What are the key learning points about atomic structure?
There have been a number of ideas over time about the structure of The smallest particle of an element. We often think of atoms as tiny spheres, but in fact they are made from smaller particles called protons, neutrons and electrons..
Atoms are made up of Subatomic particle with a positive charge and a relative mass of 1. The relative charge of a proton is +1., Uncharged subatomic particle, with a relative mass of 1. The relative charge of a neutron is 0 (neutral). and Subatomic particle, with a negative charge and a very small mass relative to protons and neutrons., which each have different masses and charges.
An A pure substance which is made from only one type of atom. Elements are listed on the periodic table. An element cannot be broken down into anything simpler by chemical means. is made up of one type of atom. Different forms of the same element can exist – these are known as Atoms which have the same number of protons (so they are atoms of the same element and have the same atomic number) but they have a different number of neutrons (so they have a different mass number)..
What were the early ideas about atoms?
All matter is made of atoms.
An early model of the atom was the The scientific idea that an atom is a sphere of positive charge, with negatively charged electrons in it..
It imagined that an atom was a sphere of positive charge, with negatively-charged electrons embedded in it.
Ernest Rutherford disproved this model and discovered that an atom had a positive The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. The plural of nucleus is nuclei. at its centre, The path that electrons take as they move around the nucleus. by negatively-charged electrons.
Then James Chadwick discovered the neutron – a particle with no charge found in the nucleus of the atom.
The fact that a neutron has no charge made it difficult to discover.
The modern model of the atom contains:
- A positively-charged nucleus that contains positive protons and neutral neutrons
- Negative electrons orbiting the nucleus in shells
An atom’s nucleus gives it most of its mass.
Test your knowledge
See if you can identify parts of the atom.
What size are atoms?
Atoms have a radius of about 0.1 nm (nanometres), or 0.0000000001 metres (1 × 10-10 m).
The radius of the nucleus of an atom is less than \(\frac{1}{10,000}\) the size of an atom.
This means the The distance from the centre of the circle to the circumference. of a nucleus is smaller than 1 x 10-14 m.
What are protons, neutrons and electrons?
Protons, neutrons and electrons are known as Objects found in atoms, including protons, neutrons and electrons..
They each have different The relative mass is the number of times heavier a particle is, compared to another., relative charges and locations in the atom.
| Subatomic particle | Relative mass | Relative charge | Location in atom |
|---|---|---|---|
| Proton | 1 | +1 | In nucleus |
| Neutron | 1 | 0 | In nucleus |
| Electron | \(\frac{1}{1840}\) | -1 | In shells orbiting the nucleus |
Key fact
When writing the relative charge of a proton or electron, it is important to include both the sign and the number.
For example, the charge on a proton is not ‘+’, but ‘+1’.
What is the atomic number and mass number?
Each element is made up of a different type of atom.
The numbers of protons, neutrons and electrons in an atom can be calculated using an element’s atomic number and mass number.
- The atomic number is the number of protons in the nucleus of an atom.
- The mass number is the total number of protons and neutrons in the nucleus of an atom.
- When the symbol for an atom is written, its mass number is placed at the top left and its atomic number at the bottom left.
The atomic number and mass number do not directly tell you the number of electrons in an atom.
However, atoms have the same number of electrons as protons, so the atomic number also gives the number of electrons in an atom.
Key fact
An atom has no overall charge as it has an equal number of protons and electrons.
The positive charge on the protons and the negative charge on the electrons cancel each other out.
How to calculate the number of protons, neutrons and electrons
The The number of protons in the nucleus of an atom. and The total number of protons and neutrons found in the nucleus of an atom. can be used to calculate the number of Subatomic particle with a positive charge and a relative mass of 1. The relative charge of a proton is +1., Uncharged subatomic particle, with a relative mass of 1. The relative charge of a neutron is 0 (neutral). and Subatomic particle, with a negative charge and a very small mass relative to protons and neutrons. in an The smallest particle of an element. We often think of atoms as tiny spheres, but in fact they are made from smaller particles called protons, neutrons and electrons..
| Number of protons = atomic number |
| Number of neutrons = mass number – atomic number |
| Number of electrons = number of protons = atomic number |
Question
The box below shows how fluorine appears on the periodic table.
Use the information in the box to calculate the number of protons, neutrons and electrons in an atom of fluorine.
Answer
Atomic number = 9, mass number = 19
Number of protons = atomic number = 9
Number of neutrons = mass number - atomic number = 19 - 9 = 10
Number of electrons = number of protons = atomic number = 9
What is electronic configuration?
Subatomic particle, with a negative charge and a very small mass relative to protons and neutrons.The path that electrons take as they move around the nucleus. the The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. The plural of nucleus is nuclei. in An energy level around the nucleus where electrons can be found orbiting..
There are rules for how the electrons are arranged in their shells.
This is known as electronic configuration, electronic structure or electronic arrangement.
The rules for electronic configuration are as follows:
- Electrons start by filling the first shell (the one closest to the nucleus).
- Once a shell is full, the electrons start filling the next shell.
- Different shells hold a different maximum number of electrons.
| Shell | Maximum electrons |
|---|---|
| First | 2 |
| Second | 8 |
| Third | 8 |
| Fourth | 2 |
How to write and draw electronic configuration
Take sodium as an example.
Sodium has 11 electrons. It has:
- 2 electrons in the first shell
- 8 electrons in the second shell
- 1 electron in the third shell
You can write sodium’s electronic configuration with numbers and commas: 2,8,1
The electronic configuration of sodium can be shown in a diagram.
The dots represent electrons.
Electrons may be shown using dots or crosses.
Key fact
When drawing electrons in the second or third shell, the first four electrons should be drawn spread out, then the remaining four electrons ‘pair-up’ with the first four.
Question
An atom of oxygen has 8 electrons.
Write the electron configuration of an oxygen atom, and show the configuration in a diagram.
Answer
- 2 electrons in the first shell.
- 6 electrons in the second shell.
The electron configuration of an oxygen atom is 2,6.
What are isotopes?
Isotopes are atoms of an element with the same atomic number, but a different mass number.
They have the same number of protons (and electrons), but a different number of neutrons.
The table below shows the number of Objects found in atoms, including protons, neutrons and electrons. in atoms of the two isotopes of chlorine.
| Atom | Atomic number | Mass number | Number of protons | Number of neutrons | Number of electrons |
|---|---|---|---|---|---|
| 35Cl | 17 | 35 | 17 | (35 – 17) = 18 | 17 |
| 37Cl | 17 | 37 | 17 | (37 – 17) = 20 | 17 |
Question
The table below gives information about four atoms which have been labelled as A, B, C and D.
Which two atoms are a pair of isotopes?
| Atom | Number of protons | Number of neutrons | Number of electrons |
|---|---|---|---|
| A | 5 | 6 | 5 |
| B | 6 | 6 | 6 |
| C | 7 | 7 | 7 |
| D | 6 | 7 | 6 |
Answer
B and D are a pair of isotopes. They are the only two atoms in the table that have the same number of protons (6), but a different number of neutrons (B has 6, D has 7).
How to calculate relative atomic mass for elements
Most elements contain a mixture of isotopes, each present in a different amount.
For example, if you collected a large number of chlorine atoms, on average 75% of them would be 35Cl, and 25% would be 37Cl.
The relative atomic mass is a calculation of the average mass of an atom of a element.
This calculation takes into account:
- the mass number of each isotope of the element
- the different abundance of each isotope present
\(relative~atomic~mass~(A_r)\) \(= \frac {(mass~number~1 \times abundance)+(mass~number~2 \times abundance)}{total~abundance}\)
Example
The relative amounts of the two chlorine isotopes are shown below.
Calculate the relative atomic mass of chlorine.
| Isotope | Abundance (%) |
|---|---|
| 35Cl | 75 |
| 37Cl | 25 |
\(relative~atomic~mass~(A_r)\) \(= \frac {(35 \times 75) + (37 \times 25)}{75~+~25}\) \(= \frac {(2625) + (925)}{100} = 35.5\)
Question
The table below shows information about the different isotopes of the element boron.
| Isotope | Abundance (%) |
|---|---|
| 10B | 20 |
| 11B | 80 |
Use the information in the table to calculate the relative atomic mass of boron.
Answer
\(relative~atomic~mass~(A_r)\)
\(= \frac {(mass~number~1 \times abundance)+(mass~number~2 \times abundance)}{total~abundance}\) \(= \frac {(10 \times 20)+(11 \times 80)}{20 + 80}\) \(= \frac {(200)+(880)}{100}\) = 10.8
Key fact
The mass of the atom compared with that of the carbon-12 isotope, which has a mass of exactly 12. These masses use the symbol Ar or RAM.is not the same as The total number of protons and neutrons found in the nucleus of an atom..
Mass numbers are used for individual atoms, and are whole numbers because they are the number of protons plus the number of neutrons.
Relative atomic mass is an average mass of an atom of an element, which takes into account all of the Atoms which have the same number of protons (so they are atoms of the same element and have the same atomic number) but they have a different number of neutrons (so they have a different mass number). and their abundance.
It is not always a Any number which does not need a decimal point. Also known as an integer..
What are compounds?
It is possible to combine atoms of different elements together.
A compound is two or more elements chemically combined.
For example, water (H2O) is a compound made up of two hydrogen atoms chemically combined to one oxygen atom.
How much do you know about atomic structure?
More on Unit 1: Structures, trends, chemical reactions, quantitative chemistry and analysis
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