The atomic structure explains everything that makes the world around us. Atoms make up all matter, from grains of sand to stars in the universe. It is an important topic in GCSE Science that helps us understand more complex issues in chemistry and physics.
This article explains the key points of atomic structure, including the components of atoms and how they interact. Quiz questions are included to test your knowledge. If you need more support, TeachTutti has a list of GCSE Science tutors who can help you grasp this and other topics in the curriculum.
Components of an atom
Atoms are the basic units of matter. There are three parts to an atom: protons, neutrons and electrons:
Protons are in the nucleus at the atom's centre. They are positively charged particles. We use the number of protons to decide the atomic number of an element and their position in the periodic table. For example, hydrogen has 1 proton while helium has 2). They affect the atom's overall mass.
Neutrons are neutral particles - they have no charge. Neutrons are also found in the nucleus and they add to the atom's mass. Being neutral, they don't affect its charge. The total neutrons can vary in atoms of the same element, leading to different isotopes e.g. Carbon-12 and Carbon-14.
Electrons are negatively charged particles that orbit the nucleus in energy shells. They are smaller than protons and neutrons, with virtually no mass and therefore no affect on the atom's mass. The arrangement of electrons affects how the atom will interact in chemical reactions.
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What are the three components of an atom?
Atomic number and mass number
We identify elements and what distinguishes them with the atomic number and mass number of an atom. They explain the structure of an atom and help distinguish between different elements and isotopes.
Atomic number
The atomic number refers to how many protons are in the atom's nucleus. This number is unique so can be used to identify an element. For instance, hydrogen atoms have one proton so the atomic number is 1, while carbon atoms have six protons so the atomic number is 6. The atomic number also how many electrons are in an atom when it's neutral (there are the same number of protons and electrons).
Mass number
The mass number is the total number of protons and neutrons in an atom. The mass number can vary between atoms of the same element as the number of neutrons can vary. This results in different isotopes e.g. Carbon-12 has 6 protons and 6 neutrons, while Carbon-14 has 6 protons and 8 neutrons.
To find the mass number of an atom, simply add the protons and neutrons together. This gives an approximation of the atom's mass because electrons are so small that their mass is negligible. The mass number is important for studying isotopes and their use in areas like medicine, archaeology and chemistry.
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What is the atomic number of an element?
Electron arrangement and energy levels
Electrons are negatively charged particles that orbit the atom's nucleus in energy levels or shells. The electron arrangement helps determine how an atom will interact with others.
Energy levels and electron shells
Electrons are arranged in shells around the nucleus. Each shell has a different energy level: shells that are closer to the nucleus have a lower energy and vice versa. The lowest energy levels are filled first by electrons before moving to the higher ones further from the nucleus. The first shell can hold 2 electrons, the second shell can hold 8, the third can hold up to 18 and so on. The pattern for electrons per shell is called the 2n² rule, where "n" is the shell number.
Electron configuration
The arrangement of electrons is called electron configuration. For example, the electron configuration of sodium (atomic number 11) is 2, 8, 1. This means there are 2 electrons in the first shell, 8 in the second and 1 in the third. This electron configuration decides the element's chemical properties, including how it bonds with other elements e.g. sodium has just 1 electron in its outermost shell, which often loses to get a stable configuration, making the element highly reactive.
The Octet Rule and Reactivity
Atoms are most stable when their outer shell is full. This often means having 8 electrons in the outer shell, called the octet rule. Atoms will gain, lose, or share electrons to get this stable configuration. For example, chlorine has seven electrons in its outer shell and often gains one to fill it, making it highly reactive. As we have seen, electron arrangements help us understand why some elements are more reactive.
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Which arrangement is correct for an atom's shells?
Ions and isotopes
Atoms can change their state in two significant ways. They can gain or lose electrons to create ions. They can also have a different number of neutrons to form isotopes.
Ions
An ion is an atom that has gained or lost electrons, which creates an electrical charge. Atoms are normally neutral, with the same amount of protons and electrons. When an atom gains extra electrons, it becomes negatively charged and is called an ion. When an atom loses electrons, it becomes positively charged and is called a cation. For example, if a sodium atom (Na) loses an electron, it creates a positively charged sodium ion (Na+). When a chlorine atom (Cl) gains an electron, it forms a negatively charged chloride ion (Cl-).
Ions are crucial in creating ionic bonds. This is when positive and negative ions attract each other to create compounds. One of these compounds is sodium chloride (NaCl), commonly known as table salt.
Isotopes
Isotopes are atoms from the same element with different numbers of neutrons. The different number of neutrons changes the mass of the atom but not its chemical makeup. For example, carbon atoms have 6 protons, but Carbon-12 has 6 neutrons, while Carbon-14 has 8 neutrons. Despite this, both isotopes behave similarly in chemical reactions.
Isotopes are important in various fields e.g. Carbon-14 is used in radiocarbon dating to find out the the age of ancient artefacts. Isotopes can be stable or radioactive, decaying over time and releasing radiation. These radioactive isotopes are used in medicine, such as cancer treatment to destroy cancerous cells.
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What is an ion?
Development of the atomic model
Atomic theory has changed over time thanks to the work of various scientists. The atomic model has gone through numerous revisions during the 19th 20th and 21st centuries, which are discussed below.
1. Dalton’s atomic theory
John Dalton created the first atomic theory in the early 19th century. He argued that atoms are tiny particles that make up all matter. Atoms of the same element are identical while atoms of other elements have different masses. Aspects of Dalton's theory are now outdated, including that we now know atoms can be divided into smaller particles.
2. Thomson’s Plum Pudding model
J.J. Thomson discovered the electron in 1897, a negatively charged particle inside the atom. He created the "plum pudding" model, which presented the atom as a positively charged sphere with electrons embedded inside, comparable to raisins in a pudding (hence the name). It was the first model that argued atoms contained smaller particles.
3. Rutherford’s Nuclear model
Ernest Rutherford’s famous gold foil experiment revolutionised the model of the atom. In 1909, He found that most of the atom’s mass and positive charge were concentrated in a small, dense nucleus at the centre, with electrons orbiting it. Rutherford's model disproved Thomson’s previous theory as it showed most of the atom was empty with a central nucleus.
4. Bohr’s Planetary model
Niels Bohr proposed that electrons orbit the nucleus in fixed energy levels or shells like planets orbit the sun. His model explained the conundrum of why electrons don't spiral into the nucleus and how atoms emit or absorb light when electrons jump between energy levels. This model introduced the concept of quantised energy levels. It marked a significant step forward in atomic theory.
5. The Quantum Mechanical model
Quantum mechanics describes electrons as probability clouds or orbitals around the nucleus. It is the current atomic model adopted by science, which was developed by scientists including Schrödinger and Heisenberg. It is a more accurate representation of how electrons behave at the subatomic level. Key to the model is the Heisenberg Uncertainty Principle, which states that we can't know the exact position and momentum of an electron simultaneously.
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Who introduced the nuclear model of the atom, which states the atom is largely empty with a dense central nucleus?
Importance of atomic structure in chemistry
Atomic structure helps us understand the properties of elements, how they interact with other elements and the creation of compounds.
1. Periodic table and element properties
Elements are organised by increasing atomic number in the periodic table. The position of an element in the table shows its atomic structure, including how many protons, neutrons and electrons it contains. Periodic groups are organised by column and the elements in each group have similar electron configurations (especially in their outer shells). The means they have similar chemical properties e.g. all elements in Group 1 have one electron in their outer shell, making them highly reactive metals.
Atomic structure can also explain trends in the periodic table, such as why atoms get larger as you progress down a column. These patterns are due to the electron arrangements and energy levels.
2. Chemical bonding
Chemical bonding happens when atoms interact to create molecules and compounds. It is more easily understood through atomic structure. There are three types of chemical bonds: ionic, covalent and metallic. Each bond varies in how the electrons are shared or transferred between atoms.
- Ionic bonding - This is the transfer of electrons between atoms. It creates positively and negatively charged ions that attract each other. Ionic bonding normally happens between metals and non-metals e.g. sodium chloride (table salt).
- Covalent bonding - The sharing of electron pairs between atoms that are normally non-metals. This creates molecules like water (H2O), where the electrons are shared between hydrogen and oxygen atoms.
- Metallic bonding - This happens between metal atoms, where electrons are shared in delocalised electrons that allow metals to conduct electricity and heat.
3. Reactions and compounds
Atomic structure is crucial to the formation of compounds and their reactions. Examples include the combustion of fuels, rusting of iron, or the complex biochemical reactions in our bodies. Everything hinges on the knowledge of how atoms interact.
For instance, the reaction of hydrogen with oxygen created water. This is a reconfiguration of atomic structures by breaking and forming of bonds. This reaction releases energy, which is why hydrogen is used as a fuel.
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Why do elements have similar chemicals when they're in the same group of the periodic table?
Final thoughts: Atomic structure revision
We have looked at the evolution of atomic models, from Dalton's theories to the quantum mechanical model currently being used. Each model has furthered our understanding of atomic structure and reinforces that science is a constantly evolving field through research and experimentation. Follow the link for a greater understanding of John Dalton and his impact on atomic theory.
Atomic structure underpins many topics that you'll encounter in chemistry. It also explains the chemical behaviour in everyday life, such as the formation of salts through ionic bonding or the use of radioactive isotopes in medical treatments. The principles of atomic structure have real-world applications all around us.
It's a good idea to test your knowledge by using worksheets and past exam questions. ChemSpider is an interactive tool that can support your learning. If you need extra support, there are qualified GCSE Science tutors at TeachTutti to provide personalised help with this and other challenging topics.
