Structure of the Atom • Topic 3 of 3

Valency, Atomic Number, Mass Number & Isotopes

Once we know how electrons are arranged in shells, we can describe an atom completely using a few key numbers.

Valence electrons and valency

The electrons present in the outermost shell of an atom are called valence electrons. The valency is the combining capacity of an atom — it tells us how many electrons an atom can lose, gain or share to complete its outermost shell (to reach a stable octet of 8, or a duplet of 2 for the first shell).

  • If the outermost shell has 1, 2 or 3 electrons, the atom tends to lose them; valency = number of valence electrons. Example: sodium (2, 8, 1) has valency 1.
  • If the outermost shell has 4, 5, 6 or 7 electrons, the atom tends to gain electrons; valency = 8 − (valence electrons). Example: chlorine (2, 8, 7) has valency 8 − 7 = 1.
  • Atoms with a full outermost shell (8 electrons, or 2 for helium) are stable and have valency 0 — these are the noble gases.

Atomic number (Z)

The atomic number Z is the number of protons in the nucleus of an atom. In a neutral atom, the number of protons equals the number of electrons, so Z also equals the number of electrons. The atomic number decides which element an atom is.

Mass number (A)

The mass number A is the total number of protons and neutrons (nucleons) in the nucleus: A = number of protons + number of neutrons. Therefore the number of neutrons = A − Z. An atom of an element X is written as AZX, for example 2311Na or 3517Cl.

Isotopes

Isotopes are atoms of the same element (same atomic number Z) that have different mass numbers A — that is, the same number of protons but different numbers of neutrons. Hydrogen has three isotopes: protium 11H, deuterium 21H and tritium 31H. Some isotopes have important uses:

  • Uranium-235 (23592U) is used as fuel in nuclear reactors and atomic energy.
  • Cobalt-60 (Co-60) is used in radiotherapy to treat cancer.
  • Iodine-131 (I-131) is used to treat goitre and diagnose thyroid disorders.

Isobars

Isobars are atoms of different elements (different atomic numbers) that have the same mass number. For example, calcium 4020Ca and argon 4018Ar are isobars.

Why atomic mass can be fractional

Most elements exist as a mixture of isotopes in fixed proportions. The atomic mass listed in the periodic table is the average mass of all the isotopes, weighted by how common each one is. Because it is an average, it is often a fraction — for example, the atomic mass of chlorine is about 35.5 u, since natural chlorine is roughly 75% 35Cl and 25% 37Cl.

Isotopes of Hydrogen (same Z, different A)
IsotopeSymbolProtons (Z)NeutronsMass No. (A)
ProtiumH-1101
DeuteriumH-2 (D)112
TritiumH-3 (T)123
Solved Examples
1
Worked Example
Find the valency of magnesium (Z = 12) from its electronic configuration.
Solution
  1. Configuration of Mg (Z = 12): 2, 8, 2.
  2. The outermost shell has 2 electrons (valence electrons).
  3. Since there are fewer than 4, the atom loses them; valency = number of valence electrons = 2.

Answer: Valency of magnesium = 2.

2
Worked Example
Find the valency of oxygen (Z = 8).
Solution
  1. Configuration of O (Z = 8): 2, 6.
  2. The outermost shell has 6 electrons.
  3. Since this is more than 4, the atom gains electrons; valency = 8 − 6 = 2.

Answer: Valency of oxygen = 2.

3
Worked Example
An atom has atomic number Z = 17 and mass number A = 35. Find the number of protons, electrons and neutrons.
Solution
  1. Number of protons = Z = 17.
  2. In a neutral atom, electrons = protons = 17.
  3. Number of neutrons = A − Z = 35 − 17 = 18.

Answer: Protons = 17, electrons = 17, neutrons = 18.

4
Worked Example
An atom is written as 2311Na. State its number of protons, electrons and neutrons.
Solution
  1. The lower number is Z = 11 = number of protons.
  2. Neutral atom ⇒ electrons = 11.
  3. The upper number is A = 23, so neutrons = A − Z = 23 − 11 = 12.

Answer: Protons = 11, electrons = 11, neutrons = 12.

5
Worked Example
Two atoms P and Q have the data P (Z = 6, A = 14) and Q (Z = 7, A = 14). Are they isotopes or isobars?
Solution
  1. Compare their atomic numbers: Z = 6 and Z = 7 are different, so they are different elements.
  2. Compare their mass numbers: both have A = 14 (same).
  3. Different elements with the same mass number are isobars.

Answer: P and Q are isobars.

6
Worked Example
Chlorine occurs as 75% 3517Cl and 25% 3717Cl. Find its average atomic mass.
Solution
  1. Average mass = (fraction of first × mass) + (fraction of second × mass).
  2. = (75/100 × 35) + (25/100 × 37).
  3. = 26.25 + 9.25 = 35.5 u.

Answer: Average atomic mass of chlorine = 35.5 u (fractional because it is a weighted average of isotopes).

Key Points

  • Valence electrons are the electrons in the outermost shell; valency is the combining capacity (= valence electrons if 1–3, or 8 − valence electrons if 4–7, and 0 for noble gases).
  • Atomic number Z = number of protons = number of electrons in a neutral atom; it identifies the element.
  • Mass number A = protons + neutrons, so neutrons = A − Z; an atom is written as AZX.
  • Isotopes have the same Z but different A (e.g. 11H, 21H, 31H); uses include U-235 (reactor fuel), Co-60 (cancer therapy), I-131 (thyroid/goitre).
  • Isobars have the same A but different Z (e.g. 4020Ca and 4018Ar); atomic mass is often fractional because it is a weighted average of isotopes.
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.The valency of sodium (configuration 2, 8, 1) is:
Explanation: Sodium has 1 valence electron, which it loses, so its valency is 1.
Q2.The number of neutrons in an atom with Z = 17 and A = 35 is:
Explanation: Neutrons = A − Z = 35 − 17 = 18.
Q3.Isotopes of an element have the same:
Explanation: Isotopes have the same atomic number (same protons) but different mass numbers.
Q4.The isotope used to treat cancer (in radiotherapy) is:
Explanation: Cobalt-60 is used in radiotherapy for treating cancer; I-131 is used for thyroid disorders and U-235 as reactor fuel.
Practice more MCQs → 📝 Assignment · 30 marks