5.1 Radioactivity

5.0

Radioactivity

5.1

Understanding the nucleus of an atom

a.

Nucleus

1.

Rutherford proposed in 1911, a nuclear model of the atom as shown in Figure 1 c, the atom has a very small dense core called the nucleus. All the positively charged particles and most of the mass of the atom are contained in the nucleus. The electrons move in orbits around the nucleus.

2.

Figure 1: Various models of the atom b.

Nucleons

1.

The particles in a nucleus are called nucleons. There are two types of nucleons, protons and neutrons. A proton is a positively charged particle. It carries a charge of +e, where e = 1.6 x 10-19 C.

Figure 2: Geiger – Marsden’s experiment: Discovery of the nucleus 2.

The number of protons in a nucleus is known as the proton number, Z.

3.

The nucleon number, A gives the total number of protons and neutrons in a nucleus. It is also known as the mass number. 1

4.

The number of neutrons in a nucleus is A+Z.

c.

Nuclide and notation

7. 8.

Nuclide is a type of nucleus with a particular proton and nucleon number. The nuclide notation for an element X is ZA X.

d.

Isotopes

1.

Isotopes are atoms of an element which have the same proton number but different nucleon numbers.

Example Questions: ☼

What is the proton number? Number of proton or atomic number

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What are the nucleons? Protons and neutrons

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What is the mass number? Total number of nucleons

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A neutral atoms consist of 17 electrons, 17 protons and 18 neutrons. What is its mass number? 17+18=35

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What is the nuclide? A type of nucleus

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A helium nucleus with proton number 2 and nucleon number 4. Write the nuclide notation of helium. 4 2 He

Questions: 1. A particular nuclide of tin has atomic number 50 and mass number 118. (a)

How many electrons are there outside the nucleus in a neutral atom of tin?

(b)

How many neutrons are there in the nucleus?

(c) How many protons are there in the nucleus? Suggest possible values for the atomic number and mass number of one

2

isotope of this tin nuclide. 2.

3.

U is an isotope of uranium. (a) What quantity is the same for the nuclei of all isotopes of uranium? (b) In each nucleus of 235 92 U , how many (i) protons (ii) neutrons, are there? 235 92

112 50

[1]

[2]

Sn is a stable isotope of tin.

What information does the symbol

112 50

Sn give us about the isotope?

[2]

4. A chlorine atom, proton number 17, can gain an electron and become a chlorine ion. A sodium atom, proton number 11, can lose an electron and become a sodium ion. (a) What is the sign of the charge on? (i) A chlorine ion, (ii) A sodium ion? Explain how you arrived at your answers. (b) How many electrons are contained in? (i) A chlorine ion, (ii) A sodium ion? 5.

[3]

[2]

Atoms P and Q are isotopes. How does the composition of neutral atom P compare with neutral atom

Answers: 1. (a) Number of electrons = 50 (b) Number of neutrons = 118 - 50 = 68 (c) Number of protons = 50 One isotope of this tin nuclide can have atomic number of 50 and mass number of 119. 2. (a) Atomic number (Proton number) (b) (i) Number of protons = Atomic number = 92 (ii) Number of neutrons = Nucleon number - Atomic number = 235 – 92 = 143

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3. The nucleus of this isotope contains 50 protons and 62 neutrons. 4. (a) (i) Negative (ii) Positive A proton and an electron carry the same amount of charge but a proton has a positive charge and an electron has a negative charge. A neutral atom contains same number of protons and electrons. If the atom gains an electron, there is excess negative charge. If the atom loses an electron, there is excess positive charge. (b) (i) A chlorine ion contains 18 electrons. (ii) A sodium ion contains 10 electrons. 5.

C

Isotopes are elements with the same number of protons but different number of neutrons. For neutral atoms, the number of protons is equal to the number of electrons.

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