2.1.1 Atomic Structure and Isotopes

• All matter is composed of

  atoms
• All elements are made up of

  atoms
• Atoms are made up of 3 types of particle

  ● protons ● neutrons ● electrons

  
• The nucleus of an atom is made up of 2 types of subatomic particle

  ● protons● neutrons
• The subatomic particle that occupies the reigon outside the nucleus is


  the electron
• Electrons are arranged


  around the nucleus in shells
• Atoms and their subatomic particles have

  tiny masses
• 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

  
• Accurate measurements show that a neutron has

  ● a slightly greater mass than a proton — by factor of 1.001375 ● so close to 1 that chemists usually assume that protons and neutrons have the same mass

  
• Charge

  ● A proton has positive charge ● An electron has a negative charge ● A neutron is neutral and has no charge— on proton is equal but opposite to the charge on an electron — the charges balance

  
• The actual charge on a single proton is

  + 1.60217733x10 - ¹⁹ C — coulombs

  
• The charge on a single electron must balance the charge on a proton so is

  -1.60217733 x 10-¹⁹ C

  
• The overall charge of an atom is zero — its neutral because


  The total positive charge from protons is cancelled by the total negative charge from electrons

  
• Neutrons hold the nucleus together despite


  the electrostatic repulsion between its positively charged protons
• Most atoms contain the same number of or slightly more neutrons than

  protons — the nucleus gets larger more and more neutrons are needed
• Atomic number is


  The number of protons in an atom— identifies the element
• As of 2014, the existence of


  114 elements had been confirmed
• Every atom of the same element contains

  the same number protons
• Different elements contain

  atoms that have different numbers protons
• The periodic table lists elements in order of the


  the number of protons in the nucleus— each element is shown with the number of protons as its atomic number or proton number

  
• Every atom of an element has the same number of

  protons ● Every atom of nitrogen atomic number 7 — contains 7 protons● Every atom of oxygen atomic number 8 — contains 8 protons
• Unlike proton the number of neutrons in the atoms of element can be


  different— usually within narrow range
• Isotopes

  are atoms of the same element with different numbers neutrons and different masses— most elements are made up of mixture of isotopes

  
• Isotopes represented using the

  chemical notation● Mass number ( nucleon number ) A — n.o protons + n.o neutrons ● Atomic number ( proton number ) Z — n.o protons

  
• To work out the n.o of neutrons in an atom

  ● subtract atomic mass from mass number // A - Z
• You can use this notation to

  work out the number of protons, neutrons and electrons in different isotopes of an element

  
• Chemists refer isotopes in different ways

  ● ¹⁶O ● ¹⁶8O● Oxygen - 16— All oxygen atoms contain 8 protons, so if the 8 is ommited like in ¹⁶O and oxygen - 16 — Still know how many protons the isotope contains
• Chemical reactions involve


  the electrons surrounding the nucleus
• Different isotopes of an element react in the same way because

  ● Different isotopes of the same element have the same number of electrons ● The number of neutrons has no effect reactions of an element — Chemical reactions involve electrons surrounding the nucleus
• There may be small differences in physical properties between isotopes

  ● higher mass isotopes of an element having :— higher melting point — higher boiling point — density— the chemical reactions are the same
• Heavy water is used to


  control processes in nuclear reactors — The H₂O molecules in normal water nearly all contain the ¹_¹H isotope of hydrogen— In heavy water all molecules of H₂O contain the ²_¹H isotope of hydrogen

  
• The ²_¹H isotope is often referred to as

  deuterium and even given its own symbol , D — formula for heavy water is often written simply as D₂0
• The chemical properties of heavy water are

  almost identical to those of normal water — has slightly different physical properties

  
• The greater density of D₂0 gives

  heavy water its name — If all water were heavy water— would see ice more often as water would freeze at a higher temperature
• Ion


  is a charged atom — the number of electrons 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 — an overall negative charge
• Ions are always shown their overall relative charge

  e.g :● Mg²+ — has two fewer electrons than protons● Cl- — has one more electron than protons

  
• The strong nuclear force holding together protons and neutrons comes at the expense of

  the loss a fraction of their mass— the small amount of mass lost is called the mass defect
• How do chemists calculate the mass of atoms if some mass is lost to hold the nucleus together ?

  ● a standard isotope is needed on which to base all atomic masses — role is taken by the carbon-12 isotope — international standard for the measurement of atomic masses● one atom carbon has a mass of 1.992 646 538x10-²⁶ kg● the atomic mass unit u is used