What do the numbers on the periodic table mean?

2019 marks the UN’s International Year of the Periodic Table or #IYPT. It’s 150 years since Dmitri Mendeleev presented his organisation of chemical elements to the world. The periodic table is used daily by chemists and other scientists as reference resource for the ingredients of the universe.

dmitri mendeleev statue dmitrimendeleev.com
Picture caption: A statue commemorating Dmitri Mendeleev surrounding by a radial presentation of his periodic table, in source: dmitrimendeleev.com

Ancient Greeks defined “elements” as one of the following four: earth, fire, water and air. They were used to explain how matter worked. Since the elucidation of atomic theory, scientists have broken the universe down into 118 chemical elements.

the four elements learner.org
Picture caption: the four classical elements and the descriptors used to classify them. Source: learner.org

Atoms can be thought of as small particles that make up everything but even they are made up of smaller components, known as sub-atomic particles, and the number of sub-atomic particles in each atoms defines what element the atom is. The three sub-atomic particles we learn about at school are protons, neutrons and electrons. Protons, neutrons and electrons are made up of even smaller components but as I am not a physicist, I’m not so concerned about looking at matter at that scale.

atomic structure explainthatstuff.com
Picture caption: the structure of an atom – a central nucleus made up of protons and neutrons surrounded by fast moving electrons. Source: explainthatstuff.com

Protons are positively charged, electrons are negatively charged and neutrons, as you might guess from its name, are neutral. The number of protons and electrons in a neutral atom are the same in order to balance the charge. An atom’s structure is made up of a central nucleus made up of protons and neutrons with electrons whizzing around the nucleus.

Because the electrons are on the exterior of the atom they are the particles that are actually involved in chemical reactions. Electrons can be completely transferred from one atom to another – in a process known as ionisation which forms ions, charged atoms – or they can be shared between two atoms to form a chemical bond. The degree of sharing between two atoms dictates what type of chemical bond has formed.

bonding weheartit.com
Picture caption: cartoon depicting the different ways atoms share valence electrons to form chemical bonds. In a covalent bond electrons are share fairly equally between atoms; in metallic bonds the electrons are not associated with specific atoms; in an ionic bond one atom loses an electron for another to gain that electron and a coordinate bond occurs when an atom donated two electrons to another atom. Source: pearlsofrawnerdism.com

Most of an atom is made up of empty space: if an atom were the size of a football stadium the nucleus would probably only take up the space of a marble sitting in the centre of the playing ground!

A standard periodic table will usually have 2 elements associated with each element: the atomic number and relative atomic mass number of that element. The atomic number can be defined as the number of protons present in the central nucleus of the element’s atom while the mass number is the total number of protons and neutrons in the nucleus. The mass number is the larger of the two numbers found associated with an element in the table.

The mass number can be quantified by a constant (a number that is shown to stay the same across numerous calculations) called Avogadro’s number. Avogadro’s number is 6.02 x 10^23. Essentially, what Avogadro’s number describes is the number of atoms you need of a particular element for the weight of your sample to match the mass number in grams.

A single unit of 6.02 x 10^23 atoms is described as a “mole”. It’s a bit like saying you have “a dozen” eggs = 12 eggs. A “mole” of atoms is 6.02 x 10^23 atoms which, if your element is carbon, would equal 12 g because 12 is the mass number of carbon.

moles hinkhousescience.com
Picture caption: a happy cartoon mole placing 6.02*10^23 carbon atoms on a scale to make 12 g of carbon (aka one mole!) credit: hinkhousescience.weebly.com

The number of neutrons does not correlate perfectly with the number of protons, its more to do with the number of neutrons required to allow that number of protons to be in close proximity with each other while maintaining stability – imagine trying to force two magnets together. They repel, much like the positively charged protons do if they get too close together.

The presence of protons in the nucleus makes the nucleus overall positively charged and the electrons whizz around within the proximity of the nucleus because they, as negatively charged particles, are attracted to that positively charged nucleus.

As an element gets bigger – with more and more protons, neutrons and electrons packed into its structure – its properties change. The smaller elements at the top of the periodic table are gases like hydrogen (atomic number 1), helium (atomic number 2) and oxygen (atomic number 6) while the heavier elements towards the bottom of the table are metals like gold (atomic number 79), lead (atomic umber 82) and uranium (atomic number 92).

phases pt periodictable.com
Picture caption: periodic table coloured according to the phase (solid, liquid, gas) an element exists in at room temperature. Most elements are solid, coloured red; some gas (blue) and only two are liquid, bromine and mercury. Source: periodictable.com

Mendeleev chose to arrange the elements by atomic number rather than mass number and very cleverly left gaps because at the time there were only 56 known elements. He was successfully able to predict the properties of the yet undiscovered elements, such as germanium, gallium and scandium.

There are other periodic tables that have additional information on them such as the size of atoms, density of elements and melting points. Trends can be seen as you go along the rows and columns of the periodic table, which I may discuss in further detail in a future post. But normally the most basic ones have the atomic and mass numbers of each element on them, as well as the symbol used to abbreviate the element’s name.

desnity pt periodictabloe.com
Picture caption: Graphic showing the increasing density of the elements going down the periodic table and also peaking in the middle of the table going left to right. Source: periodictable.com

On my Instagram and twitter account I will be posting about each element over the course of this year and will summarise each column of the periodic table in a longer blog post here. I hope you enjoy exploring the periodic table with me this year along with everyone else marking #IYPT as we learn about the stuff that makes us and everything around us.

Did you learn about the periodic table at school? Do you have a favourite element? Let me know in the comments below.


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