The ancient Greeks believed the world was made of five elements: earth, air, fire, water, and aether. Somewhere around 500 BCE, Democritus first put forward the idea that everything in the world was made of small indivisible particles called atoms. The term atomos is derived from the ancient Greek, meaning “indivisible”. In the early 1800s, John Dalton formalized atomic theory. He suggested all matter is made of small particles called atoms, that these atoms were rearranged in chemical reactions, and that these atoms have different properties.
Dmitri Mendeleev was a Russian chemist who is known as the father of the periodic table. He organized the elements known at the time into a table and left gaps in it for elements he predicted would be discovered later. The modern periodic table has 118 different elements, with 18 groups and seven periods.
Ninety-four elements are naturally occurring with 80 of them having stable isotopes. The most abundant element on Earth is oxygen, an element which is essential for life as we know it on our planet. Scientists believe that the two lightest elements were created in the Big Bang. All the other naturally occurring elements have come to exist through nuclear reactions. Stars fuse different nuclei together to produce heavier nuclei, but stars can only produce elements as heavy as 26 protons, which is iron. Elements heavier than this were created in supernovas, up to atomic number 94. Anything larger than this was created artificially by humans. Some of these supermassive elements are very unstable and break apart, or decay, in fractions of a second after they are created.
The periodic table is a way of organizing elements. In the modern periodic table, the elements are ordered by their atomic number. The atomic number indicates how many protons there are in the nucleus of an atom. The atomic mass tells us how many protons and neutrons are in the nucleus. The number of electrons are the same as the number of protons in a neutral atom. The vertical columns are known as periodic table groups. The elements in a group all have similar properties. The elements in group one for example, are all metals and all react with water. The horizontal rows are known as periods. Although elements in the same period do not have similar properties, they all share the same number of electron shells. The modern day periodic table consists of 118 different elements, starting with hydrogen with an atomic number of one, and ending with Oganesson with an atomic number of 118.
Elements are made up of three types of subatomic particles called protons, neutrons and electrons. While protons and neutrons are found in the nucleus of an atom, the electrons orbit the nucleus in shells or energy levels, located at different distances from the nucleus. Electrons are held in orbit as their negative charge is opposite to that of the nucleus. The electrons always seek the lowest energy state in an atom. In the first shell, we put a maximum of two electrons, followed by eight electrons in the second and third shells. For example, scandium has an atomic number of 21, meaning it has 21 protons. As it is a neutral atom, there are also 21 electrons. The shells are filled from the smallest going away from the center. Scandium has 21 electrons, so we need to put 21 electrons into the shells. So there will be two in the first shell, eight in the second, eight in the third, and three in the fourth. Scandium has a structure of 220.127.116.11.
The elements in group one are known collectively as the alkali metals.They are all metals that react vigorously with water. They all have one electron in their outer shell. As you move down the group from lithium to francium, the reactivity increases.
The elements in the group opposite the alkali metals are known as noble gases. They are very unreactive and have a full outer shell. They are nonflammable and have low boiling points. They include helium, neon, argon, krypton, xenon and radon.