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:
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:
Symbol Representing an Atom
Consider the following symbol:
A = 31
Z = 15
X = P
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:
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.
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
|orbit (n=)||Maximum # electrons|
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.
The second orbital (p) is arranged in three separate suborbitals in the shape of dumbbells, each oriented along different axes.
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 ½ and -½. The Pauli exclusion principle states that no two electrons in the same atom can have the same set of quantum numbers.
|Energy Level (n)||Orbital Type (l)||Number of Suborbitals||Electrons per orbital||Total # electrons per Orbital Type|
Electrons fill the orbitals starting from the one having the lowest energy. The following diagram illustrates this filling order:
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.,
|Element||Atomic Number||Electron Configuration|
|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.
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
In naming ionic compounds, the name of the metal cation is first and the name of the nonmetal anion with the suffix -ide is second. The chemical name for table salt (NaCl) is sodium chloride. Refer to the text, section 4.8, pages 86-88.
In naming covalent compounds (with two elements), the name of the first element is used with a prefix indicating the number of atoms present (e.g., H2O, H2 = dihydrogen). Then, the name of the second element, with the suffix -ide and the prefix indicating the number of atoms present (e.g., H2O, O = oxide). The chemical name for water, H2O, is dihydrogen oxide.
|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
- Balance the following chemical equations and name the type of chemical reaction for each:
- KClO3 KCl + O2
- Fe + HCl FeCl2 + H2
- CuSO4 + Fe Fe2(SO4)3 + Cu
- BaCl2 + Na2SO4 NaCl + BaSO4
- 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:
(6.98 × 1022 atoms is the correct way to express the answer in scientific notation with the correct number of significant fiqures; see Apendices A & D)
6.022 × 1023 atoms 0.116 mole × ---------------------- = 0.698 × 1023atoms mole
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 --> 2H2OThis 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
Answers to Problems
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.
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.
- Given this symbolTherefore
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
- The element with atomic number 48 is Cadmium (Cd)
- Molybdenum (Mo) has 42 protons (NOTE that the atomic number is 42)
- Tin (Sn) has an atomic mass of 118.7
- Cesium (Cs) has 55 electrons (NOTE that the atomic number is 55)
- Elements that have similar chemical properties to Boron (B) are:
- Aluminum (Al)
- Gallium (Ga)
- Indium (In)
- Thallium (Tl)
4. 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
Further explanations as to how to balance these equations:
- 2 KClO3 2 KCl + 3 O2
- Fe + 2 HCl FeCl2 + H2
Single replacement reaction
- 3 CuSO4 + 2 Fe Fe2(SO4)3 + 3 Cu
Single replacement reaction
- BaCl2 + Na2SO4 2 NaCl + BaSO4
Double replacement reaction
- C3H8 + 5 O2 3 CO2 + 4 H2O
- 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
- 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
- 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
- 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
- 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
- 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
(or 1.03 × 1024 atoms)
6.022 × 1023 atoms 1.71 mole × ------------------------- = 10.3 × 1023 atoms mole Return
- 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
- The sum of the values of the superscripts is equal to the number of electrons in the atom. REMEMBER that this number is also equal to the atomic number for the atom.
- REMEMBER that only electrons are gained or lost.
I.e., to determine the number of protons and electrons found in an ion (using O-2 & Ca+2 as examples):
- First determine the atomic number of the element. That will tell you how many protons are in the ion and the number of electrons in a neutral atom.For O (oxygen) the atomic number is 8. I.e., it has 8 protons and 8 electrons.
For Ca (calcium) the atomic number is 20. I.e., it has 20 protons and 20 electrons.
- Then, if the ion has a negative charge, it has gained electrons. If the ion has a positive charge, it has lost electrons.For O-2, the charge is -2, therefore, it has gained 2 electrons. I.e., it has 8 protons and 10 electrons.
For Ca+2, the charge is +2, therefore, it has lost two electrons. I.e., it has 20 protons and 18 electrons.
NOTE that the number of protons did not change !!