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2.1-2.2

Atomic number, nucleon number and particles

Your guide: Miss SanaCounts atoms the way you count rupees: carefully.

Atomic number, nucleon number and particles

Two atoms can look alike. The nucleus tells them apart.

Let us count protons, neutrons and electrons.

1 · Moseley and the atomic number

In 1913, Moseley hit different elements with cathode rays. They gave X-rays of fixed frequencies.

The square root of the frequency was directly proportional to the atomic number Z.

So Z is a basic property of an element. It is also called the proton number.

Protons plus neutrons make the nucleon number A. It is also the mass number.

The book gives A = Z + N. So N = A − Z.

2 · Counting particles in atoms and ions

For 27Al: Z = 13 and A = 27. Neutrons: 27 − 13 = 14.

Al3+ loses three electrons. Electrons: 13 − 3 = 10. Protons stay 13.

35Cl gains one electron to make Cl−. Electrons: 17 + 1 = 18. Neutrons: 35 − 17 = 18.

Table 2.1: O2− has 8 neutrons, 8 protons and 10 electrons. S2− has 16, 16 and 18. P3− has 16, 15 and 18.

3 · Particles in an electric field

Neutrons have no charge. They go straight and are not deflected.

Protons are positive. They bend towards the negative plate.

Electrons are negative. They bend towards the positive plate, and much more.

The reason: an electron is 1/1836 of the mass of a proton, as the book says.

Table 2.2: proton mass 1.0073 amu, neutron 1.0087 amu, electron 5.4858 × 10−4 amu.

Step 1 / 7

Notes, short questions and MCQs

Read the full notes: key terms, model answers and MCQs with answers

Atomic number, nucleon number and particles

Two atoms can look alike. The nucleus tells them apart.

Let us count protons, neutrons and electrons.

1 · Moseley and the atomic number

In 1913, Moseley hit different elements with cathode rays. They gave X-rays of fixed frequencies.

The square root of the frequency was directly proportional to the atomic number Z.

So Z is a basic property of an element. It is also called the proton number.

Protons plus neutrons make the nucleon number A. It is also the mass number.

The book gives A = Z + N. So N = A − Z.

2 · Counting particles in atoms and ions

For 27Al: Z = 13 and A = 27. Neutrons: 27 − 13 = 14.

Al3+ loses three electrons. Electrons: 13 − 3 = 10. Protons stay 13.

35Cl gains one electron to make Cl−. Electrons: 17 + 1 = 18. Neutrons: 35 − 17 = 18.

Table 2.1: O2− has 8 neutrons, 8 protons and 10 electrons. S2− has 16, 16 and 18. P3− has 16, 15 and 18.

3 · Particles in an electric field

Neutrons have no charge. They go straight and are not deflected.

Protons are positive. They bend towards the negative plate.

Electrons are negative. They bend towards the positive plate, and much more.

The reason: an electron is 1/1836 of the mass of a proton, as the book says.

Table 2.2: proton mass 1.0073 amu, neutron 1.0087 amu, electron 5.4858 × 10−4 amu.

Key terms

Atomic number Z
The number of protons in the nucleus.
Nucleon number A
Protons plus neutrons. Also the mass number.
Cation
An atom that has lost electrons.
Anion
An atom that has gained electrons.

Short questions with model answers

  1. Q1. Quick Check 2.1 (a): neutrons in 39K (Z = 19), 35Cl (Z = 17) and 40Ar (Z = 18).

      39 − 19 = 20, 35 − 17 = 18 and 40 − 18 = 22.

    • Q2. Exercise Q.2 (e): an atom has Z = 17 and A = 35. Find the particles, then for the −1 ion.

        Atom: 17 protons, 35 − 17 = 18 neutrons, 17 electrons. Ion: 17 protons, 18 neutrons, 17 + 1 = 18 electrons.

      Common mistakes

      • ✗ “The mass number counts electrons too.”

        ✓ It counts only protons and neutrons.

      • ✗ “A cation gains electrons.”

        ✓ A cation loses electrons. Al3+ has 13 − 3 = 10.

      MCQs

      1. 1. How many neutrons are in 39K with Z = 19?

        1. (a) 19
        2. (b) 20
        3. (c) 39
        4. (d) 58
        Show answer

        (b) 39 − 19 = 20.

      2. 2. How many electrons are in Al3+? (Z = 13)

        1. (a) 16
        2. (b) 13
        3. (c) 10
        4. (d) 3
        Show answer

        (c) 13 − 3 = 10.

      3. 3. How many electrons are in S2−? (Z = 16)

        1. (a) 14
        2. (b) 16
        3. (c) 18
        4. (d) 32
        Show answer

        (c) 16 + 2 = 18.

      4. 4. In an electric field, which particle bends the most?

        1. (a) Neutron
        2. (b) Proton
        3. (c) Electron
        4. (d) All the same
        Show answer

        (c) It is the lightest, about 1/1836 of a proton.

      5. 5. Which relation did Moseley find?

        1. (a) Frequency is proportional to Z.
        2. (b) Square root of frequency is proportional to Z.
        3. (c) Frequency is proportional to A.
        4. (d) Frequency does not depend on Z.
        Show answer

        (b) The book states this in section 2.1.

      Quick revision

      • Z is the number of protons. A is protons plus neutrons.
      • Neutrons: N = A − Z.
      • A cation loses electrons. An anion gains electrons.

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