1.1 The periodic table: groups and periods  (Page 2/2)

Periods in the periodic table

A period is a horizontal row in the periodic table of the elements. Some of the trends that can be observed within a period are highlighted below:

• As you move from one group to the next within a period, the number of valence electrons increases by one each time.
• Within a single period, all the valence electrons occur in the same energy shell. If the period increases, so does the energy shell in which the valence electrons occur.
• In general, the diameter of atoms decreases as one moves from left to right across a period. Consider the attractive force between the positively charged nucleus and the negatively charged electrons in an atom. As you move across a period, the number of protons in each atom increases. The number of electrons also increases, but these electrons will still be in the same energy shell. As the number of protons increases, the force of attraction between the nucleus and the electrons will increase and the atomic diameter will decrease.
• Ionisation energy increases as one moves from left to right across a period. As the valence electron shell moves closer to being full, it becomes more difficult to remove electrons. The opposite is true when you move down a group in the table because more energy shells are being added. The electrons that are closer to the nucleus 'shield' the outer electrons from the attractive force of the positive nucleus. Because these electrons are not being held to the nucleus as strongly, it is easier for them to be removed and the ionisation energy decreases.
• In general, the reactivity of the elements decreases from left to right across a period.
• The formation of halides follows the general pattern: $\mathrm{X}{\mathrm{Cl}}_{\mathrm{n}}$ (where $\mathrm{X}$ is any element in a specific group and $\mathrm{n}$ is the number of that specific group.). For example, the formula for the halides of group 1 will be $\mathrm{X}\mathrm{Cl}$ , for the second group the halides have the formula $\mathrm{X}{\mathrm{Cl}}_2$ and in the third group the halides have the formula $\mathrm{X}{\mathrm{Cl}}_3$ . This should be easy to see if you remember the valency of the group and of the halides.

The formation of oxides show a trend as you move across a period. This should be easy to see if you think about valency. In the first group all the elements lose an electron to form a cation. So the formula for an oxide will be ${\mathrm{X}}_2\mathrm{O}$ . In the second group (moving from left to right across a period) the oxides have the formula $\mathrm{X}\mathrm{O}$ . In the third group the oxides have the formula ${\mathrm{X}}_2{\mathrm{O}}_3$ .

Several other trends may be observed across a period such as density, melting points and boiling points. These trends are not as obvious to see as the above trends and often show variations to the general trend.

Electron affinity and electronegativity also show some general trends across periods. Electron affinity can be thought of as how much an element wants electrons. Electron affinity generally increases from left to right across a period. Electronegativity is the tendency of atoms to attract electrons. The higher the electronegativity, the greater the atom attracts electrons. Electronegativity generally increases across a period (from left to right). Electronegativity and electron affinity will be covered in more detail in a later grade.

You may see periodic tables labeled with s-block, p-block, d-block and f-block. This is simply another way to group the elements. When we group elements like this we are simply noting which orbitals are being filled in each block. This method of grouping is not very useful to the work covered at this level.

Using the properties of the groups and the trends that we observe in certain properties (ionization energy, formation of halides and oxides, melting and boiling points, atomic diameter) we can predict the the properties of unknown elements. For example, the properties of the unfamiliar elements Francium (Fr), Barium (Ba), Astatine (At), and Xenon (Xe) can be predicted by knowing their position on the periodic table. Using the periodic table we can say: Francium (Group 1) is an alkali metal, very reactive and needs to lose 1 electron to obtain a full outer energy shell; Barium (Group 2) is an alkali earth metal and needs to lose 2 electrons to achieve stability; Astatine (Group 7) is a halogen, very reactive and needs to gain 1 electron to obtain a full outer energy shell; and Xenon (Group 8) is a noble gas and thus stable due to its full outer energy shell. This is how scientists are able to say what sort of properties the atoms in the last period have. Almost all of the elements in this period do not occur naturally on earth and are made in laboratories. These atoms do not exist for very long (they are very unstable and break apart easily) and so measuring their properties is difficult.

Exercise: elements in the periodic table

Refer to the elements listed below:

• Lithium ( $\mathrm{Li}$ )
• Chlorine ( $\mathrm{Cl}$ )
• Magnesium ( $\mathrm{Mg}$ )
• Neon ( $\mathrm{Ne}$ )
• Oxygen ( $\mathrm{O}$ )
• Calcium ( $\mathrm{Ca}$ )
• Carbon ( $\mathrm{C}$ )

1. belongs to Group 1
2. is a halogen
3. is a noble gas
4. is an alkali metal
5. has an atomic number of 12
6. has 4 neutrons in the nucleus of its atoms
7. contains electrons in the 4th energy level
8. has only one valence electron
9. has all its energy orbitals full
10. will have chemical properties that are most similar
11. will form positive ions