Atoms and isotopes

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Inside the Atom

Atoms are the building blocks of all matter and are incredibly small, with a radius of about 1 × 10⁻¹⁰ m.
At the centre of an atom is a tiny, dense, positively charged nucleus, which contains almost all of the mass of the atom.
The radius of the nucleus is over 10,000 times smaller than the radius of the whole atom.
Negatively charged electrons orbit around the nucleus, and almost all of the atom is empty space.
The nucleus contains protons (positive charge, relative mass 1) and neutrons (no charge, relative mass 1).
Electrons have a negative charge and almost no mass — about 1/2000 the mass of a proton or neutron.

Atomic Number and Mass Number

The atomic number (or proton number) is the number of protons in an atom; it determines which element the atom is.
In a neutral atom, the number of electrons equals the number of protons, so the overall charge is zero.
The mass number (or nucleon number) is the total number of protons and neutrons in the nucleus.
Number of neutrons = mass number − atomic number.
For example, sodium has atomic number 11 and mass number 23, so it has 23 − 11 = 12 neutrons.
Electrons are not included in the mass number because their mass is negligible compared to protons and neutrons.

Nuclear Notation

Atoms can be represented using nuclear notation, where the mass number is written as a superscript and the atomic number as a subscript before the chemical symbol.
For example, ²³⁸₉₂U shows uranium with mass number 238 and atomic number 92.
The top number is the mass number (total protons + neutrons).
The lower number is the atomic number (number of protons).
From the notation you can work out the number of neutrons by subtracting the lower number from the top number.

Nuclide notation

Isotopes

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.
Isotopes of an element have the same atomic number but different mass numbers.
The number of neutrons does not affect the chemical properties (such as charge) but does affect the mass of the atom.
Different isotopes of an element have the same number of electrons and protons.
For example, chlorine has two common isotopes: chlorine-35 and chlorine-37, with 18 and 20 neutrons respectively.
The relative atomic mass of chlorine is given as 35.5 because chlorine-35 is about three times more abundant than chlorine-37.
Some isotopes are unstable due to an imbalance of protons and neutrons.

Isotopes of hydrogen

Electron Energy Levels

Electrons orbit the nucleus at specific distances called energy levels (or shells).
The first energy level can hold up to 2 electrons; the second and third can each hold up to 8 electrons.
Electrons further from the nucleus have more energy — higher energy levels are at greater distances.
Electrons can absorb electromagnetic radiation and move to a higher energy level, further from the nucleus.
When an electron moves back down to a lower energy level, it emits electromagnetic radiation.
The colours in the visible spectrum are produced when electrons move down energy levels and emit light.

Positive Ions

An ion is an atom or particle with a non-zero charge.
Atoms become positive ions when they lose one or more outer electrons.
Electrons can be removed by friction (rubbing), by absorbing electromagnetic radiation, or during chemical reactions.
When radiation passes close to atoms, it can knock electrons out, leaving the atom with an overall positive charge.
Ions are often more chemically reactive than neutral atoms because of their positive charge.
Protons are locked in the nucleus and cannot move between atoms; only electrons can be transferred.

The atomic structure of ions

The Plum Pudding Model

Before the discovery of the electron, atoms were thought to be tiny, indivisible solid spheres.
J.J. Thomson discovered the electron at the end of the 19th century and proposed the Plum Pudding model.
In the Plum Pudding model, the atom is a ball of positive charge with negative electrons embedded in it — like plums in a pudding.
This model made the atom neutral overall because the positive 'dough' balanced the negative electrons.
The Plum Pudding model was replaced in 1909 when new experimental evidence from α scattering could not be explained by it.

Rutherford Scattering and the Nuclear Model

In 1909, Ernest Rutherford's students Geiger and Marsden fired α particles at a thin gold foil.
They expected α particles to pass straight through with only small deflections.
Instead, most α particles passed straight through, some were deflected, and a few bounced straight back.
The bouncing back could not be explained by the Plum Pudding model, so a new model was needed.
Rutherford proposed the nuclear model: nearly all mass is concentrated in a tiny, positively charged nucleus at the centre, with electrons orbiting at a distance.
The nuclear model replaced the Plum Pudding model because it could better explain the experimental observations.

Rutherford's Alpha Scattering Experiment

Bohr's Model and Later Developments

In 1913, Niels Bohr improved the nuclear model by suggesting that electrons orbit at specific distances called energy levels.
Bohr's model could explain the absorption and emission of electromagnetic radiation.
Theoretical calculations from Bohr's model agreed with experimental observations.
Later experiments showed that the positive charge of the nucleus could be subdivided into smaller particles — protons.
In 1932, James Chadwick proved the existence of neutrons in the nucleus, about 20 years after the nucleus was accepted.
The atomic model has changed over time as new evidence from experiments has led to improved models.

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1.What is the name used to describe nuclides which have the same number of protons but different numbers of neutrons?
Easy
- AIons
- BIsotopes
- CIsomers
- DAllotropes
2.Which of the following correctly describes the structure of an atom?
Easy
- AA positively charged nucleus containing protons and neutrons, surrounded by negatively charged electrons
- BA negatively charged nucleus containing protons and neutrons, surrounded by positively charged electrons
- CA neutral nucleus containing protons and electrons, surrounded by neutrons
- DA positively charged nucleus containing protons and electrons, surrounded by neutrons
3.Which of the following statements about the nucleus of an atom is correct?
Easy
- AIt contains most of the mass of the atom and is positively charged
- BIt contains most of the mass of the atom and is negatively charged
- CIt contains very little of the mass of the atom and is positively charged
- DIt contains very little of the mass of the atom and is neutral
4.Which of the following statements about isotopes are correct? (select all that apply)
Medium
- AIsotopes have the same number of protons
- BIsotopes have different numbers of neutrons
- CIsotopes have different numbers of electrons
- DIsotopes have the same mass number
- EIsotopes of the same element have the same chemical properties
5.An atom of sodium has an atomic number of 11 and a mass number of 23. How many neutrons are in the nucleus?
Easy
- A11
- B12
- C23
- D34
6.Which of the following statements about positive ions is correct?
Medium
- AA positive ion is formed when an atom gains electrons
- BA positive ion is formed when an atom loses electrons
- CA positive ion is formed when an atom loses protons
- DA positive ion is formed when an atom gains protons
7.In the plum pudding model, the atom is a ball of positive charge with negative electrons embedded in it.
Easy
True or false?
8.In the Bohr model of the atom, electrons orbit the nucleus at specific distances called energy levels.
Easy
True or false?

Soal lainnya

9.Match each subatomic particle to its relative charge.
Medium
- Proton
- Neutron
- Electron
- +1
- 0
10.Place the parts of an atom in order of increasing distance from the centre of the atom.
Medium
- Nucleus
- First energy level
- Second energy level
- Third energy level
11.Which of the following statements about positive ions are correct? (select all that apply)
Hard
- AA positive ion forms when an atom loses one or more electrons.
- BA positive ion forms when an atom gains one or more protons.
- CA positive ion has more protons than electrons.
- DA positive ion is a different element from the neutral atom.
- EA positive ion has a non-zero overall charge.
12.Match each atomic model to its key feature.
Medium
- Plum pudding model
- Nuclear model
- Bohr model
- A ball of positive charge with negative electrons embedded in it
- A positively charged nucleus with electrons orbiting at a distance
- Electrons orbit the nucleus at specific distances called energy levels
13.In a neutral atom, the number of electrons is equal to the number of protons.
Easy
True or false?
14.An electron in an atom absorbs electromagnetic radiation. What happens to the electron?
Medium
- AIt moves to a higher energy level, further from the nucleus.
- BIt moves to a lower energy level, closer to the nucleus.
- CIt leaves the atom and becomes a positive ion.
- DIt emits electromagnetic radiation and moves down an energy level.
15.Which statement best describes the difference between the plum pudding model and the nuclear model?
Medium
- AThe plum pudding model has a positive nucleus; the nuclear model has electrons embedded in a positive ball.
- BThe plum pudding model has electrons embedded in a positive ball; the nuclear model has a positive nucleus with electrons orbiting at a distance.
- CThe plum pudding model has a negative nucleus; the nuclear model has a positive nucleus.
- DThe plum pudding model has electrons orbiting energy levels; the nuclear model has electrons embedded in a positive ball.
16.A neutral atom of an element has a nucleus with a radius of about 7.2 × 10⁻¹⁵ m. The radius of the whole atom is about 1.4 × 10⁻¹⁰ m. Which of the following gives the ratio of the radius of the atom to the radius of its nucleus, in standard form?
Hard
- A2.0 × 10⁴
- B2.0 × 10⁵
- C5.1 × 10⁻⁶
- D5.1 × 10⁶
17.An atom of an isotope of uranium is represented by the symbol ²³⁸₉₂U. Which of the following statements about this atom is correct?
Medium
- AIt has 92 neutrons and 146 protons.
- BIt has 92 protons and 146 neutrons.
- CIt has 92 protons and 238 neutrons.
- DIt has 238 protons and 92 neutrons.
18.An atom that loses one or more electrons becomes a positive ion.
Easy
True or false?
19.Match each atomic model with its key feature.
Medium
- Plum pudding model
- Nuclear model
- Bohr model
- Electrons orbit the nucleus at specific distances called energy levels.
- The atom is a ball of positive charge with negative electrons embedded in it.
- The atom has a small, dense, positively charged nucleus with electrons orbiting around it.
20.Place the following atomic models in the order in which they were proposed, from earliest to latest.
Medium
- Plum pudding model
- Nuclear model
- Bohr model
- Solid sphere model
21.An atom of an element has an atomic number of 20 and a mass number of 44. It loses two electrons to form an ion. Which of the following correctly describes the ion?
Challenge
- AIt has 20 protons, 24 neutrons and 18 electrons.
- BIt has 20 protons, 24 neutrons and 22 electrons.
- CIt has 20 protons, 22 neutrons and 18 electrons.
- DIt has 22 protons, 20 neutrons and 18 electrons.
22.In the α particle scattering experiment, a beam of α particles was directed at a very thin sheet of gold foil. Which observation provides evidence that the nucleus is positively charged?
Medium
- AMost α particles passed straight through the foil with no deflection
- BA few α particles were deflected through large angles or bounced straight back
- CThe α particles all slowed down as they passed through the foil
- DThe gold foil became positively charged after the experiment
23.Which of the following conclusions about the structure of the atom can be drawn from the results of the α particle scattering experiment? (select all that apply)
Medium
- AMost of the atom is empty space
- BThe atom contains a small, dense nucleus
- CThe nucleus contains most of the mass of the atom
- DElectrons are embedded in a ball of positive charge
- EThe nucleus is negatively charged
24.In the α particle scattering experiment, most of the α particles passed straight through the gold foil.
Easy
True or false?
25.Match each observation from the α particle scattering experiment to the conclusion it supports.
Medium
- Most α particles passed straight through the foil
- A few α particles were deflected through large angles
- α particles were deflected by the foil rather than passing through unaffected
- The atom is mostly empty space
- The nucleus is small, dense and positively charged
- The atom contains charged particles that can exert a force on α particles
26.Place the following steps in the order in which they occur when an electron absorbs electromagnetic radiation and then emits it.
Medium
- Electromagnetic radiation is absorbed by an electron
- The electron moves up to a higher energy level
- The electron is unstable at the higher energy level
- The electron moves back down to a lower energy level
- A wave of electromagnetic radiation is emitted
27.How many protons does this carbon atom contain?
Medium
- A6
- B8
- C14
- D20
28.An atom of an isotope of an element is represented by the symbol ³⁵₁₇Cl. Which of the following correctly describes the particles in this atom?
Medium
- A17 protons, 17 neutrons and 17 electrons
- B17 protons, 18 neutrons and 17 electrons
- C18 protons, 17 neutrons and 18 electrons
- D17 protons, 18 neutrons and 18 electrons
29.Which of the following statements about the isotopes chlorine-35 and chlorine-37 are correct? (select all that apply)
Medium
- AThey have the same number of protons
- BThey have the same number of neutrons
- CThey have different numbers of neutrons
- DThey have the same number of electrons
- EThey have different numbers of protons
30.Isotopes of the same element always have the same number of neutrons.
Easy
True or false?
31.A neutral atom of an element has an atomic number of 20 and a mass number of 44. It loses two electrons to form an ion. Which of the following correctly describes the ion?
Hard
- A20 protons, 24 neutrons and 18 electrons
- B20 protons, 22 neutrons and 20 electrons
- C18 protons, 24 neutrons and 18 electrons
- D22 protons, 20 neutrons and 20 electrons
32.A sample of chlorine contains the isotopes chlorine-35 and chlorine-37. The relative atomic mass of the sample is 35.5. Which statement best explains this value?
Hard
- AChlorine-35 is about three times more abundant than chlorine-37
- BChlorine-37 is about three times more abundant than chlorine-35
- CThe two isotopes are present in equal amounts
- DSome chlorine atoms contain 35.5 neutrons
33.An atom of an element has an atomic number of 13 and a mass number of 27. It loses three electrons to form an ion. Which of the following correctly gives the number of protons, neutrons and electrons in the ion?
Hard
- A13 protons, 14 neutrons, 10 electrons
- B13 protons, 14 neutrons, 13 electrons
- C10 protons, 14 neutrons, 10 electrons
- D13 protons, 17 neutrons, 10 electrons
34.Which of the following is the correct relative charge of a proton, a neutron and an electron respectively?
Medium
- A+1, 0, −1
- B−1, 0, +1
- C+1, −1, 0
- D0, +1, −1
35.When an electron absorbs electromagnetic radiation, it moves to a lower energy level closer to the nucleus.
Medium
True or false?
36.In the Bohr model of the atom, electrons orbit the nucleus at specific distances called energy levels. Which of the following is the main advantage of the Bohr model over the earlier nuclear model?
Hard
- AIt showed that the nucleus contains protons and neutrons
- BIt could explain the absorption and emission of electromagnetic radiation
- CIt showed that the atom is mostly empty space
- DIt showed that electrons are embedded in a ball of positive charge
37.A sample of an element contains two isotopes. Isotope A has a mass number of 10 and isotope B has a mass number of 11. The relative atomic mass of the sample is 10.8. Which statement best explains this value?
Hard
- AIsotope B is more abundant than isotope A
- BIsotope A is more abundant than isotope B
- CThe two isotopes are present in equal amounts
- DSome atoms contain 10.8 neutrons
38.Which of the following statements about the plum pudding model and the nuclear model are correct? (select all that apply)
Hard
- AIn the plum pudding model, negative electrons are embedded in a ball of positive charge
- BIn the nuclear model, the mass of the atom is concentrated in the nucleus
- CIn the plum pudding model, the atom is mostly empty space
- DIn the nuclear model, electrons orbit the nucleus at a distance
- EIn the nuclear model, the nucleus is negatively charged
39.An atom of an element has an atomic number of 11 and a mass number of 23. It loses one electron to form an ion. Which of the following correctly describes the ion?
Medium
- A11 protons, 12 neutrons and 10 electrons
- B11 protons, 12 neutrons and 11 electrons
- C10 protons, 12 neutrons and 10 electrons
- D11 protons, 11 neutrons and 10 electrons
40.The number of neutrons in an atom affects its mass but not its charge.
Medium
True or false?
41.What is the name of the particle discovered by James Chadwick in 1932?
Medium
42.Complete the sentence about the structure of an atom.
Easy
An atom consists of a positively charged nucleus containing ____ and ____, surrounded by negatively charged ____.
43.An atom of carbon-14 has an atomic number of 6. How many neutrons are in its nucleus?
Medium
44.Complete the sentence about isotopes.
Easy
Isotopes are atoms of the same element that have the same number of ____ but a different number of ____.
45.An atom of chlorine-37 has an atomic number of 17. How many neutrons are in its nucleus?
Medium
46.What is the name of the particle in the nucleus that has no charge?
Easy
47.An atom of an isotope of uranium is represented by the symbol ²³⁵₉₂U. Calculate the number of neutrons in this atom.
Hard
48.Complete the sentence about the charge of an atom.
Medium
In a neutral atom, the number of electrons is equal to the number of ____, so the total charge is ____.
49.What is the name of the particle that has no charge and is found in the nucleus of an atom?
Medium
50.Complete the sentence about the size of an atom.
Easy
The radius of an atom is about ____ m.
51.How many times smaller is the radius of a nucleus compared to the radius of the whole atom?
Medium
52.An atom of lithium has an atomic number of 3 and a mass number of 7. How many neutrons are in its nucleus?
Medium
53.Explain why the α particle scattering experiment led to the replacement of the plum pudding model with the nuclear model.
Hard
54.Describe the main advantage of the Bohr model over the nuclear model.
Hard
55.In the α particle scattering experiment, α particles were fired at a thin gold foil. Explain why the results of this experiment led to the plum pudding model being replaced by the nuclear model.
Hard
56.Describe what happens to an electron in an atom when it absorbs electromagnetic radiation, and what happens when it later emits electromagnetic radiation.
Hard
57.Explain how the results of the α particle scattering experiment provided evidence for the nuclear model of the atom.
Hard
58.An atom of carbon-14 has an atomic number of 6. How many neutrons are in its nucleus?
Medium
- A9
- B8
- C7
- D10
59.An atom of chlorine-37 has an atomic number of 17. How many neutrons are in its nucleus?
Medium
- A20 neutrons
- B18 neutrons
- C22 neutrons
- D24 neutrons
60.An atom of an isotope of uranium is represented by the symbol ²³⁵₉₂U. Calculate the number of neutrons in this atom.
Hard
- A171
- B129
- C143
- D157
61.How many times smaller is the radius of a nucleus compared to the radius of the whole atom?
Medium
- A12000
- B10000
- C9000
- D11000
62.An atom of lithium has an atomic number of 3 and a mass number of 7. How many neutrons are in its nucleus?
Medium
- A5
- B6
- C4
- D3

Kartu flash

What is the overall electrical charge of an atom, and why?

Zero, because the number of electrons equals the number of protons, so the positive and negative charges cancel out.

What is the approximate radius of an atom?

About 1 × 10⁻¹⁰ m.

How does the radius of a nucleus compare with the radius of the whole atom?

The nucleus is over 10,000 times smaller than the atom.

Where is most of the mass of an atom concentrated?

In the nucleus, even though the nucleus is tiny compared with the whole atom.

What particles make up the nucleus of an atom?

Protons (positive charge) and neutrons (no charge).

Describe the charge and relative mass of a proton.

A proton has a positive charge and a relative mass of one unit.

Describe the charge and relative mass of a neutron.

A neutron has no charge and a relative mass of one unit.

Describe the charge and mass of an electron.

An electron has a negative charge and almost no mass — about 1/2000 the mass of a proton or neutron.

What is the atomic number of an atom?

The number of protons in the atom; it is also equal to the number of electrons in a neutral atom.

What is the mass number of an atom?

The total number of protons and neutrons in the nucleus.

How do you calculate the number of neutrons in an atom?

Number of neutrons = mass number − atomic number.

In nuclear notation, what do the top and bottom numbers represent?

The top number is the mass number (protons + neutrons) and the lower number is the atomic number (protons).

What are isotopes?

Atoms of the same element with the same number of protons but different numbers of neutrons.

How does the number of neutrons affect an atom's chemical properties?

It does not affect the chemical properties or charge — it only changes the mass of the atom.

Why is chlorine's mass number often given as 35.5?

Chlorine has two isotopes, chlorine-35 and chlorine-37, and chlorine-35 is about three times more abundant, giving an average closer to 35.

How does an atom become a positive ion?

It loses one or more outer electrons, leaving it with more protons than electrons and so a net positive charge.

What is an ion?

An atom or particle with a non-zero charge.

Why can protons not move between atoms when an object becomes charged?

Protons are locked in the nucleus, so only electrons can be transferred between atoms or objects.

What happens when an electron absorbs electromagnetic radiation?

It gains energy and moves up to a higher energy level, further from the nucleus.

What happens when an electron moves back down to a lower energy level?

It emits a wave of electromagnetic radiation.

How are electrons arranged in an atom?

They orbit the nucleus at particular distances called energy levels; higher energy levels are further from the nucleus.

How many electrons can occupy the first, second and third energy levels?

Up to 2 in the first level, up to 8 in the second and up to 8 in the third.

Describe the Plum Pudding model of the atom.

A ball of positive charge with small negative electrons embedded in it, like plums in a pudding.

What did the α particle scattering experiment show?

Most α particles passed straight through the gold foil, some were deflected, and a few bounced straight back.

What conclusions did Rutherford draw from the α scattering results?

Nearly all the mass is concentrated in a tiny central nucleus, the nucleus is positively charged, and electrons orbit at a distance.

Why did the nuclear model replace the Plum Pudding model?

The Plum Pudding model could not explain why some α particles bounced back off the gold foil, but the nuclear model could.

What did Niels Bohr add to the nuclear model?

He suggested that electrons orbit the nucleus at specific distances in distinct energy levels.

Why was the Bohr model accepted over the earlier nuclear model?

It explained experimental findings better, including the absorption and emission of electromagnetic radiation, and its theoretical calculations matched experimental results.

Who discovered the proton, and what is it?

Ernest Rutherford discovered that the positive charge of a nucleus could be broken into smaller particles, each with the same charge as a hydrogen nucleus — these are protons.

Who discovered the neutron and when?

James Chadwick proved the existence of neutrons in 1932, about 20 years after the nucleus became an accepted idea.

What are the properties of a neutron as discovered by Chadwick?

A neutron has a mass similar to a proton and no charge — it is neutral.

Why do scientific models of the atom change over time?

New experimental evidence may not fit the current model, so the model is changed or replaced by one that explains the observations better.

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