A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes

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Notes de leçon

Atoms and Elements

All substances are made of tiny particles called atoms.
An atom is the smallest part of an element that can exist.
Each atom consists of a central nucleus containing protons and neutrons, with electrons orbiting in shells.
An element is a pure substance made of only one type of atom.
Each element has a unique chemical symbol, e.g. O for oxygen, Na for sodium.
There are about 100 different elements, shown in the periodic table.

A collection of identical atoms representing an element

Atomic Structure and Symbols

The atomic number (Z) is the number of protons in an atom's nucleus; it is unique to each element.
The mass number (A) is the total number of protons and neutrons in the nucleus.
In a neutral atom, the number of electrons equals the number of protons.
Atoms have no overall charge because the positive protons and negative electrons balance.
Chemical symbols are written with the mass number as a superscript and atomic number as a subscript, e.g. ¹²₆C for carbon-12.
For ions, the number of electrons changes but the number of protons stays the same.

The structure of an atom

Relative Mass and Charge of 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 of about 1/1836 (negligible).
Almost all of the mass of an atom is concentrated in the nucleus.
The nucleus is very small compared to the overall size of the atom: radius of atom ~0.1 nm, nucleus <1/10,000 of that.

Isotopes

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Isotopes have the same atomic number but different mass numbers.
The chemical properties of isotopes are the same because they have the same electron arrangement.
For example, carbon-12 and carbon-13 are isotopes of carbon.
The relative atomic mass of an element is the average mass of its isotopes, taking into account their abundances.

Calculating Relative Atomic Mass

Relative atomic mass (Ar) is the weighted average mass of the isotopes of an element compared to 1/12 the mass of a carbon-12 atom.
It is calculated using the formula: Ar = (mass of isotope 1 × abundance 1 + mass of isotope 2 × abundance 2 + …) / total abundance.
Abundances are often given as percentages.
For example, chlorine has isotopes ³⁵Cl and ³⁷Cl in abundances 75% and 25%, giving Ar = (35×75 + 37×25)/100 = 35.5.
Relative atomic mass is not always a whole number because it is an average.

Electronic Structure

Electrons occupy energy levels (shells) around the nucleus.
The first shell can hold up to 2 electrons; the second and third shells can hold up to 8 electrons each.
Electrons fill the lowest available energy levels first (innermost shells).
The electronic structure can be written as numbers, e.g. sodium (2,8,1) or shown in a diagram.
The number of electrons in the outer shell determines the element's chemical properties.

Development of the Atomic Model

Early models described atoms as solid spheres that could not be divided.
J.J. Thomson discovered the electron in 1897 and proposed the plum pudding model: a ball of positive charge with negative electrons embedded in it.
Rutherford's α scattering experiment showed that most α particles passed straight through gold foil, but some were deflected or bounced back.
This led to the nuclear model: a tiny, dense, positively charged nucleus at the centre, with electrons orbiting outside.
Niels Bohr improved the nuclear model by suggesting electrons orbit in fixed shells at specific distances.
James Chadwick later provided evidence for neutrons in the nucleus.

Size and Scale of Atoms

Atoms have a radius of about 0.1 nm (1 × 10⁻¹⁰ m).
The nucleus is less than 1/10,000 of the atom's radius (about 1 × 10⁻¹⁴ m).
This means most of an atom is empty space.
The mass of an atom is concentrated in the nucleus.
The prefix nano means 10⁻⁹.

Diapos

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1.Which subatomic particles led to J.J. Thomson's plum pudding model?
Easy
- Aprotons
- Bneutrons
- Celectrons
- Dnucleus
2.The diagram shows the structure of an atom using the current model of the atom. How does this diagram show that this is an atom of lithium?
Easy
- Ait has three electrons
- Bit has three protons
- Cit has three protons and two neutrons
- Dit has two neutrons
3.Which of these statements is correct about atoms?
Easy
- Aelectrons are positively charged
- Bthe number of protons is equal to the number of neutrons
- Cmost of the mass of an atom is concentrated in the nucleus
- Delectrons are found in the nucleus
4.Match each subatomic particle to its relative charge.
Medium
- proton
- neutron
- electron
- +1
- 0
5.Atoms are made up of subatomic particles. Which statement about subatomic particles is true?
Medium
- ANeutrons and electrons are found in the nucleus. Protons are found in shells.
- BProtons and neutrons are found in the nucleus. Electrons are found in shells.
- CProtons and electrons are found in the nucleus. Neutrons are found in shells.
- DNeutrons are found in the nucleus. Protons and electrons are found in shells.
6.The nucleus is very small compared to the overall size of an atom.
Easy
True or false?
7.The nucleus is larger than the atom.
Easy
True or false?
8.Which two statements best describe what the gold foil experiment revealed about the structure of the atom? (select all that apply)
Medium
- AThe nucleus is positively charged
- BElectrons are evenly distributed throughout the atom
- CThe nucleus is small and contains most of the atom's mass
- DProtons and neutrons are evenly distributed
- EThe atom is a solid sphere that cannot be divided

Plus de questions

9.The diagram shows the structure of an atom using the current model of the atom. How does this diagram show that this is an atom of beryllium?
Medium
- AIt has 9 electrons
- BIt has 9 protons
- CIt has 4 protons
- DIt has 5 neutrons
10.Atoms have no overall charge. Which statement describes why atoms have no overall charge?
Easy
- AAtoms have the same number of protons and neutrons
- BAtoms have the same number of protons and electrons
- CAtoms have the same number of neutrons and electrons
- DAtoms have more protons than electrons
11.Which statement about the nucleus is true?
Medium
- AThe nucleus has negligible mass
- BThe nucleus contains protons and electrons
- CThe mass of an atom is concentrated in the nucleus because it contains protons and electrons
- DThe mass of an atom is concentrated in the nucleus because it contains protons and neutrons
12.The diagram shows the particles in the atom of an element. Particle Y is a proton. What is particle Z?
Easy
- Aan electron
- Ba molecule
- Ca neutron
- Da nucleus
13.Which of these has the smallest mass?
Easy
- Aan electron
- Ba neutron
- Ca nucleus
- Da proton
14.The relative atomic mass of chlorine is 35.5. Which is the correct reason why it is not a whole number?
Medium
- Achlorine contains half a proton
- Brelative atomic mass is the average mass of all of the isotopes of that element
- Crelative atomic mass is calculated from the mass of the protons divided by the mass of the neutrons
- Da chlorine molecule has a relative atomic mass of 71, so each atom has a relative atomic mass of 35.5
15.Which of the following statements about the relative charges of subatomic particles are correct? (select all that apply)
Easy
- AAn electron has a relative charge of -1
- BA neutron has a relative charge of +1
- CA proton has a relative charge of +1
- DA neutron has a relative charge of 0
- EA proton has a relative charge of -1
16.Match each subatomic particle to its relative mass.
Medium
- Proton
- Neutron
- Electron
- 1
- 1/1836 (negligible)
17.The number of protons in an atom is its atomic number.
Easy
True or false?
18.Arrange the following atomic models in the order they were proposed, from earliest to latest.
Medium
- Plum pudding model
- Nuclear model
- Bohr model
19.Which statement about the structure of an atom is correct?
Medium
- AElectrons are found in the nucleus
- BProtons and neutrons are found in the nucleus, and electrons orbit in shells
- CProtons and electrons are found in the nucleus, and neutrons orbit in shells
- DNeutrons are found in the nucleus, and protons and electrons orbit in shells
20.Which of the following statements about relative atomic mass are correct? (select all that apply)
Medium
- AIt is an average value that takes account of the abundance of isotopes
- BIt is always a whole number
- CIt is calculated using the masses and abundances of the isotopes of an element
- DIt is the same as the mass number of the most common isotope
21.An atom of an element has 17 protons and 18 neutrons. What is its mass number?
Medium
- A17
- B18
- C35
- D34
22.An atom of an element has 19 protons and 20 neutrons. What is its mass number?
Medium
- A19
- B20
- C39
- D58
23.The relative atomic mass of magnesium is 24.3. Magnesium has three isotopes: Mg-24, Mg-25 and Mg-26. Which statement explains why the relative atomic mass is not a whole number?
Medium
- AIt is the average mass of the isotopes, weighted by their abundances.
- BIt is the mass of the most abundant isotope.
- CIt is the sum of the masses of the isotopes divided by three.
- DIt is the mass of the isotope with the middle mass number.
24.A sample of chlorine contains 75% chlorine-35 and 25% chlorine-37. What is the relative atomic mass of this sample?
Medium
- A35.5
- B36.0
- C36.5
- D37.0
25.A sample of boron contains 20% boron-10 and 80% boron-11. What is the relative atomic mass of this sample?
Medium
- A10.2
- B10.8
- C11.0
- D10.5
26.An atom of an element has 16 protons and 18 neutrons. What is its atomic number?
Medium
- A16
- B18
- C34
- D2
27.An ion of an element has 13 protons, 14 neutrons and 10 electrons. What is the charge on the ion?
Hard
- A3+
- B3−
- C1+
- D1−
28.An atom of an element has 8 protons, 8 neutrons and 10 electrons. What is the charge on the ion?
Hard
- A2+
- B2−
- C0
- D1−
29.Which of the following is the correct electronic structure of a chlorine atom (atomic number 17)?
Medium
- A2,8,7
- B2,8,8
- C2,7,8
- D2,8,17
30.Which of the following is the correct electronic structure of a potassium atom (atomic number 19)?
Medium
- A2,8,8,1
- B2,8,9
- C2,8,8,2
- D2,8,7,2
31.The radius of an atom is about 0.1 nm. The radius of its nucleus is about 1 × 10⁻¹⁴ m. Approximately how many times smaller is the radius of the nucleus than the radius of the atom?
Medium
- A10
- B100
- C1,000
- D10,000
32.An atom of an element has 11 protons and 12 neutrons. What is its mass number?
Medium
- A11
- B12
- C23
- D1
33.A sample of an element contains two isotopes. Isotope 1 has a mass of 10 and an abundance of 20%. Isotope 2 has a mass of 11 and an abundance of 80%. What is the relative atomic mass of the element?
Hard
- A10.2
- B10.8
- C11.0
- D10.5
34.A sample of copper contains 69% copper-63 and 31% copper-65. What is the relative atomic mass of this sample?
Medium
- A63.6
- B64.0
- C64.4
- D63.0
35.In a mass spectrometer, a sample of an element gives two peaks: one at m/z = 20 with a relative abundance of 90%, and one at m/z = 22 with a relative abundance of 10%. What is the relative atomic mass of the element?
Challenge
- A20.2
- B20.5
- C21.0
- D22.0
36.An atom of an element has 20 protons and 20 neutrons. What is its mass number?
Hard
- A20
- B40
- C60
- D0
37.An atom of an element has 35 protons and 45 neutrons. What is its mass number?
Hard
- A35
- B45
- C80
- D10
38.An atom of an element has 29 protons and 35 neutrons. What is its atomic number?
Medium
- A29
- B35
- C64
- D6
39.An atom of an element has 92 protons and 146 neutrons. What is its mass number?
Hard
- A92
- B146
- C238
- D54
40.Which of the following statements about isotopes is correct?
Medium
- AIsotopes have the same number of protons but different numbers of neutrons.
- BIsotopes have the same number of neutrons but different numbers of protons.
- CIsotopes have the same mass number but different atomic numbers.
- DIsotopes have the same number of electrons but different numbers of protons.
41.Complete the sentence about atoms.
Easy
The size of the nucleus is very ____ in comparison with the overall size of an atom.
42.Complete the sentence about isotopes.
Easy
Isotopes are atoms that have the same number of ____ but have a different number of ____.
43.Use the words from the box to complete the sentences about the description of subatomic particles.
Medium
Protons have a relative charge of ____ and a relative mass of ____. Neutrons have a relative charge of ____.
44.Complete the following table about the subatomic particles found in two isotopes of carbon.
Medium
carbon-12: ____ protons, ____ neutrons, ____ electrons; carbon-13: ____ protons, ____ neutrons, ____ electrons
45.Use the words from the box to complete the sentences about the description of the structure of an atom.
Medium
The nucleus is ____ charged due to the presence of ____. There is ____ attraction between the nucleus and ____ charged electrons orbiting around the nucleus.
46.Use words from the box to complete the sentences about chlorine.
Medium
35-chlorine has the ____ number of 35 and the ____ number of 17. One atom of chlorine has 17 ____ and electrons.
47.An atom of carbon-12 has 6 protons and 6 neutrons. What is its mass number?
Medium
48.Complete the sentence about the electronic structure of an atom.
Medium
Electrons occupy the lowest available ____ levels, which are also called ____.
49.What is the chemical symbol for sodium?
Easy
50.Explain how the results of the α particle scattering experiment led to the nuclear model of the atom.
Medium
51.Describe the difference between the plum pudding model and the nuclear model of the atom.
Medium
52.Explain why the model of the atom changed from the plum pudding model to the nuclear model.
Medium
53.Explain why atoms have no overall charge.
Medium
54.Explain how the relative atomic mass of an element is calculated from its isotopes.
Hard
55.An atom of carbon-12 has 6 protons and 6 neutrons. What is its mass number?
Medium
- A11
- B12
- C13
- D14

Cartes mémo

What is an atom?

An atom is the smallest part of an element that can exist. It is the building block of all matter and still retains the properties of the element.

What are the three subatomic particles found in an atom?

Protons, neutrons and electrons.

Where are protons and neutrons located in an atom?

In the nucleus at the centre of the atom.

Where are electrons found in an atom?

Electrons move very fast around the nucleus in orbital paths called shells.

Why is almost all of the mass of an atom concentrated in the nucleus?

The nucleus contains the protons and neutrons, which carry nearly all the mass. The mass of an electron is negligible.

What is the relative charge of a proton?

What is the relative charge of an electron?

-1

What is the relative charge of a neutron?

0 (it is neutral).

What is the relative mass of a proton and of a neutron?

Both have a relative mass of 1. The electron's mass is negligible in comparison.

Why do atoms have no overall electrical charge?

In an atom, the number of electrons equals the number of protons. The negative and positive charges cancel out.

What is the atomic number of an atom?

The number of protons in the nucleus of an atom. It is unique to each element.

What is the mass number of an atom?

The total number of protons and neutrons in the nucleus (also called the nucleon number).

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

Mass number − atomic number (protons + neutrons minus protons).

What is an element?

A pure substance that consists of only one type of atom, distinguished by its unique number of protons.

How is each element represented?

By a unique chemical symbol, such as O for oxygen or Na for sodium. There are about 100 different elements, shown in the periodic table.

What are the rules for writing a chemical symbol?

A symbol has one or two letters. A single letter is uppercase (C, O); with two letters the first is uppercase and the second is lowercase (Na, Mg).

What is an isotope?

Atoms of the same element that have the same number of protons but a different number of neutrons.

What is relative atomic mass?

An average value that takes account of the abundance of the isotopes of the element, so it is not always a whole number.

Why is the relative atomic mass of chlorine 35.5 and not a whole number?

It is the average mass of all the isotopes of chlorine, weighted by their abundance.

How are electrons arranged in an atom?

Electrons occupy the lowest available energy levels (innermost available shells). The electronic structure can be shown by numbers or a diagram.

What did J.J. Thomson's discovery of the electron lead to?

It showed atoms were not just solid spheres and led to the plum pudding model — a ball of positive charge with negative electrons embedded in it.

What did Rutherford's α particle scattering experiment show?

Most α particles passed straight through, some were deflected a little and a few bounced back. This showed atoms are mostly empty space with a tiny, dense, positively charged nucleus at the centre.

How did the nuclear model differ from the plum pudding model?

The plum pudding model had positive charge spread throughout the atom with electrons embedded in it. The nuclear model replaced this with a tiny central nucleus containing the mass, with electrons outside it.

What did Niels Bohr contribute to the atomic model?

He suggested that electrons orbit the nucleus in fixed shells (energy levels) at specific distances, which explained why electrons do not spiral into the nucleus.

Who provided the evidence for the existence of neutrons?

James Chadwick, about 20 years after the nucleus became an accepted scientific idea.

Why has the model of the atom changed over time?

New experimental evidence led to scientific models being changed or replaced — for example, the discovery of the electron and the results of the α scattering experiment.

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