Question

Arrange calcium, strontium, arsenic, bromine, and chlorine in order of (a) increasing atomic size and (b) increasing first ionization energy.

Short answer

(a) Increasing atomic size: As < Cl < Br < Ca < Sr (b) Increasing first ionization energy: Sr < Ca < As < Br < Cl

Step-by-step solution

Locate the elements in the periodic table

To properly understand their periodic trends, we should first locate calcium (Ca), strontium (Sr), arsenic (As), bromine (Br), and chlorine (Cl) in the periodic table. Calcium (Ca) – Group 2, Period 4 Strontium (Sr) – Group 2, Period 5 Arsenic (As) – Group 15 (or Group 5A), Period 4 Bromine (Br) – Group 17 (or Group 7A), Period 4 Chlorine (Cl) – Group 17 (or Group 7A), Period 3 2. Arrange the elements in increasing order of atomic size

Determine the order of atomic size

We know that atomic size increases from top to bottom within a group and decreases from left to right across a period. Based on these trends and the placement of the elements in the periodic table, the increasing order of atomic size is: As < Cl < Br < Ca < Sr 3. Arrange the elements in increasing order of ionization energy

Determine the order of ionization energy

Ionization energy increases from left to right across a period and decreases from top to bottom within a group. With these trends and the position of the elements in the periodic table, the increasing order of first ionization energy is: Sr < Ca < As < Br < Cl

Key concepts

Atomic Size

Atomic size, or atomic radius, refers to the distance from the nucleus of an atom to the outermost electrons. It tells us how big an atom is. When you look at the periodic table, you'll notice that atomic size

• increases as you move down a group (column), because additional layers of electrons are added away from the nucleus.
• decreases as you move across a period (row) from left to right, due to the increasing number of protons pulling the electrons closer to the nucleus, making the atom more compact.

In the exercise, we see these trends reflected in the question of ordering elements by atomic size:
1. Arsenic (As) is smaller than chlorine (Cl) because arsenic is further to the left in the same period.
2. Chlorine (Cl) is smaller than bromine (Br) as they belong to the same group, and chlorine is higher up.
3. Bromine (Br) is smaller than calcium (Ca) because they are from adjacent groups on the same period.
4. Calcium (Ca) is smaller than strontium (Sr) as both are in Group 2, with calcium being higher in the group.

First Ionization Energy

First ionization energy is the energy required to remove the outermost electron from a neutral atom in its gaseous state. It essentially shows how tightly an atom holds onto its outermost electron:

• Ionization energy tends to increase as you move across a period from left to right since atoms have more protons pulling on the electrons, making them harder to remove.
• It decreases as you move down a group due to increasing distance between the nucleus and the outermost electrons, reducing the pull on the electrons.

Applying these principles to the exercise:
1. Strontium (Sr) has the lowest ionization energy, as it is the lowest in its group.
2. Calcium (Ca) follows, being above strontium in the same group.
3. Arsenic (As) has a higher energy requirement than both strontium and calcium, but less than bromine and chlorine, as it is located to their left.
4. Bromine (Br) and chlorine (Cl) require more energy, with chlorine needing the most due to its position further to the right and higher up in the group.

Group and Period in Periodic Table

The periodic table is organized into vertical columns called groups and horizontal rows known as periods. Understanding this layout helps in predicting element properties:

• Groups, such as the ones for calcium and strontium (Group 2), or bromine and chlorine (Group 17), contain elements with similar chemical characteristics because they have the same number of valence electrons.
• Periods, like Period 3 where chlorine resides, or Period 4 where arsenic, bromine, and calcium are found, indicate elements with the same number of electron shells.

By locating elements in their respective groups and periods, one can predict and explain trends, such as atomic size and ionization energy discussed earlier. The exercise showcases these typical trends, illustrating how moving across periods or down groups affects atomic and chemical behaviors.