sample is vapourised and ionised to form positive ions
  • 3. ions are accelerated. Heavier ions are more difficult to deflect so each weight is seperated
  • 4. the ions are detected on the mass spectrum as a mass/charge ratio (m/z). the greater the abundance, the larger the signal.
  • for ions with a charge of one, this ratio is the equivalent to the relative isotopic mass.
  • Relative atomic mass = (Percentage abundance1 x mass number1Percentage abundance2 x mass number2)/100
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Ionic Charges

Simple ions from the periodic table

  • atoms on the left side of the table lose electrons to form cations, while atoms from the right side barr noble gasses gain electrons to form anions
  • some metals can form different ions and these can be shown by roman numerals so copper(I) has a charge of 1+ and copper(II) has a charge of 2+
  • silver only can form an ion of 1+, while zinc can only form 2+ and aluminum can only form 3+

Binary compunds

  • Binary compounds contain two elements only
  • it is written as (first element) (second element appended by -ide)
  • in ionic compounds, the metal ion always comes first.

Polyatomic ions

  • an ion can contain more than two elements bonded together. These ions are polyatomic
  • see polyatomic ions revision sheet to see all common ions and charges.
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Diatomic molecules

Some elements only exist as a molecule when unbonded. Most are diatomic:

  • H2
  • N2
  • O2
  • F2
  • Cl2
  • Br2

Some exist as other forms, such as Pand S8 (it is normal to write it as S since otherwise you have to multiply the equation by a factor of 8)

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