The Nuclear Atom

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The Nuclear Atom

Atoms consist of a small, dense, positively charged nucleus surrounded by electrons.
The nucleus contains protons and neutrons, collectively called nucleons.
Almost all of the atom's mass is concentrated in the nucleus because protons and neutrons are much heavier than electrons.
Electrons occupy the space outside the nucleus, often described as a cloud of negative charge.
The atom is held together by the electrostatic attraction between the positive nucleus and the negative electrons.
The nucleus is very small compared to the overall size of the atom, so most of the atom is empty space.

The structure of an atom

Subatomic Particles

Protons have a relative charge of +1 and a relative mass of 1.
Neutrons have a relative charge of 0 and a relative mass of 1.
Electrons have a relative charge of –1 and a relative mass that is negligible (about 1/1836 of a proton).
The actual charge on an electron is –1.602189 × 10⁻¹⁹ C and on a proton is +1.602189 × 10⁻¹⁹ C.
Relative masses and charges are used because the actual values are extremely small.
These values can be found in the IB Data Booklet.

Atomic Number and Mass Number

The atomic number (Z) is the number of protons in the nucleus of an atom.
In a neutral atom, the atomic number is also equal to the number of electrons.
The mass number (A) is the total number of protons and neutrons (nucleons) in the nucleus.
Number of neutrons = mass number – atomic number.
Each element is represented by its chemical symbol with the mass number as a superscript and atomic number as a subscript.
The mass (nucleon) number and atomic (proton) number are given for each element in the Periodic Table.

Mass number (A) and atomic number (Z) notation with the chemical symbol (X).

Protons, Neutrons and Electrons in Atoms and Ions

All atoms and ions of the same element have the same number of protons.
A neutral atom has equal numbers of protons and electrons, so it has no overall charge.
A positive ion (cation) forms when electrons are lost, so it has fewer electrons than protons.
A negative ion (anion) forms when electrons are gained, so it has more electrons than protons.
The charge on an ion is the difference between the number of protons and electrons.
Number of protons = mass number – number of neutrons.
Number of electrons in an ion = atomic number – charge (for positive ions) or + charge (for negative ions).

The structure of the carbon atom

Isotopes

Isotopes are atoms of the same element with the same number of protons and electrons but different numbers of neutrons.
Isotopes have the same atomic number but different mass numbers.
For example, carbon-12 and carbon-14 both have 6 protons but contain 6 and 8 neutrons respectively.
Isotopes have the same chemical properties because they have the same electron configuration.
Isotopes have different physical properties due to differences in mass, such as density, melting/boiling point, and rate of diffusion.
Isotopes can be represented as carbon-12 or ¹²C, and carbon-14 or ¹⁴C.

Atomic number and mass number

Relative Atomic Mass

The relative atomic mass (Ar) is the ratio of the average mass of the atoms of an element to the unified atomic mass unit.
It is defined as the average mass of one atom of an element compared to one twelfth of the mass of an atom of carbon-12.
The relative atomic mass is calculated using the percentage abundances of the isotopes.
The equation is: Ar = (percentage abundance × mass) + (percentage abundance × mass) + … ÷ 100.
Percentage abundances are either given or can be read from a mass spectrum.
Round the final answer only after completing the full calculation, to the required number of decimal places.

The Periodic Table key showing where relative atomic mass (Ar) and atomic number (Z) are given for each element, using carbon as the example.

Mass Spectrometry (HL)

A mass spectrometer is used to find the percentage abundance of isotopes in an element.
The basic processes are: vaporisation, ionisation to form positive ions, acceleration, and detection.
Ions are detected as a mass-to-charge ratio (m/z).
The mass spectrum shows peaks corresponding to different isotopes.
The relative atomic mass can be calculated from the mass spectrum using the percentage abundances.
Specific details of the processes in mass spectrometry are not assessed.

Stages in a mass spectrometer

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練習問題

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1.Which statement correctly defines the atomic number of an element?
Easy
- AThe number of protons in the nucleus of an atom
- BThe total number of protons and neutrons in an atom
- CThe number of electrons in the outermost shell of an atom
- DThe number of neutrons in the nucleus of an atom
2.Which row correctly describes the relative charge and relative mass of a proton, neutron, and electron?
Easy
- AProton: +1, 1; Neutron: 0, 1; Electron: –1, negligible
- BProton: +1, 1; Neutron: 0, 0; Electron: –1, negligible
- CProton: +1, 1; Neutron: 0, 1; Electron: –1, 1
- DProton: 0, 1; Neutron: +1, 1; Electron: –1, negligible
3.The phosphide ion, ³²₁₅P³⁻, is used in medicine as a radiotherapy treatment for some forms of cancer. What is the composition of the phosphide ion?
Easy
- A15 protons, 17 neutrons, 18 electrons
- B15 protons, 17 neutrons, 15 electrons
- C17 protons, 15 neutrons, 18 electrons
- D15 protons, 32 neutrons, 18 electrons
4.Which row correctly describes the characteristics of the nucleus and surrounding space in an atom?
Easy
- ANucleus: small, dense and positive; Surrounding space: mainly empty space
- BNucleus: large, dense and positive; Surrounding space: densely populated with electrons
- CNucleus: small, dense and neutral; Surrounding space: mainly empty space
- DNucleus: small, dense and positive; Surrounding space: densely populated with electrons
5.A neutral atom contains both positive protons and negative electrons. Which statement best explains why the atom has no overall charge?
Medium
- AThe charge on an electron is equal and opposite to the charge on a proton.
- BOne proton has a mass 1840 times greater than one electron.
- CThe difference in charge between electrons and protons is balanced by the neutrons.
- DElectrons are spread out in shells around the nucleus while protons are concentrated inside the nucleus.
6.The isotope ⁶⁰₂₇Co is used in the treatment of cancer cells in the body. Which statements about this isotope are correct? I. The charge on the nucleus is +27. II. There are 33 neutrons in the nucleus. III. It has the same number of neutrons as other isotopes of cobalt.
Medium
- AI and II only
- BI and III only
- CII only
- DI, II and III
7.Which row correctly describes the subatomic particles found in ²⁶Mg²⁺?
Medium
- AProtons: 12, Neutrons: 14, Electrons: 10
- BProtons: 10, Neutrons: 14, Electrons: 12
- CProtons: 12, Neutrons: 26, Electrons: 10
- DProtons: 14, Neutrons: 12, Electrons: 12
8.An element consists of two isotopes, ⁶⁹X and ⁷¹X, with abundances of 60% and 40% respectively. What is the relative atomic mass of element X?
Medium
- A69.8
- B69.2
- C70.0
- D70.2

もっと問題

9.Which of the following statements about isotopes are correct? (Select all that apply.)
Medium
- AIsotopes of the same element have the same number of protons.
- BIsotopes of the same element have the same number of neutrons.
- CIsotopes of the same element have the same electron configuration.
- DIsotopes of the same element have different mass numbers.
- EIsotopes of the same element have different chemical properties.
10.Which of the following statements about the mass spectrometer and its use in determining relative atomic mass are correct? (Select all that apply.)
Medium
- AThe sample is vapourised before being ionised.
- BThe sample is ionised to form positive ions.
- CThe ions are detected as a mass-to-charge ratio, m/z.
- DThe relative atomic mass is calculated from the percentage abundances of the isotopes.
- EThe mass spectrometer measures the mass of neutral atoms directly.
11.The mass number of an atom is the total number of protons and neutrons in its nucleus.
Easy
True or false?
12.Isotopes of the same element have identical physical properties.
Easy
True or false?
13.Match each subatomic particle with its correct relative charge.
Easy
- Proton
- Neutron
- Electron
- +1
- 0
14.Arrange the steps of mass spectrometry in the correct order.
Medium
- Sample is vapourised
- Sample is ionised to form positive ions
- Ions are accelerated
- Ions are detected as m/z signals
15.Which option correctly describes the relative charges and masses of the subatomic particles?
Medium
- Aproton +1, neutron 0, electron –1; relative mass of an electron 1/1840
- Bproton 0, neutron +1, electron +1; relative mass of an electron 1
- Cproton +1, neutron 0, electron –1; relative mass of an electron 1
- Dproton 0, neutron +1, electron –1; relative mass of an electron 1
16.X is a particle with 18 electrons and 20 neutrons. Three species are shown below: I. ⁴⁰Ca²⁺, II. ³⁹K⁺, III. ³⁸Ar. Which of the three species could particle X be?
Medium
- AI only
- BI and II only
- CII and III only
- DI, II and III
17.The elements of the periodic table have different isotopes. What is different about the nuclei of stable isotopes?
Medium
- Athe same number of protons, but different number of neutrons
- Bthe same number of protons, and the same number of neutrons
- Ca different number of protons, and a different number of neutrons
- Da different number of protons, and the same number of neutrons
18.In which of the following species are the numbers of protons, neutrons and electrons all different?
Medium
- A²³Na⁺
- B²⁷Al
- C¹⁹F⁻
- D³²S²⁻
19.Which statements correctly describe the distribution of mass and charge in the atom? I. The negative charge is concentrated in one area outside the nucleus. II. The mass is concentrated inside the nucleus. III. The negative charge is spread around outside the nucleus.
Medium
- AI and III
- BI and II
- CII and III
- DI, II and III
20.The mass spectrum of element X is shown below. Which of the following statements is correct?
Hard
- AX has a relative atomic mass between 70 and 71
- BThe three isotopes of X are separated after being converted to anions
- CThe most abundant isotope of X contains 71 neutrons
- DThe isotope of X with mass 72 will be deflected the most
21.Which statement explains the similarity in chemical behaviour for isotopes of the same element?
Hard
- Athey may have the same electronic configuration, depending on the reaction
- Bthey have the same electronic configuration
- Ctheir mass numbers are different
- Dthey have different numbers of neutrons
22.What is the total number of protons and electrons in two moles of hydrogen gas?
Hard
- A4
- B8
- C2.4 × 10²⁴
- D4.8 × 10²⁴
23.The reaction H(g) + H⁺(g) → H₂⁺(g) occurs in gas clouds. Which row gives the correct atomic structure of the H⁺ ion?
Medium
- Aprotons: 1, neutrons: 0, electrons: 0
- Bprotons: 1, neutrons: 1, electrons: 0
- Cprotons: 1, neutrons: 0, electrons: 1
- Dprotons: 0, neutrons: 1, electrons: 0
24.Iron has several naturally occurring isotopes. Their abundances and isotopic masses are: ⁵⁴Fe 5.845% (53.9396 amu), ⁵⁶Fe 91.754% (55.9349 amu), ⁵⁷Fe 2.119% (56.9354 amu), ⁵⁸Fe 0.282% (57.9333 amu). What is the average atomic mass of iron?
Medium
- A56.1858 amu
- B54.200 amu
- C59.270 amu
- D55.845 amu
25.The mass spectrum of element X shows peaks at m/z = 70, 71, and 72 with relative abundances 60%, 30%, and 10% respectively. Which statement is correct?
Medium
- AX has a relative atomic mass between 70 and 71.
- BThe three isotopes of X are separated after being converted to anions.
- CThe most abundant isotope of X contains 71 neutrons.
- DThe isotope of X with mass 72 will be deflected the most.
26.What is the total number of protons and electrons in two moles of hydrogen gas (H₂)?
Hard
- A4
- B8
- C2.4 × 10²⁴
- D4.8 × 10²⁴
27.A sample of carbon contains 94% ¹²C and 6% ¹⁴C. What is the relative atomic mass of this carbon sample? Give your answer to 2 decimal places.
Hard
- A12.12
- B12.24
- C13.00
- D12.00
28.Boron contains two isotopes, ¹⁰B and ¹¹B, with relative abundances of 20% and 80% respectively. What is the relative atomic mass of boron?
Medium
- A10.20
- B10.80
- C11.00
- D10.50
29.A sample of oxygen contains the isotopes ¹⁶O (99.76%), ¹⁷O (0.04%), and ¹⁸O (0.20%). What is the relative atomic mass of oxygen to 2 decimal places?
Medium
- A16.00
- B17.18
- C16.09
- D17.00
30.A sample of antimony contains two isotopes: ¹²¹Sb and ¹²³Sb. The mass spectrum shows relative abundances of 60% and 40% respectively. What is the relative atomic mass of antimony?
Medium
- A121.8
- B122.0
- C122.2
- D121.6
31.A sample of rhodium contains 85% ¹⁰¹Rh and 15% ¹⁰²Rh. What is the relative atomic mass of rhodium? Give your answer to 2 decimal places.
Hard
- A101.15
- B101.85
- C101.50
- D102.00
32.Which statement explains why isotopes of the same element have similar chemical behaviour?
Medium
- AThey may have the same electronic configuration, depending on the reaction.
- BThey have the same electronic configuration.
- CTheir mass numbers are different.
- DThey have different numbers of neutrons.
33.The relative atomic mass of an element is the ratio of the average mass of its atoms to one twelfth of the mass of an atom of carbon-12.
Easy
True or false?
34.Which isotope of X is the most abundant?
Medium
- Am/z = 20
- Bm/z = 21
- Cm/z = 22
- Dm/z = 20 and 22 equally
35.In a mass spectrometer, a sample is first vapourised and then ionised. Which of the following best describes why the sample must be ionised?
Medium
- Aso that the ions can be accelerated and detected by their mass-to-charge ratio
- Bso that the sample becomes a gas and can be introduced into the instrument
- Cso that the isotopes can be separated by their different boiling points
- Dso that the ions can be neutralised before detection
36.A mass spectrum of an element shows peaks at m/z = 20, 21 and 22 with relative abundances 90%, 0.3% and 9.7% respectively. Which isotope is the most abundant?
Medium
- Athe isotope with m/z = 20
- Bthe isotope with m/z = 21
- Cthe isotope with m/z = 22
- Dall isotopes are equally abundant
37.In a mass spectrometer, ions are accelerated and then deflected by a magnetic field. Which ion would be deflected the most?
Hard
- Aan ion with a small mass and a high charge (small m/z)
- Ban ion with a large mass and a high charge (large m/z)
- Can ion with a small mass and no charge
- Dan ion with a large mass and a low charge (large m/z)
38.Which of the following statements about the processes in a mass spectrometer are correct? (Select all that apply.)
Medium
- AThe sample is vapourised before being ionised.
- BThe sample is ionised to form positive ions.
- CThe ions are separated according to their mass-to-charge ratio.
- DThe ions are detected as negative ions.
- EThe sample must be a solid.
39.In a mass spectrometer, the ions are separated according to their mass-to-charge ratio.
Medium
True or false?
40.Match each stage of mass spectrometry with its correct description.
Medium
- Vaporisation
- Ionisation
- Acceleration
- Detection
- sample is converted to a gas
- sample is converted to positive ions
- ions are speeded up by an electric field
- ions produce a signal at a particular m/z
41.Complete the definition of isotopes.
Easy
Isotopes are atoms of the same element with the same number of ____ but different numbers of ____.
42.What is the name of the particle that has a relative charge of 0 and a relative mass of 1?
Easy
43.Calculate the relative atomic mass of a sample of rhodium containing 85% of ¹⁰¹Rh and 15% of ¹⁰²Rh. Give your answer to 2 decimal places.
Medium
44.Complete the table for the subatomic particles.
Medium
A proton has a relative charge of ____ and a relative mass of ____.
45.Calculate the mass, in grams, of one molecule of nitrogen (N₂). Use the masses: proton = 1.672622 × 10⁻²⁷ kg, neutron = 1.674927 × 10⁻²⁷ kg, electron = 9.109383 × 10⁻³¹ kg. Give your answer to 3 significant figures.
Hard
46.Calculate the relative atomic mass of element X from the mass spectrum. Give your answer to 2 decimal places.
Medium
47.Calculate the relative atomic mass of a sample of antimony containing 60% of ¹²¹Sb and 40% of ¹²³Sb. Give your answer to 2 decimal places.
Medium
48.Iron has the following naturally occurring isotopes: ⁵⁴Fe (5.845%, 53.9396 amu), ⁵⁶Fe (91.754%, 55.9349 amu), ⁵⁷Fe (2.119%, 56.9354 amu), and ⁵⁸Fe (0.282%, 57.9333 amu). Calculate the average atomic mass of iron. Give your answer to 3 decimal places.
Medium
49.A sample of carbon contains 94% ¹²C and 6% ¹⁴C. Calculate the relative atomic mass of this carbon sample. Give your answer to 2 decimal places.
Medium
50.A sample of magnesium contains three isotopes: 24Mg (79.0%), 25Mg (10.0%) and 26Mg (11.0%). Calculate the relative atomic mass of this sample to 2 decimal places.
Medium
51.Complete the sentences about the mass spectrometer.
Medium
The sample is first ____ and then ____ to form positive ions. The ions are separated according to their ____ ratio.
52.Explain why isotopes of the same element have the same chemical properties but different physical properties.
Medium
53.Oxygen consists of three stable isotopes: oxygen-16, oxygen-17, and oxygen-18. State the particles present in each isotope and outline what differences would be expected in the chemical reactivity of the three isotopes.
Hard
54.Explain how the relative atomic mass of an element can be determined from its mass spectrum.
Hard
55.Calculate the relative atomic mass of a sample of rhodium containing 85% of ¹⁰¹Rh and 15% of ¹⁰²Rh. Give your answer to 2 decimal places.
Medium
- A101.15
- B111.27
- C91.03
- D121.39
56.Calculate the mass, in grams, of one molecule of nitrogen (N₂). Use the masses: proton = 1.672622 × 10⁻²⁷ kg, neutron = 1.674927 × 10⁻²⁷ kg, electron = 9.109383 × 10⁻³¹ kg. Give your answer to 3 significant figures.
Hard
- A4.68e-23 g
- B1 g
- C2 g
- D-1 g
57.Calculate the relative atomic mass of element X from the mass spectrum. Give your answer to 2 decimal places.
Medium
- A24.2
- B20.2
- C18.2
- D22.2
58.Calculate the relative atomic mass of a sample of antimony containing 60% of ¹²¹Sb and 40% of ¹²³Sb. Give your answer to 2 decimal places.
Medium
- A121.8
- B109.6
- C134
- D146.2
59.Iron has the following naturally occurring isotopes: ⁵⁴Fe (5.845%, 53.9396 amu), ⁵⁶Fe (91.754%, 55.9349 amu), ⁵⁷Fe (2.119%, 56.9354 amu), and ⁵⁸Fe (0.282%, 57.9333 amu). Calculate the average atomic mass of iron. Give your answer to 3 decimal places.
Medium
- A67.015 amu
- B50.26 amu
- C55.845 amu
- D61.43 amu
60.A sample of carbon contains 94% ¹²C and 6% ¹⁴C. Calculate the relative atomic mass of this carbon sample. Give your answer to 2 decimal places.
Medium
- A13.12
- B14.12
- C11.12
- D12.12
61.A sample of magnesium contains three isotopes: 24Mg (79.0%), 25Mg (10.0%) and 26Mg (11.0%). Calculate the relative atomic mass of this sample to 2 decimal places.
Medium
- A26.75
- B29.18
- C24.32
- D21.89

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