Lecture 4 Atoms and Atomic Structure

Protons, Neutrons and Electrons

The atom is the smallest building block of an element. The atom is composed of a core called the nucleus where most of the atoms mass is concentrated. The nucleus is composed of two types of particles:

  1. Protons which are positively charged
  2. Neutrons which are neutral, i.e., are not charged

Each of these particles has a mass specified as 1 atomic mass unit (amu). An amu is equivalent to 1.67 10-24 g. The number of protons in all atoms of the same element is always the same. This number is given the name atomic number. The number of neutrons may very slightly. The atomic mass (or atomic weight) of an element is the average of the masses of all atoms of the same element.
NOTE: The number of electrons and protons in atoms of the same element will always be the same. However, the number of neutrons may vary. Isotope is the term given to an atom of the same element but having a different number of neutrons in its nucleus.

Moving in orbitals around the nucleus are smaller atomic particles called electrons, which are negatively charged. The mass of an electron is only 0.00054 amu. An atom of an element in its natural state contains the same number of electrons as protons, thus making it neutral.

An atom may gain or lose electrons when reacting with atoms of other elements. When this occurs, the atom is known as an ion; and it would have a positive charge if electrons are lost, or a negative charge if electrons are gained. An example is when sodium (Na) reacts with chlorine (Cl) to form common table salt (NaCl). Sodium loses one of its electrons to chlorine; thus the sodium atoms become positively charged ions (Na+), and the chlorine atoms become negatively charged ions (Cl-).

If two or more electrons are lost or gained, that number is placed before the sign. For example:

Sample problem :
  1. Determine the number of protons and electrons in the following ions:

    1. P-3

    2. Li+


Positively charged ions are called cations and negatively charged ions are called anions.

Symbol Representing an Atom

where X represents the symbol for the element; A represents the mass number (i.e., neutrons plus protons); Z represents the atomic number (i.e., the number of protons); C represents the charge, if any.

Consider the following symbol:

A = 31
Z = 15
X = P
  1. The element is Phosphorus, symbol P (=X)
  2. The atomic number is 15 (=Z), thus it contains 15 protons
  3. The mass number is 31 (=A), thus it contains a total of 31 protons plus neutrons. Since it contains 15 protons, it can be determined by subtracting 15 from 31 that it contains 16 neutrons
  4. Since no value is given for 'C', there is no charge on this atom and it, therefore, contains 15 electrons

Sample problem :
  1. Given the following symbol, determine the element and the number of protons, electrons and neutrons:




The Periodic Table


The periodic table [See text - Figure 3 (2nd edt.) / Figure 4 (3rd edt.)] lists the elements in order of their atomic number. The element's symbol is in the middle of the box. The atomic number is found above the element's symbol. The atomic mass is usually located below the element's symbol and it is a decimal number. Note the illustration below:

Figure 4.1

Depending on the publisher of the table, other information may be given in the box (e.g., name of the element). E.g., Periodic Table of Elements (other links to periodic tables are listed in the table of contents at the left). You might find it helpful to have the window with this periodic table available for the rest of this chapter, so that you can refer to it easily. (i.e., When you click on this link, a new window will open with the periodic table. Minimize this window and you can refer to it easily.)

The Periodic Law states that if the elements are arranged in order of increasing atomic mass, they exhibit a periodicity of their chemical properties. The periodic table was developed by Dmitri Mendeleev. Note that this law has been adjusted to arrangement in increasing atomic number [with modern measuring techniques, it was found that there are a few places where the atomic mass is out of order compared to atomic number; e.g., Cobalt (Co) and Nickel (Ni)].

All known elements are listed in the Periodic Table, organized on the basis of their physical and chemical properties.

  1. Grouped as metals, non-metals, and metalloids
    1. Metals:
      • shiny
      • opague
      • good conductors of electricity and heat
      • ductile (i.e., can be reshaped or bent without breaking)
      • most are solids
    2. Non-metals:
      • may be transparent
      • very poor conductors of electricity and heat
      • brittle
      • may be solid, liquid or gas
    3. Metalloids:
      • have both metal and non-metal characteristics
      • weak conductors of electricity (thus, used as semiconductors in computers)
  2. Arranged in rows, each called a period
      Properties of elements gradually change across each row (called periodic trend); e.g., atomic size decreases from left to right
  3. Arranged in columns, each called a group or family
      In each group, the properties of the elements are quite similar
      • Group 1 - Alkali metals - make alkaline mixtures which maybe used in preparation of soaps
      • Group 2 - Alkaline-earth metals - also form alkaline mixtures; are fire resistant
      • Groups 3-12 - Transition metals - do not form alkaline mixtures, are harder and less reactive with water than elements in groups 1 & 2
          Inner transition metals
          • Lanthanides (of 6th period) - all have similiar properties; tend to be found mixed together at same locations on earth; difficult to purify
          • Actinides (of 7th period) - all have similar properties; difficult to purify; those heavier than Uranium are not found in nature (they are synthesized in the laboratory)
      • Group 16 - Chalcogens (Greek for 'ore-forming) - top two elements, oxygen and sulfur, are commonly found in ores
      • Group 17 - Halogens (Greek for 'salt-forming) - have tendency to form salts
      • Group 18 - Noble gases - unreactive gases which tend not to combine with other elements

Sample problem :
  1. Practice reading the Periodic Table (inside front cover of text or Periodic Table of Elements)

    1. What are the name and symbol for the element having the atomic number of 48 ?

    2. How many protons are there in a Molybdenum (Mo) atom ?

    3. What is the atomic mass of Tin (Sn) ?

    4. How many electrons are their in an atom of Cesium (Cs) ?

    5. Name an element that possesses similar chemical properties as Boron.




Model of the Atom


Niels Bohr (a 20th century physicist) developed a model of the atom showing how the electrons are confiqured about the nucleus. His model placed electrons in different orbits having specific energies. Each orbit was specified by an integer called quantum number, n. The higher this number, the greater the distance between the electron and nucleus and the greater the energy. Bohr also specified that each orbit could only hold a certain maximum number of electrons which was determined by n using the formula 2 n2

Table 4.1
orbit (n=) Maximum # electrons
1 2
2 8
3 18

Through mathematical equations developed by the Austrain pysicist Erwin Schr&oulm;dinger, the orbit of the Bohr model was replaced with orbitals. In these orbitals, electrons moved within a shaped volume of space. The first orbital (designated by the letter s) is spherical in shape.

Figure 4.2

The second orbital (p) is arranged in three separate suborbitals in the shape of dumbbells, each oriented along different axes.

Figure 4.3

In this updated model, the electrons are given a set of quantum numbers designated by the letters n, l, ml, ms. The first (n) is the principal quantum number and corresponds to Bohr's orbit numbers (i.e., 1, 2, 3, etc). The second (l) is the angular momentum quantum number and corresponds to the orbital types (i.e., s = 0, p = 1, d = 2, f = 3). These values depend on n; i.e., 0, 1, 2, .. n-1. The third (ml) is often called the magnetic orbital quantum number and corresponds to the suborbital. Its values are dependent on l; i.e., l, l-1, l-2, ... 0, -1, -2, ...., -l. The fourth (ms) is the spin quantum number. Its values include and -. The Pauli exclusion principle states that no two electrons in the same atom can have the same set of quantum numbers.

Table 4.2
Energy Level (n) Orbital Type (l) Number of Suborbitals Electrons per orbital Total # electrons per Orbital Type
1 s 1 2 2
2 s 1 2 2
  p 3 2 6
3 s 1 2 2
  p 3 2 6
  d 5 2 10
4 s 1 2 2
  p 3 2 6
  d 5 2 10
  f 7 2 14


Electrons fill the orbitals starting from the one having the lowest energy. The following diagram illustrates this filling order:

Figure 4.4

The electron configuration of an atom of an element is designated using the number and letter designation in the above diagram with the number of electrons occupying that orbital designated as superscripts. E.g.,

Table 4.3
Element Atomic Number Electron Configuration
Hydrogen (H) 1 1s1
Helium (He) 2 1s2
Lithium (Li) 3 1s2 2s1
Carbon (C) 6 1s2 2s2 2p2
Neon (Ne) 10 1s2 2s2 2p6
Magnesium (Mg) 12 1s2 2s2 2p6 3s2
Potassium (K) 19 1s2 2s2 2p6 3s2 3p6 4s1


All elements in the same column of the periodic table have the same electron configuration in their highest energy level (i.e., the one with the highest principal quantum number (n). Note in the above table that H, Li, and K (all in the same column in the periodic table) have one electron in their highest s orbital. These electrons are called valence electrons and are important in determining the elements properties. The valence electrons are the electrons that are involved in the reaction between two or more atoms. A general rule is that atoms are 'satisfied' when there are eight electrons in their highest energy level.

All the elements in group VIIIA (Noble Gases) have eight (two for Helium) valence electrons. These elements are chemically inert; i.e., they do not react readily with other elements to form compounds.

Sample problem :
  1. What is the electron configuration of the following: (NOTE Table 4.3)

    1. Fluorine (F)
    2. Sodium (Na)
    3. Aluminum (Al)
    4. Calcium (Ca)




Compounds & Chemical Reactions


From Lecture 2: A compound is a substance composed of two or more elements; and from the law of definite proportions, the amounts of each element in the compound will be in some proportion to each other.

A compound is represented using the chemical symbols of the elements it contains along with numbers that indicate the proportion of the atoms of each element in the compound. This representation is known as the chemical formula (or molecular formula). E.g., consider

Types of Compounds

Be sure to look over sample problems in text

Chemical Reactions

Table 4.4 - Types of Chemical Reactions
Combination A + B A B
Decomposition A B A + B
Single replacement A + B C A C + B
(where A is a metal)
A + B C B A + C
(where A is a nonmetal)
Double replacement A B + C D A D + C B
Combustion A B + O2 A O + B O

Chemical reactions are represented by chemical equations where A, B, C, and D above are replaced by molecular formulas or element symbols. For the chemical equation to be correct, the number of atoms of each element on the left side most equal to the number of atoms of the corresponding element on the right side (This confirms to the law of conservation of mass). E.g.,

The reaction of hydrogen with oxygen to form water (dihydrogen oxide):

2 H2 + O2 ==> 2 H2O

[Note that gases are usually always represented as diatomic molecules; i.e., consist of two atoms of the element]

Sample problem :

Be sure to look over sample problems in text

  1. Balance the following chemical equations and name the type of chemical reaction for each:

    1. KClO3 KCl + O2

    2. Fe + HCl FeCl2 + H2

    3. CuSO4 + Fe Fe2(SO4)3 + Cu

    4. BaCl2 + Na2SO4 NaCl + BaSO4

    5. C3H8 + O2 CO2 + H2 O




The Mole Concept


The mole concept gives the relationship between the mass of a substance and the number of atoms present. One mole of a substance contains 6.022 1023 atoms (This value is known as Avogadro's number). The mass of one mole of a substance is equal to its atomic mass in grams, for elements, or molecular mass, for molecules. [Molecular mass is the sum of the atomic masses of all the atoms in the molecule.]

Table 4.5
Substance Atomic Mass Mass in grams of one mole Number of atoms
Oxygen (O) 16.00 16.00 6.022 1023
Copper (Cu) 63.55 63.55 6.022 1023
Sodium chloride (NaCl) 58.44 (22.99+35.45) 58.44 6.022 1023

To determine the number of atoms in a sample of a substance of known mass, the number of moles of that substance must be determined.

Example : Determine the number of atoms in a pure silver ring having a mass of 12.5 grams.
(1) The atomic mass of silver (Ag) is 107.9, therefore,
           1 mole Ag = 107.9 g.
(2) Calculate the number of moles in the 12.5 gram ring:

      1 mole      
  12.5 g --------- = 0.116 mole  
      107.9 g      
(3) Calculate the number of atoms in 0.116 mole:

      6.022 1023 atoms    
  0.116 mole ---------------------- = 0.698 1023atoms
(6.98 1022 atoms is the correct way to express the answer in scientific notation with the correct number of significant fiqures; see Apendices A & D)

Balanced chemical equations show the number of moles of reactant(s) it takes to obtain the number of moles of product(s): e.g.,

2H2 + O2 --> 2H2O
This equation tells us that 2 moles of hydrogen (H2) react with 1 mole of oxygen (O2) to form 2 moles of water (2H2O). From this information, we can determine how much water will be produced from a given amount of hydrogen and/or oxygen; or how much hydrogen and oxygen it would take to make some specific amount of water.

Example: Determine the mass of water produced from 64 grams of oxygen and an unlimited amount of hydrogen.

First determine the number of moles of oxygen (O2) in 64 grams (atomic mass of oxygen equals 16.0, rounded off to the nearest tenth of a gram):
one mole of O2 = 2 x 16.0 g. = 32 g./mole, therefore
      1 mole O2      
  64 g.O2 --------- = 2 moles O2  
      32 g.O2      

From the balanced equation we see that 1 mole of oxygen will produce 2 moles of water. Therefore, 2 moles of oxygen (i.e., 2 x 1 mole) will produce 4 moles of water (2 x 2 moles) with an unlimited amount of hydrogen. [The number of moles of hydrogen that will be used is 4 moles (2 x 2 moles)]

Since a water molecule contains 2 atoms of hydrogen and 1 atom of oxygen, the atomic mass of one mole of water is 2 x 1.0 g. H + 1 x 16.0 g. O = 18 g. H2O. Then
      18.0 g.H2O      
  4 mole H2O --------- = 72.0 g.H2O  
      1 mole H2O      


Sample problems :

Be sure to look over sample problems in text

  1. Determine the number of moles and the number of atoms in 100 grams of table salt (NaCl).

  2. Answer

  3. Determine the number of grams of hydrogen and oxygen needed to produce 108 grams of water.

  4. Answer






Answers to Problems



    1. P-3

      P (Phosphorus) has an atomic number of 15. Therefore, it has 15 protons.
      A charge of -3 indicates that it has gained 3 electrons. Therefore, it has 18 electrons.

    2. Li+

      Li (Lithium) has an atomic number of 3. Therefore, it has 3 protons.
      A charge of + indicates that it has lost 1 electron. Therefore, it has 2 electrons.


  1. Given this symbol

    A = 56
    Z = 26
    C = 3+
    X = Fe
    • The element is Iron, symbol Fe (=X)
    • The atomic number is 26 (=Z), thus it contains 26 protons
    • The mass number is 56 (=A), thus it contains a total of 56 protons
      plus neutrons. Since it contains 26 protons,
      subtract 26 from 56 to find that it contains 30 neutrons
    • The charge is 3+ (=C), therefore there are 3 less electrons
      (negatively charged) than there are protons
      (positively charged); i.e., 26 - 3 = 23 electrons


    1. The element with atomic number 48 is Cadmium (Cd)
    2. Molybdenum (Mo) has 42 protons (NOTE that the atomic number is 42)
    3. Tin (Sn) has an atomic mass of 118.7
    4. Cesium (Cs) has 55 electrons (NOTE that the atomic number is 55)
    5. Elements that have similar chemical properties to Boron (B) are:
      • Aluminum (Al)
      • Gallium (Ga)
      • Indium (In)
      • Thallium (Tl)


    a. Fluorine (Fl) 1s2 2s2 2p5
    b. Sodium (Na) 1s2 2s2 2p6 3s1
    c. Aluminum (Al) 1s2 2s2 2p6 3s23p1
    d. Calcium (Ca) 1s2 2s22p6 3s23p6 4s2


    1. 2 KClO3 2 KCl + 3 O2
      Decomposition reaction

    2. Fe + 2 HCl FeCl2 + H2
      Single replacement reaction

    3. 3 CuSO4 + 2 Fe Fe2(SO4)3 + 3 Cu
      Single replacement reaction

    4. BaCl2 + Na2SO4 2 NaCl + BaSO4
      Double replacement reaction

    5. C3H8 + 5 O2 3 CO2 + 4 H2O
      Combustion reaction
Further explanations as to how to balance these equations:
  1. KClO3 --> KCl + O2
    The K and Cl are ok at this point, so go to the Oxygen. Since there are 3 O's on the left and 2 on the right, you would place a 2 in front of the KClO3 and a 3 in front of the O2 to give 6 oxygens on each side.:
    2KClO3 --> KCl + 3O2
    Now you'll see that the K and Cl are unbalanced with 2 of each on the left and 1 each on the right. By placing a 2 in front of the KCl on the right, these will again be balanced:
    2KClO3 --> 2KCl + 3O2

  2. Fe + HCl --> FeCl2 + H2
    On this one, you see that the Fe is balanced, but there are 2 Cl's on the right and 1 on the left, so place a 2 in front of the HCl on the left and the Cl's and H's will be balnced:
    Fe +2HCl --> FeCl2 + H2

  3. CuSO4 + Fe --> Fe2(SO4)3 + Cu
    The Cu is ok, but there needs to be 2 Fe's on the left:
    CuSO4 + 2Fe --> Fe2(SO4)3 + Cu
    In this reaction, the SO4 ion stays as SO4, so you would treat that as one entity and since there are 3 SO4's on the right, you would place 3 in front of the CuSO4 to balance the SO4's:
    3CuSO4 + 2Fe --> Fe2(SO4)3 + Cu
    Now this changes the number of Cu's on the left to three, so place a 3 in front of the Cu on the right and the complete reaction will be balanced:
    3CuSO4 + 2Fe --> Fe2(SO4)3 + 3Cu

  4. BaCl2 + Na2SO4 --> NaCl + BaSO4
    The Ba is ok at this point, but there needs to be 2 Na's and 2 Cl's on the right. Since these are together as NaCl on the right, you need only place a 2 in front of NaCl giving:
    BaCl2 + Na2SO4 --> 2NaCl + BaSO4

  5. C3H8 + O2 --> CO2 + H2O
    Start with the Carbon where there are 3 on the left and 1 on the right, placing 3 in front of the CO2 on the right will balance the Carbons:
    C3H8 + O2 --> 3CO2 + H2O
    Since there are H's connected to the carbon on the left, balance the H's next. Seeing that there are 8 on the left, place a 4 in front of the H2O to give 8 on the right:
    C3H8 + O2 --> 3CO2 + 4H2O
    Now count up the Oxygen's on each side: 2 on the left and 10 on the right. Place a 5 in front of the O2 on the left to give 10 on the left and they will be balanced:
    C3H8 + 5O2 --> 3CO2 + 4H2O


  1. Atomic mass Na = 22.99
    Atomic mass Cl = 35.45
    Atomic mass NaCl = 58.44 ; i.e., there are 58.44 grams in one mole of NaCl

          1 mole      
      100 g --------- = 1.71 mole  
          58.44 g      
          6.022 1023 atoms    
      1.71 mole ------------------------- = 10.3 1023 atoms
    (or 1.03 1024 atoms)

  1. First determine the number of moles of water (H2O) in 108 grams (atomic mass of water equals 18.0):
    one mole of H2O = 2 x 1.0 g.of H + 16.0 g. of O = 18 g./mole of H2O, therefore
          1 mole H2O      
      108 g.H2O ------------- = 6 moles H2O  
          18.0 g.H2O      

    From the balanced equation

    2H2 + O2 --> 2H2O

    we see that 2 moles of water are produced from 2 moles of hydrogen and 1 mole of oxygen. Therefore, 6 moles of water (i.e., 3 x 2 moles) are produced from 6 moles of hydrogen (3 x 2 moles) and 3 moles of oxygen (3 x 1 mole)

    Since a mole of hydrogen contains 2 atoms of hydrogen, the atomic mass of one mole of hydrogen is 2 x 1.0 g. H = 2.0 g. H2; and a mole of oxygen contains 2 atoms of oxygen, the atomic mass of one mole of oxygen is 2 x 16.0 g. O = 32.0 g. O2. Then

          2.0 g.H2      
      6 mole H2 ------------- = 12.0 g.H2  
          1 mole H2      

          32.0 g.O2      
      3 mole O2 ------------- = 96.0 g.O2  
          1 mole O2      

    Note how the Law of Conservation of Mass is obeyed by these figures: i.e.,
    12.0 g. hydrogen + 96.0 g. oxygen = 108 g. water or
    108 g. total reactants = 108 g. total product





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