Atoms, Ions and Compounds

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.

Isotopes

• 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 ^a_zH, 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

Carbon-12

• 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

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 number₁ + Percentage abundance₂ x mass number₂)/100

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.

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 P₄ and S₈ (it is normal to write it as S since otherwise you have to multiply the equation by a factor of 8)