Doc Brown's chemistry notes on atomic and electron structure

3. Atoms, sub-atomic particles, important definitions of atomic number, mass number, explaining nuclide notation and diagrams

• Reminders
• What sub–atomic particles make up atoms? What is their mass and charge?
• The diagram above of a 'portrait of an atom' gives some idea on the structure of an atom (sometimes called the Bohr Atomic Model), it also includes some important definitions and notation used to describe atomic structure
  • REMINDER of the three fundamental particles you need to know are ...
  • proton: particle mass = 1, electric charge = +1, the charged particle in the nucleus
  • neutron: particle mass = 1, charge = 0, uncharged particle in the nucleus
  • electron: particle mass = 1/1850 ~1/2000, electric charge = –1,
    • Electrons are NOT in the nucleus but exist in electronic energy levels around the nucleus (a sort of orbit, often described as a shell, see later).
  • The nucleus of protons and neutrons is tiny, even compared to the tiny atom!
    • So most of the volume of an atom is empty space, BUT it is where the tiny electrons are.
    • In fact the diameter of the nucleus of protons plus neutrons is about a ten thousandth of the diameter of the whole atom!
    • Since the nucleus is composed of positive protons and neutral neutrons, the nucleus itself must be positive.
    • A neutral atom carries no overall charge because the number of positive protons equals the number of negative electrons (both singly charged + and -), and this information is given by the atomic/proton number.
      • A neutral helium atom has 2 protons and 2 electrons and a uranium atom has 92 protons and 92 electrons. 2+ balances 2- and 92+ balances 92-, easy!
      • Since a neutron is electrically neutral, the number of neutrons in the nucleus of an atom cannot affect the total positive nuclear charge of the protons or the number of negative electrons in the atom.

• Reminders of the 'Bohr' model
• Protons and neutrons are the 'nucleons' or 'sub–atomic' particles present in the minute positive nucleus and the negative electrons are held by the positive protons in 'orbits' called energy levels or shells.
  • Some important evidence for this 'picture' is obtained from alpha particle scattering experiments (see section 7).
  • Although the nucleus must be positive because of the positive protons (neutrons are neutral) an individual atom is neutral because the number of electrons equals the number of protons – so the charges 'cancel out'.
  • An ion particle carries an overall surplus electric charge - positive or negative - so how are they formed?
  • If electrons are removed from an atom you get a positive ion from excess positive charge, and, if electrons are added to an atom, you get a negative ion from excess negative charge.
  • An ion, by definition, cannot be neutral and the number of protons cannot equal the number of electrons.

• Two important definitions
• (1)The atomic number (Z) is the number of protons in the nucleus and is also known as the proton number of the particular element and it is this number that decides what element a particular atom is.
  • Each element has its own atomic number, so all the atoms of a particular element have the same atomic number.
  • In a neutral atom, the number of electrons equals the atomic/proton number.
• It is the proton/atomic number (Z) that determines the number of electrons an element has, its specific electron structure and therefore the specific identity of a particular element in terms of its physical and chemical properties.
• It cannot be overemphasised that it is the electronic structure that determines the chemical character of an element, hence the proton/atomic number determines everything about a particular element.
• (2) The mass number (A) is also known as the nucleon number, is the number of particles in the nucleus of a particular atom–isotope (notes on 4_71atom4.htm – definition and examples).
  • Therefore the mass number or nucleon number = sum of the protons plus neutrons in the nucleus.
  • Since the mass of a proton or neutron equals 1, the mass number equals the mass of a particular atom to the nearest whole number.

  • Relative Atomic Mass is dealt with on a separate calculation page

• The neutron number (N) = mass number – proton/atomic number
• In an individual atom the number of protons (+) equals the number of electrons (–), that is the number of positive charges is equal to the number of negative charges to make the atom neutral.

• Nuclide notation for atomic structure
(both neutral atoms and ions)
• Nuclide notation is a standard shorthand method used in chemistry and physics to represent the specific composition of an atomic nucleus. It displays the element's chemical symbol alongside its mass number and atomic number to define a distinct nuclear species.
• In the example in the diagram for lithium–7, ⁷₃Li is its nuclide atomic notation ...
• 
• before the chemical symbol of the element Li
  • is the top left number  = nucleon number/mass number = 7 (3 protons + 4 neutrons).
  • and the bottom left number = proton number/atomic number = 3 (3 protons in nucleus).
  • therefore the number of neutrons = nucleon/mass number - proton/atomic number = 7 - 3 = 4,
  • AND it must have 3 electrons, because number of protons (+) = number of electrons (-) in a neutral atom.
• Similarly how to interpret the following nuclear symbols ... is described for some neutral atoms ...
• , atom of hydrogen–1, symbol H, mass 1, just 1 proton and one electron, but NO neutrons (unique, hydrogen-1 is the only atom with NO neutrons)
• , atom of helium–4, symbol He, mass 4,  2 protons, 4 – 2 = 2 neutrons and 2 electrons
• , atom of sodium, symbol Na, mass 23, 11 protons, 23 – 11 = 12 neutrons and 11 electrons

• atom of iron–56, mass 56, 26 protons, 30 neutrons (56 – 26) and 26 electrons

• If the proton number and electron number are different, the atom has an overall surplus or deficiency of electrical charge, resulting in an electrically charged particle called an ion e.g.
  • IONS and sub-atomic particles and nuclide notation
  • If an atom loses 1 or 2 electrons, the protons produce an excess of 1 or 2 units of positive charge.
    • The excess positive charge on the positive ion is written as + or 2+ etc.
  • If an atom gains 1 or 2 electrons, the extra electrons produce an excess of 1 or 2 units of negative charge.
    • The excess negative charge on the negative ion is written as - or 2- etc.
  • So, reading the symbols below, as above, BUT, now taking into account the electrical charge on the ion ..
  • the positive sodium ion Na⁺,11 protons, 10 electrons (11-1), 12 neutrons (23-11)
  • the positive magnesium ion Mg²⁺,12 protons, 10 electrons (12-2), 12 neutrons (24-12)
  • the negative chloride ion Cl^–,17 protons, 18 electrons (17+1), 20 neutrons (37-17)
  • the negative sulfide ion S^2-,16 protons, 18 electrons (16+2), 16 neutrons (32-16)
  • for more details and examples see ionic bonding notes.

• The electrons are arranged in specific energy levels according to a set of rules

  • (dealt with in section 5).

• This description of an atom consisting of the relatively minute nucleus of protons and neutrons surrounded by electrons in particular shells or energy levels is sometimes referred to as the Bohr Model of the atom, after the great Danish scientist Niels Bohr (1885–1962), one of the most brilliant founders of modern atomic theory.

• Other examples of interpreting the nuclide notation and definition reminders:

  • Top left number is the nucleon number or mass number (A = sum of protons + neutrons = nucleons)

  • Bottom left number is the atomic number or proton number (Z = protons in nucleus)

  • Electrons = protons if the atom is electrically neutral i.e. NOT an ion.

  • The neutron number N = A – Z i.e. mass/nucleon number – atomic/proton number

    • Therefore from the following 'full' atomic symbols, assuming we are dealing with electrically neutral atoms, the number of sub-atomic particles for the following atoms will be as follows ...

    • Cobalt atom (isotope cobalt–59), mass 59, 27 protons, 32 neutrons (59 – 27), 27 electrons

    • Californium atom (isotope californium–246), mass 246, 98 protons, 148 neutrons (246 – 98), 98 electrons

    • So, at this point we had better explain, slightly belatedly, what isotopes are!

Quizzes - practice exam questions

• Some basic practice exam questions on atomic structure (at end of index page)

• GCSE level Foundation (easier) Atomic Structure multiple choice QUIZ

• GCSE level Higher (harder) Atomic Structure multiple choice QUIZ

• gap-fill worksheet on Atomic Structure

• matching pair quiz on Atomic and electronic structure 1. fundamental particles

• matching pair quiz on Atomic and electronic structure 2. periodic table