Cations and Anions

Q26: Describe the process by which positive (cation) and negative (anions) are formed from atoms, including the role of electrons in this transformation?

Cations & Anions

What is a Cation?

A cation is a positively charged ion. It forms when an atom loses one or more electrons. Since protons remain the same, the atom gets a net positive charge.

Example: Sodium (Na)

  • Sodium atom (Na) has 11 protons and 11 electrons (neutral charge).
  • It loses one electron, becoming Na⁺ with 11 protons and 10 electrons.
  • Since protons are more than electrons, Na⁺ has a +1 charge.

What is an Anion?

An anion is a negatively charged ion. It forms when an atom gains extra electrons, making electrons more than protons.

Example: Chlorine (Cl)

  • Chlorine atom (Cl) has 17 protons and 17 electrons (neutral charge).
  • It gains one extra electron, becoming Cl⁻ with 17 protons and 18 electrons.
  • Since electrons are more than protons, Cl⁻ has a -1 charge.

Formation of Cations

Describe the formation of Na+ + and Mg2+ cations.

(a) Formation of Na+ ion:

Na: 1s2 2s2 2p6 3s1 → Na+: 1s2 2s2 2p6

(b) Formation of Mg2+ ion:

Mg: 1s2 2s2 2p6 3s2 → Mg2+: 1s2 2s2 2p6

Describe the formation of cations for the following metal atoms
(a) Li (atomic no 3)
(b) Al (atomic no 13)

Formation of L i+ and A l3+ Cations

(a) Formation of Li+ ion:

Li: 1s2 2s1 → Li+: 1s2

(b) Formation of Al3+ ion:

Al: 1s2 2s2 2p6 3s2 3p1 → Al3+: 1s2 2s2 2p6

Describe the formation of anions for the following non-metal atoms..
(a) oxygen (atomic no.8)
(b) Fluorine (atomic no 9)

Formation of Anions

(a) Formation of anion by oxygen atom:

Oxygen belongs to Group VIA on the periodic table. It has six electrons in its valence shell and needs two electrons to achieve noble gas configuration.
O: 1s2 2s2 2p4 + 2e- → O2-: 1s2 2s2 2p6 (octet)

(b) Formation of anion by fluorine atom:

Fluorine belongs to Group VIIA on the periodic table. It has seven electrons in its valence shell and requires one electron to complete its octet.
F: 1s2 2s2 2p5 + 1e- → F-: 1s2 2s2 2p6 (octet)

Describe the formation of anions for the following non-metal atoms.
(a) Sulphur (atomic no.16).
(b) Chlorine (atomic no.17)

Formation of Sulfur and Chlorine Anions

(a) Sulfur (Atomic No. 16):

S + 2e- → S2-
Sulfur Electron Configuration:
S: 1s2 2s2 2p6 3s2 3p4
S2-: 1s2 2s2 2p6 3s2 3p6

(b) Chlorine (Atomic No. 17):

Cl + 1e- → Cl-
Chlorine Electron Configuration:
Cl: 1s2 2s2 2p6 3s2 3p5
Cl-: 1s2 2s2 2p6 3s2 3p6

PlantUML DiagramPlantUML DiagramPlantUML Diagram

Q28: How can we assign symbols for atoms and ions?

Symbols for Atoms and Ions

Atoms and ions are represented by symbols. These symbols use one or two letters. They may also have numbers:

  • A mass number at the top left
  • An atomic number at the bottom left
  • A charge at the top right

For example, the symbol for a magnesium ion (Mg²⁺) has:

  • Mass number: 24 (upper left)
  • Atomic number: 12 (lower left)
  • Charge: +2 (upper right)

Student Learning Outcome

  • Radioactivity can change the number of protons and the identity of an atom.
  • Some isotopes are radioactive.
  • Isotopes are useful in carbon dating and medical imaging.

3.6 Cations and Anions

Cations

Definition and Formation

Positively charged ions formed when an atom loses one or more electrons.
Usually formed from metal atoms.
Aim to achieve a stable electron configuration similar to noble gases.

Characteristics

Number of protons< Number of electrons, resulting in a positive charge.

Example 3.1: Formation of Cations

  • Na+ formation:
    Na: 1s² 2s² 2p⁶ 3s¹ → Na+: 1s² 2s² 2p⁶
  • Mg2+ formation:
    Mg: 1s² 2s² 2p⁶ 3s² → Mg2+: 1s² 2s² 2p⁶

Problem Solving Strategy for Cation Formation

  1. Identify the element's group in the periodic table.
  2. Determine the number of valence electrons.
  3. Represent the loss of valence electrons using electronic configuration or electron dot structure.

Anions

Definition and Formation

Negatively charged ions formed when an atom gains one or more electrons.
Usually occurs in atoms with high electron affinity.
Aim to achieve a stable electron configuration similar to noble gases.

Characteristics

Number of electrons < Number of protons, resulting in a negative charge.

Example 3.1: Formation of Anions

  • O2- formation:
    O: 1s² 2s² 2p⁴ + 2e⁻ → O2-: 1s² 2s² 2p⁶
  • F- formation:
    F: 1s² 2s² 2p⁵ + e⁻ → F-: 1s² 2s² 2p⁶

Problem Solving Strategy for Anion Formation

  1. Write the electronic configuration or dot structure.
  2. Determine the number of electrons needed to achieve an octet configuration.
  3. Represent the addition of electrons.

Concept Assessment Exercises

  • Exercise 3.2: Describe cation formation for Li and Al.
  • Exercise 3.3: Describe anion formation for S and Cl.