Atomic Structure

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.

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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:

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

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 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.

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.

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:

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.