Trends in the Periodic Table
Structure of the Periodic Table
- Rows: 7 periods (horizontal)
- Columns: 18 groups (vertical)
- Elements are arranged by atomic number (number of protons)
Period and Group Significance
- Period: The period number of an element tells you the highest principal energy level (or electron shell) that contains electrons in that atom. Both sodium (Na) and chlorine (Cl) are in Period 3. Their electron configurations show this:
- Example: Sodium (Na) in Period 3 has electron configuration 2, 8, 1
- Chlorine (Cl) in Period 3 has electron configuration 2, 8, 7
Sodium (Na): 2, 8, 1 The "2" represents the first energy level (with 2 electrons), the "8" represents the second energy level (with 8 electrons), and the "1" represents the third energy level (with 1 electron). The highest occupied energy level is the third, corresponding to Period 3.
Chlorine (Cl): 2, 8, 7 Similarly, chlorine's electron configuration shows electrons in the first, second, and third energy levels. The highest occupied energy level is still the third, again corresponding to Period 3.
The key is that the largest number in the electron configuration corresponds to the period.
- Group: Indicates the number of electrons in the outer shell
- Example: Carbon (C), atomic number 6, Period 2, Group 14, has electron configuration 2, 4
The group number of an element indicates the number of electrons in its outermost electron shell, also known as the valence shell. These valence electrons are crucial for determining the element's chemical behavior.
Let's use sodium (Na) and chlorine (Cl) as examples to illustrate the relationship between group number and outer shell electrons:
Sodium (Na):
- Atomic Number: 11 (meaning 11 protons and 11 electrons)
- Period: 3 (meaning its electrons occupy up to the third energy level)
- Group: 1 (meaning it has 1 electron in its outermost shell)
- Electron Configuration: 2, 8, 1. The "1" at the end shows 1 electron in the third (outermost) energy level. The group number (1) directly reflects this single valence electron.
Chlorine (Cl):
- Atomic Number: 17 (meaning 17 protons and 17 electrons)
- Period: 3 (meaning its electrons occupy up to the third energy level)
- Group: 17 (meaning it has 7 electrons in its outermost shell)
- Electron Configuration: 2, 8, 7. The "7" at the end signifies 7 electrons in the third (outermost) energy level. Again, the group number (17) tells us directly how many valence electrons are present (we take the ones digit, 7, in the modern periodic table).
In both cases, the group number directly reflects (in the modern table) the number of valence electrons found in the outermost energy level of the atom. These valence electrons are key for understanding how these atoms will interact and bond with other atoms.
Chemical Reactions and Group Properties
- Chemical reactions involve electron movement
- Elements in the same group have similar chemical properties due to the same number of outer shell electrons
Trends Across Periods (Left to Right)
- Change from Metals to Non-Metals
- Decrease in Atomic Radius
- More protons pull electrons closer
- Increase in First Ionisation Energy
- Energy needed to remove the outermost electron
- Increase in Electronegativity
- Attraction of a bonded atom for the electron pair in a covalent bond
Trends Down a Group
- Becoming More Metallic
- Example: Group 14 - Carbon (non-metal) vs Lead (metal)
- Increase in Atomic Radius
- Extra electron shell added for each subsequent element
- Decrease in First Ionisation Energy
Study Focus: Key Groups
- Group 1: Alkali Metals
- More reactive down the group as it's easier to lose the outer shell electron
- Group 17: Halogens
- Less reactive down the group as fewer shells lead to less attraction for gaining electrons
- Group 18: Noble Gases
- Full outer shell, very unreactive
- Increase in densities and boiling points down the group
Summary
- Understanding periodic trends is crucial for predicting element behavior in chemical reactions.
- Groups reflect similarities in reactivity and properties due to shared electron configurations.