Pure substances can be classified as elements and compounds. Elements are made of atoms and our understanding of the nature of the atom has evolved over two centuries. The atom is composed of a nucleus that is extremely small compared to the outer boundary of the atom, but contains nearly all the mass of the atom (so it is super dense). The nucleus contains protons and neutrons. The number of protons is unique to each element and is the same as the atomic number on the periodic table. The number of neutrons changes within atoms of the same element to make different isotopes of the element. The mass number is the number of protons and neutrons in each isotope, while the periodic table lists the average atomic mass of the element (measured in amu, atomic mass units). The average atomic mass is determined by using the percent natural abundance of each isotope of the element and the mass of each isotope in a calculation called a weighted average. Electrons surround the nucleus and form the boundary of the atom called the electron cloud. The number of electrons equals the number of protons for a neutral atom, because the negative charges of the electrons match the positive charge of the protons in the nucleus. Changing the number of electrons forms an ion of the atom. Gaining electrons increases the negative charge to make an anion and losing electrons leaves a higher positive charge to form a cation.

Electron configuration of atoms is important for understanding the chemical and physical properties of elements. Quantum theory is used to precisely determine the nature of all electrons in any atom. Each electron—first, second, third, etc.—has a unique set of quantum numbers used with wave equations. Quantum numbers can be translated into electron orbitals, which represent “containers” for the probability of finding an electron about the nucleus. Creating an electron configuration that lists the energy level, or shell, with the type of orbital, or subshell, and the number of electrons in each suborbital, shows that similarities in electron configuration appear in each family of the periodic table. Orbital diagrams are figures that show the electron configuration, and also indicate the number of subshell available and the spin of electrons that fill the orbitals. This more detailed listing of electrons can be used to explain more specific details of the order of electrons in atoms, such as the exception of Cr, chromium, and Cu, copper.

The changes in the arrangement of electrons changes the properties of the elements, but because the pattern repeats with the addition of a new energy level, there are trends in the properties of elements on the periodic table, periodic trends. Ionization energy, which is the energy needed to remove a single electron, increases from bottom to top of the periodic table and increases from left to right. Electronegativity follows a similar pattern and atomic size is a trend in the opposite direction, increasing from top to bottom and right to left.

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