Over 2000 years ago, the Greek philosopher Democritus made the following famous quote:Nothing exists except atoms and empty space; everything else is opinion. He was partly right. Everything, from rocks to stars to blood to batteries, is made up of atoms. But Democritus believed that atoms were the smallest unit possible - even the word itself, atom, comes from the Greek…
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Over 2000 years ago, the Greek philosopher Democritus made the following famous quote:
Nothing exists except atoms and empty space; everything else is opinion.
He was partly right. Everything, from rocks to stars to blood to batteries, is made up of atoms. But Democritus believed that atoms were the smallest unit possible - even the word itself, atom, comes from the Greek for indivisible. We now know better, and the study of particles inside the atom created the field of atomic structure.
Atomic structure is the arrangement of subatomic particles within the atom. An atom is the smallest unit of ordinary matter within an element.
Atoms contain three types of subatomic particles: protons, neutrons and electrons.
Protons
Protons are positively charged particles. They are pretty small - one proton is only about kg, but we tend to measure their mass on a particular scale called the carbon-12 scale. On this scale, a carbon-12 atom has a mass of exactly 12 and a proton has a mass of approximately 1. We’ll explore what exactly a carbon-12 atom is later on in this article.
You’ll find protons densely packed together in the nucleus in the centre of the atom. They’re quite important, because once you know the number of protons in an atom, you know where in the periodic table you’ll find it and what element it is a part of.
Neutrons
Neutrons are neutral particles. They also have a relative mass of about 1 and are found alongside protons in the nucleus. Neutron numbers can vary between atoms of the same element without much effect on their chemical properties.
Electrons
Electrons are negative particles. If you thought protons were tiny, you’ll be in for a shock - electrons have an actual mass ofor a relative mass of 1/1840 on the carbon-12 scale. They aren’t found alongside protons and neutrons. Instead, electrons spend their time in energy levels, also known as shells, orbiting the nucleus. Energy levels increase in energy as they get further from the nucleus and electrons will always try to be in the lowest energy level possible. Take a look at "Electron shells, sub-shells and orbitals" for more information.
The number of electrons in an atom determines its chemical properties and how it reacts.
.
Fig. 1 The structure of an atom. The electrons orbit the positive nucleus, which contains a dense mass of protons and neutrons.
The table below gives a summary of the relative charges, masses and locations within the atom of the different subatomic particles:
Particle
Mass
Charge
Location within atom
Proton
1
+1
Inside nucleus
Neutron
1
0
Inside nucleus
Electron
0
-1
In shells orbiting nucleus
Although atoms may seem crowded with all their subatomic particles, they are actually mostly empty space - turns out Democritus wasn’t too wrong after all!
Finding the atomic structure of an element
It’s all very well and good being told you have an atom of a certain element, but how do you know its atomic structure and exactly how many of each type of fundamental particle it has? This is where the periodic table comes in handy. Let’s take a look at carbon as an example:
Fig. 2: Carbon.
6 → Atomic number
12 → Mass number
The atomic number, Z, gives the number of protons in the atom and therefore tells you which element the atom belongs to. The mass number, A, gives the combined total number of protons and neutrons in the atom. Therefore, you can find the number of neutrons by subtracting A from Z. In an uncharged atom, the number of electrons is always equal to the number of protons, so is also given by the atomic number. In our example above, this particular carbon atom has six protons, six electrons and six neutrons.
Atomic structure of lithium
Fig. 3: Lithium
Lithium has a mass number of 7 (to one decimal place) and an atomic number of 3. This means it has:
Three protons.
Four neutrons.
Three electrons.
Atomic structure of oxygen
Fig. 4: Oxygen.
Oxygen has a mass number of 16 and an atomic number of 8. Therefore it has:
Eight protons.
Eight neutrons.
Eight electrons.
Taking our example of carbon again, we can represent atomic and mass numbers in the following way:
More simply, the mass number of an atom can be represented by writing it after the element name. For example, carbon-12 has a mass number of 12.
For more information on subatomic particles, see "Fundamental particles" and "Isotopes and mass number".
What are ions and isotopes?
What would happen if an atom lost an electron? Uncharged atoms have, by definition, the same number of protons and electrons. Because electrons are negative particles, if an atom loses an electron it will become positively charged. This is known as an ion.
Ions are atoms that have either gained or lost electrons to form a charged particle.
Ions have different chemical properties due to their different electron configurations.
What about if an atom lost a neutron? This wouldn’t have much effect on how it reacts, because neutrons are neutral particles and don't influence the atom’s chemical properties. However, it would change its mass. We call this particle an isotope.
Isotopes are atoms of the same element with differing numbers of neutrons.
As their electron configurations are the same, isotopes have similar chemical properties.
Ions and isotopes are explored in more detail in the article "Ions and isotopes".
What is relative atomic mass?
If you take a look at the periodic table, shown below, you’ll see that mass numbers are rarely whole numbers. This is because they take into account the proportions of isotopes naturally found in samples of the element. For example, you often find chlorine atoms with different masses. Some have a mass number of 35 and some have a mass number of 37. The two different atoms are isotopes of each other. The average mass of all the naturally found isotopes of an element is known as an element’s relative atomic mass.
Relative atomic mass is the average mass of an atom in an element compared to 1/12 of the mass of a carbon-12 atom.
To find out the relative atomic mass of chlorine, for example, we multiply the mass of each chlorine isotope by its percentage abundance and add them all together. If there is 75% chlorine-35 and 25% chlorine-37 in a sample, the relative atomic mass will be (0.75*35) + (0.25*37) = 35.5.
If you want to find out more about how we find out the mass and abundance of isotopes in a sample, see "Mass spectrometry".
Fig.5: The periodic table. Iron is shown as an example of atomic mass 55.845. Note that this is not a whole number.
How are electrons arranged in the atom?
Electrons are found in shells orbiting the nucleus, which are also known as energy levels. Shells contain sub-shells and orbitals. The configuration of the electrons gives each element its chemical properties. Electrons fill the orbitals following certain rules, such as filling shells of the lowest energy level first, in order to achieve the lowest energy state possible. Although the filling rules can seem quite complicated, it is a logical system. We won’t go into too much detail now, but the following table gives a quick overview of the different sub-shells and the number of orbitals and electrons they can hold:
Sub-shell
Number of orbitals
Maximum number of electrons
S
1
2
P
3
6
D
5
10
For more information on electron filling and shells, see "Electron configuration".
How have ideas about the atom changed over time?
The Ancient Greeks were some of the first to believe in the existence of atoms, but as we now know, they were only partially correct about their nature. Ideas about the atom have changed throughout history thanks to new scientific discoveries, and some particularly influential scientists include:
Democritus.
John Dalton.
J J Thompson.
Ernest Rutherford.
Democritus
As you know, Democritus believed that atoms were the smallest particle possible and were therefore indivisible. He also believed that atoms were indestructible and constantly in motion.
John Dalton
Dalton, a physicist and chemist born in 1766, also suggested that atoms could not be broken down. He further expanded on the idea to suggest that all atoms in an element were identical and atoms of different elements had different masses.
JJ Thompson
Thompson, born in 1856, discovered that atoms contain subatomic particles which we now know as electrons. He proposed that negatively charged electrons were scattered randomly throughout the positive atom like plums in a plum pudding.
Ernest Rutherford
Between 1908 and 1924, Rutherford and his team carried out a series of experiments. They fired positively charged particles at an extremely thin sheet of gold leaf. They discovered multiple things thanks to their observations.
Firstly, they discovered that most of the particles passed straight through the gold leaf. This meant the atom was mostly empty space.
Secondly, they saw that some of the particles were reflected by the gold leaf. As like charges repel each other, this meant there was a tiny, densely charged positive mass in the centre of the atom. Rutherford termed this the nucleus. He proposed that the positive particles in an atom were confined to its centre and the negative electrons were found in rings orbiting the nucleus.
Atomic Structure - Key takeaways
Atoms consist of protons, neutrons and electrons.
An element’s mass number gives its total number of protons and neutrons, whereas the atomic number gives its number of protons.
Protons and neutrons are found in the nucleus of an atom whereas electrons are found in shells orbiting the nucleus.
Electrons fill shells according to rules in order to achieve the lowest energy state possible.
Views about the atom have changed over time due to new scientific discoveries.
References
Fig. 1: Atomic Structure Animation (https://upload.wikimedia.org/wikipedia/commons/thumb/2/2b/Atom_animation.gif/256px-Atom_animation.gif) by ԱննաԼյուբա (https://commons.wikimedia.org/w/index.php?title=User:%D4%B1%D5%B6%D5%B6%D5%A1%D4%BC%D5%B5%D5%B8%D6%82%D5%A2%D5%A1&action=edit&redlink=1) is licensed by CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/deed.en).
Fig. 3: Lithium (https://commons.wikimedia.org/wiki/File:Lithium.svg) by Albedo-ukr (https://commons.wikimedia.org/wiki/User:Albedo-ukr) is licensed by CC BY-SA 2.5 (https://creativecommons.org/licenses/by-sa/2.5/).
Fig. 4: Oxygen (https://upload.wikimedia.org/wikipedia/commons/thumb/a/a3/Oxygen.svg/512px-Oxygen.svg.png) by Albedo-ukr (https://commons.wikimedia.org/wiki/User:Albedo-ukr) is licensed by CC BY-SA 2.5 (https://creativecommons.org/licenses/by-sa/2.5/).
Fig. 5: Periodic Table of Elements (https://upload.wikimedia.org/wikipedia/commons/4/4d/Periodic_table_large.svg) by 2012rc (https://commons.wikimedia.org/w/index.php?title=User:2012rc&action=edit&redlink=1) is licensed by CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/deed.en).
Frequently Asked Questions about Atomic Structure
The electronic structure of an atom is its internal arrangement of electrons in shells and subshells.
Atoms contain positive protons and neutrons found in a central nucleus, and electrons orbiting the nucleus in shells.
The electronic configuration of an atom is worked out through the filling of shells and subshells, in order to achieve the lowest energy state possible.
An atom is the smallest unit of an element, consisting of protons, neutrons and electrons. An ion is an atom that has gained or lost an electron to form a charged particle.
Final Atomic Structure Quiz
Atomic Structure Quiz - Teste dein Wissen
Question
Which statement is correct?
Show answer
Answer
Protons have a relative mass of 1
Show question
Question
Complete the following sentence: electrons are found ___________.
Show answer
Answer
In shells orbiting the nucleus.
Show question
Question
What does mass number represent?
Show answer
Answer
The total number of protons and neutrons in an atom.
Show question
Question
What does atomic number represent?
Show answer
Answer
The number of protons in an atom.
Show question
Question
Which particle determines the chemical properties of an atom?
Show answer
Answer
Electron
Show question
Question
Which particle determines the element an atom belongs to?
Show answer
Answer
Proton
Show question
Question
What is an ion?
Show answer
Answer
An atom that has either gained or lost an electron to form a charged particle.
Show question
Question
What is an isotope?
Show answer
Answer
Isotopes are atoms of the same element with differing numbers of neutrons.
Show question
Question
Why do ions have differing chemical properties?
Show answer
Answer
Because they have different electron configurations.
Show question
Question
Define relative atomic mass.
Show answer
Answer
The average mass of an atom in an element compared to 1/12 of the mass of a carbon-12 atom.
Show question
Question
A sample of copper contains 69% copper-63 and 31% copper-65. Calculate its relative atomic mass, giving your answer to 1 decimal place.
Show answer
Answer
63.6
Show question
Question
Compare and contrast JJ Thompson and Ernest Rutherford's views about the atom.
Show answer
Answer
Both believed in negatively charged subatomic particles called electrons.
Thompson believed that electrons were randomly scattered throughout the rest of the positively charged atom.
Rutherford believed that the electrons orbited in rings around a nucleus densely packed with positive protons.
Rutherford believed that most of the atom was empty space.
Show question
Question
List three of Democritus’ beliefs about the atom.
Show answer
Answer
Atoms were indivisible.
Atoms were indestructible.
Atoms were constantly moving.
Atoms were the smallest unit possible.
Everything was made up of atoms.
Show question
Question
What is an atom?
Show answer
Answer
The smallest unit of an element.
Show question
Question
What are the three fundamental particles?
Show answer
Answer
Proton, neutron and electron.
Show question
Question
What is the relative mass of a proton on the carbon-12 scale?
Show answer
Answer
1
Show question
Question
Which sentence is correct?
Show answer
Answer
Neutrons are neutral and have a relative mass of 1.
Show question
Question
Which sentence is correct?
Show answer
Answer
Protons and electrons have opposite charges.
Show question
Question
What is the relative mass of an electron?
Show answer
Answer
1/1840
Show question
Question
Where are protons found in an atom?
Show answer
Answer
In the nucleus.
Show question
Question
Where are electrons found in an atom?
Show answer
Answer
In shells orbiting the nucleus
Show question
Question
Where are neutrons found in an atom?
Show answer
Answer
In the nucleus.
Show question
Question
Compare electrostatic forces with strong nuclear forces.
Show answer
Answer
Both electrostatic forces and strong nuclear forces are found in an atom.
Electrostatic forces occur between electrons and the nucleus
The strong nuclear force acts between protons and neutrons, within the nucleus.
The strong nuclear force is a lot stronger than electrostatic forces.
Show question
Question
Protons and neutrons are collectively referred to as nucleons. Give a reason for this.
Show answer
Answer
They are both found in the nucleus.
Show question
Question
2s is a lower energy shell than 2p. Predict which will get filled with electrons first.
Show answer
Answer
2s will get filled first.
Show question
Question
The electron shell 2s is further away from the nucleus than 1s. Predict which has a higher energy level.
Show answer
Answer
2s
Show question
Question
Which element contains eight protons?
Show answer
Answer
Oxygen
Show question
Question
When two hydrogen nuclei fuse, a proton is turned into a neutron. What element is the overall product?
Show answer
Answer
The overall product has one proton so is an isotope of hydrogen.
Show question
Question
Predict what chemical, physical or other changes will occur when a lithium atom gains a proton
Show answer
Answer
It becomes beryllium, a new element.
Show question
Question
Predict what chemical, physical or other changes will occur when a lithium atom gains an electron.
Show answer
Answer
It changes chemical properties as it has a different electron configuration.
Show question
Question
Predict what chemical, physical or other changes will occur when a lithium atom gains a neutron.
Show answer
Answer
It has slightly different physical properties as it has increased in mass.
Show question
Question
What is an element's mass number?
Show answer
Answer
The combined total number of protons and neutrons in its nucleus.
Show question
Question
What is an element’s atomic number?
Show answer
Answer
The number of protons in its nucleus.
Show question
Question
What symbol represents mass number?
Show answer
Answer
A
Show question
Question
What symbol represents atomic number?
Show answer
Answer
Z
Show question
Question
An atom has eight protons. How many electrons does it have?
Show answer
Answer
8
Show question
Question
An atom has six protons. How many neutrons does it have?
Show answer
Answer
5
Show question
Question
What is an ion?
Show answer
Answer
An atom that has gained or lost an electron to form a charged particle.
Show question
Question
How do isotopes of the same element differ?
Show answer
Answer
Isotopes of the same element have the same number of protons and electrons but different numbers of neutrons.
Show question
Question
Compare ions and isotopes.
Show answer
Answer
Ions and isotopes of the same element both have the same number of protons.
Isotopes will have different numbers of neutrons but the same number of electrons whereas ions have different numbers of electrons.
Isotopes are neutral overall whereas ions are charged particles.
Show question
Question
Aluminium can form an ion with a charge of +3. Represent this using roman numerals.
Show answer
Answer
Al(III)
Show question
Question
Beryllium can react to form an ion by losing two electrons. What is the charge on this ion?
Show answer
Answer
2+
Show question
Question
Define relative atomic mass.
Show answer
Answer
The average mass of an atom of an element in a sample compared to 1/12th of the mass of a ¹²C atom.
Show question
Question
Work out the relative atomic mass of a sample of copper containing 69% 63Cu and 31% 65Cu, to 1 decimal place.
Show answer
Answer
63.6
Show question
Question
What is mass spectrometry?
Show answer
Answer
An analytical technique used to determine the mass to charge ratio of ions.
Show question
Question
Give 2 uses for mass spectrometry.
Show answer
Answer
Identify molecules in a sample.
Work out the relative atomic mass of an element.
Show question
Question
What are the 4 stages of TOF mass spectrometry?
Show answer
Answer
Ionisation.
Acceleration.
Flight.
Detection.
Show question
Question
How does the velocity of ions relate to their mass in TOF spectrometry?
Show answer
Answer
As all ions are accelerated to the same kinetic energy, lighter ions have a faster velocity.
Show question
Question
Why is electrospray ionisation known as a ‘soft’ technique?
Show answer
Answer
It produces very little fragmentation.
Show question
Question
A molecule is ionised in TOF mass spectrometry. The resulting graph has peaks at 60, 28 and 22.
What is the peak at 60 known as?
What are the other peaks caused by?
Predict which ionisation technique was used to ionise the molecule.
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kg, but we tend to measure their mass on a particular scale called the carbon-12 scale. On this scale, a carbon-12 atom has a mass of exactly 12 and a proton has a mass of approximately 1. We’ll explore what exactly a carbon-12 atom is later on in this article.
or a relative mass of 1/1840 on the carbon-12 scale. They aren’t found alongside protons and neutrons. Instead, electrons spend their time in energy levels, also known as shells, orbiting the nucleus. Energy levels increase in energy as they get further from the nucleus and electrons will always try to be in the lowest energy level possible. Take a look at "Electron shells, sub-shells and orbitals" for more information.
Fig. 3: Lithium
