Atomic Structure

Atomic Structure

Atomic structure is a fascinating topic that has been studied for over 2000 years. Democritus, a Greek philosopher, once said that nothing exists except atoms and empty space; everything else is just opinion. He was partly right! Almost everything around us, from rocks to stars to blood to batteries, is made up of atoms. However, Democritus believed that atoms were the smallest unit possible. We now know better! The study of particles inside the atom created the field of atomic structure.

Atomic structure refers to the arrangement of subatomic particles within the atom. An atom is the smallest unit of ordinary matter within an element. Atoms are made up of three types of subatomic particles: protons, neutrons, and electrons. If you're curious about the world around you, understanding the basics of atomic structure is a great place to start. So, let's delve deeper into this topic!


Protons are tiny particles that carry a positive charge. Even though they only weigh about kg, their mass is measured on a 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 talk about carbon-12 atoms in more detail later on in this article.

In the centre of an atom, you'll find the nucleus, where protons are tightly packed together. The number of protons in an atom is crucial because it determines which element it belongs to on the periodic table. Knowing the number of protons in an atom is the key to unlocking its identity!


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 are negatively charged particles, and they are much lighter than protons. In fact, they have a relative mass of only 1/1840 on the carbon-12 scale. Unlike protons and neutrons, electrons don't hang out in the nucleus. Instead, they occupy energy levels, also known as shells, that surround the nucleus. The further away from the nucleus an electron is, the higher its energy level. Electrons always try to occupy the lowest energy level possible. For more information on electron shells, sub-shells, and orbitals, check out "Electron shells, sub-shells and orbitals."

The number of electrons in an atom determines how it behaves chemically and how it interacts with other atoms. Overall, atoms may seem crowded with all their subatomic particles, but they are mostly empty space. Democritus wasn't too far off the mark after all! To summarize, here's a helpful table that shows the relative charges, masses, and locations of the different subatomic particles in an atom:

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:

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

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

Oxygen has an atomic number of 8, which means it has 8 protons, and it has a mass number of 16, which means it has 8 neutrons and 8 electrons.

Ions are atoms that have gained or lost electrons, resulting in a charged particle. They have different chemical properties compared to their neutral counterparts due to their altered electron configurations. On the other hand, isotopes are atoms of the same element that have varying numbers of neutrons. Isotopes have similar chemical properties because their electron configurations remain the same. However, they differ in their mass and stability. If an atom loses a neutron, it wouldn't significantly affect its chemical properties because neutrons are neutral particles. However, it would change the mass of the atom, resulting in a different isotope. To learn more about ions and isotopes, check out the article "Ions and isotopes."

What is relative atomic mass?

Mass spectrometry is an analytical technique used to measure the mass-to-charge ratio of ions in a sample. The results are presented as a mass spectrum, which shows the relative abundances of the ions on the y-axis and their m/z ratios on the x-axis. Mass spectroscopes consist of five basic parts: a high vacuum system, a sample handling system, an ion source, an analyzer, and a detector or recorder .

In mass spectrometry, atoms or molecules are ionized using a high-energy electron beam and then separated based on their mass-to-charge ratios (m/z). The sample is typically in an aqueous or organic solution and is immediately vaporized by a heater.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 an element, we multiply the mass of each isotope by its percentage abundance and add them all together.

The periodic table

How are electrons arranged in the atom?

Electrons are negatively charged particles that occupy shells or energy levels around the nucleus of an atom. The shells are arranged in increasing energy levels, with the innermost shell having the lowest energy level. Each shell can hold a maximum number of electrons, and these electrons fill the orbitals in the shell according to certain rules.

The shells are divided into sub-shells, which are designated by letters such as s, p, d, and f. The sub-shells have different shapes and orientations, and the number of sub-shells in a shell depends on the shell's principal quantum number, n.

The s sub-shell has a spherical shape and can hold 1 or 2 electrons. The p sub-shell has a dumbbell shape and can hold up to 6 electrons. The d sub-shell has a more complex shape and can hold up to 10 electrons, while the f sub-shell can hold up to 14 electrons.

The electron configuration of an atom describes how the electrons are arranged in the different sub-shells and shells. The configuration of the electrons in an atom determines its chemical properties, such as its reactivity and ability to form bonds with other atoms.

The filling of the sub-shells follows certain rules, such as the Aufbau principle, which states that electrons fill the lowest energy level first before moving to higher levels. The Pauli exclusion principle states that each orbital can hold a maximum of two electrons with opposite spins, while Hund's rule states that electrons fill the orbitals singly before pairing up.

In summary, the electron configuration of an atom is determined by the number of electrons in each sub-shell and shell, and this configuration helps to explain the chemical properties of the element.

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.


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

The discovery of the nucleus by Rutherford and his team in the early 20th century revolutionized our understanding of atomic structure. The nucleus is a tiny, densely charged positive mass found at the center of the atom, and it contains protons and neutrons. Electrons, which are negatively charged particles, are found in shells orbiting the nucleus.

The number of protons in the nucleus of an atom determines its atomic number and its chemical properties. The total number of protons and neutrons in the nucleus gives the atom's mass number. Isotopes of an element have the same atomic number but different mass numbers due to a different number of neutrons in the nucleus.

The arrangement of electrons in the shells around the nucleus determines the chemical properties of an element. Electrons fill the shells according to certain rules, such as the Aufbau principle, Pauli exclusion principle, and Hund's rule, to achieve the lowest energy state possible.

Our understanding of atomic structure has evolved over time, as new scientific discoveries have been made. For example, the development of quantum mechanics in the early 20th century led to a better understanding of the behavior of electrons in atoms and molecules.

In summary, the discovery of the nucleus by Rutherford and his team was a major breakthrough in our understanding of atomic structure. The nucleus contains protons and neutrons, while electrons are found in shells orbiting the nucleus. The arrangement of electrons in the shells determines the chemical properties of an element, and our understanding of atomic structure continues to evolve with new scientific discoveries.

Atomic Structure

What is the electronic structure of atoms?

The electronic structure of an atom is its internal arrangement of electrons in shells and subshells.

How can you describe the structure of an atom?

Atoms contain positive protons and neutrons found in a central nucleus, and electrons orbiting the nucleus in shells.

How can you find the electronic configuration of an atom?

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.

What is the difference between an atom and an ion?

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.

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