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Cambridge IGCSE0625

The nuclear model of the atom

Physics 0625 Chapter Notes

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The nuclear model of the atom - The atomThe nuclear model of the atom - The nucleus
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In this chapter

  1. 1Structure of the atom
  2. 2Positive and negative ions
  3. 3The alpha-particle scattering experiment
  4. 4What the observations show
  5. 5Protons, neutrons and electrons
  6. 6Proton number and nucleon number
  7. 7Isotopes
  8. 8Nuclear fission and fusion
  9. 9Charge and mass of the nucleus

1. Structure of the atom

An atom has a positively charged nucleus. The nucleus contains protons and neutrons.

The negatively charged electrons are in orbit around the nucleus.

Examiner insight

“Candidates should know that electrons orbit the nucleus, that they are negatively charged and can be removed from the nucleus.”

Worked example 11 mark

What is the charge on the nucleus of an atom and what is the charge on the electrons in orbit around the nucleus?

A nucleus negative, electrons negative · B nucleus negative, electrons positive · C nucleus positive, electrons negative · D nucleus positive, electrons positive

  1. 1

    C: the nucleus is positive and the electrons are negative. [1]

2. Positive and negative ions

A neutral atom becomes an ion when it gains or loses electrons.

An atom forms a positive ion by losing electrons. An atom forms a negative ion by gaining electrons.

Examiner insight

“Some candidates did not recognise that removing an electron from a neutral atom leaves an ion that is positively charged.” “Few candidates recognised that atoms form positive ions by losing electrons.”

Worked example 11 mark

How do atoms form positive ions?

A by gaining electrons · B by gaining neutrons · C by losing electrons · D by losing protons

  1. 1

    C: by losing electrons. [1]

3. The alpha-particle scattering experiment

A beam of positively charged alpha particles is directed at a thin sheet of gold in a vacuum.

Most of the alpha particles pass straight through the sheet. A small number are deflected.

These observations led to the nuclear model of the atom.

4. What the observations show

Most alpha particles pass straight through, so the atom is mostly empty space. A small number are deflected. So the positive charge is concentrated in a very small nucleus at the centre of the atom. The nucleus contains most of the mass of the atom.

The positively charged alpha particles are deflected by a repulsive force from the nucleus.

Examiner insight

“Common incorrect responses referred to alpha particles or nuclei instead of atoms, or concluded that there is lots of empty space between atoms.” “Weaker candidates often referred to atoms instead of nuclei or made vague statements, for example that like charges repel instead of stating clearly that alpha particles are positively charged.”

Worked example 11 mark

When a beam of positively charged alpha particles is directed towards a thin sheet of gold in a vacuum, the following observations are made.

1 Most of the alpha particles pass straight through the sheet of gold. 2 A small number of alpha particles are deflected through angles greater than 4°.

Which row links each observation to its appropriate conclusion?

A 1: the atom consists mainly of empty space · 2: the negative charge is concentrated at the centre of the atom B 1: the atom consists mainly of empty space · 2: the positive charge is concentrated at the centre of the atom C 1: the negative charge is concentrated at the centre of the atom · 2: the atom consists mainly of empty space D 1: the positive charge is concentrated at the centre of the atom · 2: the atom consists mainly of empty space

  1. 1

    B: observation 1 shows empty space, observation 2 shows positive charge at the centre. [1]

Recap

  • An atom has a positively charged nucleus. Negatively charged electrons orbit it.
  • A positive ion forms when an atom loses electrons.
  • A negative ion forms when an atom gains electrons.
  • Extended: most alpha particles pass straight through, so the atom is mostly empty space.
  • Extended: a few are deflected, so the nucleus is small and positively charged.
  • Extended: the nucleus contains most of the mass of the atom.

5. Protons, neutrons and electrons

The nucleus is made of protons and neutrons. Electrons orbit the nucleus. They are not in it.

The relative charges are +1 for a proton, 0 for a neutron and –1 for an electron. The nucleus is positive and the electrons are negative.

Examiner insight

“The most common error was to state that electrons were one of the particles found in the nucleus, whilst others simply gave nucleons as the name of a particle in the nucleus.”

Worked example 11 mark

What is the charge on the nucleus of an atom and what is the charge on the electrons in orbit around the nucleus?

A negative, negative · B negative, positive · C positive, negative · D positive, positive

  1. 1

    C: the nucleus is positive and the electrons are negative. [1]

6. Proton number and nucleon number

The proton number (atomic number) Z is the number of protons in the nucleus. The nucleon number (mass number) A is the number of protons plus neutrons.

The nuclide notation puts A above Z in front of the symbol. The number of neutrons is A - Z.

^(A)_ZX

neutrons = A - Z

Examiner insight

“A common error was to state that the number of neutrons was 241.” “A poor understanding of how to use nuclide notation was evident here.”

Worked example 12 marks

An isotope of uranium has 92 protons and 143 neutrons in each nucleus.

Complete the nuclide notation for this isotope.

  1. 1

    nucleon number = 92 + 143 = 235 [1]

  2. 2

    ²³⁵₉₂U [1]

7. Isotopes

Isotopes of an element have the same number of protons but a different number of neutrons. So they have the same proton number and different nucleon numbers.

An element may have more than one isotope. An isotope need not be radioactive.

Examiner insight

“The most common error was to state that the number of neutrons was the same, and that the number of protons differed.” “A significant proportion of candidates of all abilities thought that isotopes must be radioactive/unstable.”

Worked example 12 marks

Iodine-123 is another isotope of iodine.

State the meaning of the term, isotope.

  1. 1

    isotopes have the same number of protons in the nucleus [1]

  2. 2

    and a different number of neutrons in the nucleus [1]

8. Nuclear fission and fusion

In nuclear fission a neutron hits an unstable nucleus, such as a uranium nucleus. The nucleus splits into smaller nuclei. A large amount of energy is released.

In nuclear fusion nuclei join to form a larger nucleus. Fusion takes place in stars, where hydrogen is converted into helium.

In both, the total mass of the products is less than that of the original nuclide, and energy is released. In an equation the proton numbers and the nucleon numbers balance.

²₁H + ³₁H → ⁴₂He + ¹₀n

Examiner insight

“It was important that the reference was to nuclei splitting and not atoms, isotopes or elements splitting.” “The two main causes of error were candidates who explained nuclear fusion by using a word such as “fuse” or “fusing” or who made no reference to the involvement of nuclei.”

9. Charge and mass of the nucleus

The relative charge on a nucleus equals its proton number. The relative mass of a nucleus equals its nucleon number.

So a nucleus with proton number 11 and nucleon number 23 has relative charge +11 and relative mass 23.

Examiner insight

“However, the majority of weaker candidates had the misconception that the number of protons is calculated by subtracting the atomic number from the mass number, and therefore chose option B.”

Worked example 11 mark

A nuclide has the symbol ²³₁₁Na. Which statement about all atoms of this nuclide is correct?

A There are 11 protons in the nucleus. · B There are 23 neutrons in the nucleus. · C There are 11 electrons in the nucleus. · D There are 34 nucleons in the nucleus.

  1. 1

    A: the lower number, 11, is the proton number, so there are 11 protons. [1]

Recap

  • The nucleus holds protons and neutrons. Relative charges: proton +1, neutron 0, electron –1.
  • Proton number Z = number of protons. Nucleon number A = protons + neutrons. Neutrons = A - Z.
  • Isotopes: same number of protons, different number of neutrons.
  • Extended: fission is the splitting of a nucleus. Fusion is the joining of nuclei. Both release energy.
  • Extended: relative charge of a nucleus = proton number. Relative mass = nucleon number.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Name the two types of particle found in the nucleus of an atom.1 mark
  2. State the charge on the nucleus of an atom and the charge on an electron.2 marks
  3. Describe the structure of an atom.3 marks
  4. State how a neutral atom becomes a positive ion.1 mark
  5. A neutral atom has 5 electrons. It loses 2 electrons. State the number of electrons in the ion, and whether the ion is positive or negative.2 marks
  6. A fluorine atom has 9 electrons. It forms an ion with 10 electrons. State whether the ion is positive or negative. Explain your answer. State whether the nucleus changes.3 marks
  7. In the alpha-particle scattering experiment, most alpha particles pass straight through the gold sheet. State what this shows about the atom.1 mark
  8. A small number of the alpha particles are deflected by the gold sheet. State two things this shows about the nucleus of an atom.2 marks
  9. A student directs alpha particles at a thin sheet of silver in a vacuum. The student writes two statements. Statement 1: Most alpha particles pass straight through, so there is lots of empty space between the atoms. Statement 2: A few alpha particles are deflected by the electrons, because like charges repel. Each statement contains a mistake. Write a correct conclusion for statement 1. For statement 2, name the part of the atom that deflects the alpha particles and explain the deflection.4 marks
  10. A student directs 12 000 alpha particles at a thin sheet of copper in a vacuum. 11 997 of them pass straight through the sheet. The rest are deflected. (a) Calculate the fraction of the alpha particles that are deflected. Give your answer as a decimal. [2] (b) State what your answer to (a) shows about the nucleus. Explain your answer. [2] (c) State two other things about the nucleus that the scattering of alpha particles gives evidence for. [2]6 marks
  11. State the two types of particle found in the nucleus of an atom.1 mark
  12. Complete the table to give the relative charge of each particle. | particle | relative charge | | --- | --- | | proton | ? | | neutron | ? | | electron | ? |3 marks
  13. A nucleus of argon has the symbol ⁴⁰₁₈Ar. State the number of protons and the number of neutrons in it.2 marks
  14. A nucleus of copper (Cu) contains 29 protons and 34 neutrons. Give its nucleon number and write its nuclide notation.2 marks
  15. Chlorine has two isotopes, chlorine-35 and chlorine-37. The proton number of chlorine is 17. State one similarity and one difference between their nuclei, and the number of neutrons in each.4 marks
  16. The table shows four nuclei. Which two are isotopes of the same element? Give a reason. | nucleus | protons | neutrons | | --- | --- | --- | | P | 12 | 12 | | Q | 12 | 13 | | R | 13 | 12 | | S | 14 | 14 |2 marks
  17. A nucleus has proton number 26 and nucleon number 56. State its relative charge and its relative mass.2 marks
  18. Describe what happens in nuclear fission.3 marks
  19. In a star, two nuclei of helium-3 undergo nuclear fusion. The products are one nucleus of helium-4 and two identical particles, X. ³₂He + ³₂He → ⁴₂He + 2X Determine the nucleon number and the proton number of X. State what particle X is. State how the total mass of the products compares with the total mass of the two helium-3 nuclei.4 marks
  20. A nucleus of plutonium-239 has proton number 94. It absorbs one neutron and then splits into two smaller nuclei, P and Q, and some neutrons. Nucleus P has relative charge +54 and relative mass 134. Nucleus Q contains 63 neutrons. Determine the proton number of Q, the nucleon number of Q and the number of neutrons released.5 marks

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