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General Chemistry - Elements

There are 103 elements, and they are all set out on the periodic table. The smallest existing unit of any element is one atom of that element. All atoms have a nucleus, which contains protons and neutrons. Each proton has a positive charge of +1. Neutrons have no charge. Outside its nucleus, the atom contains electrons and each electron has a charge of -1. In every atom, the normal number of electrons outside the nucleus is equal to the number of protons inside the nucleus, so the atom is electrically neutral. Knowing that an atom has a positive charge in its nucleus and negative charge outside its nucleus, you can see that the atom is held together by the attractive force of opposite charges. 

For any atom, the number of protons in the nucleus is called the atomic number. The atomic number is shown by a subscript next to the symbol of the element, and it tells you what element the atom is. 

Isotopes :
For any atom, the number of protons or electrons is not necessarily equal to the number of neutrons. If two atoms of the same element differ in their numbers of neutrons, they are called isotopes. Isotopes have the same atomic number and the same net charge, but they differ in their mass numbers. 

The mass of an electron is negligible; therefore, an atom’s mass derives entirely from its protons and neutrons. The mass number for any atom is equal to the sum of its protons and neutrons. The mass number is represented by the superscript of an element. 

Now, remember that the mass number is not the same as atomic weight. In nature, elements typically occur as combinations of their isotopes, so the atomic weight of an element represents the average weight per atom within the combination of isotopes of that element that usually occurs in nature. 

The unit of measurement for atomic mass is atomic mass units, AMU. 

Ions :
If an atom loses an electron, it acquires a charge of +1 and is called a positively charged ion. If an atom gains an extra electron, it acquires a charge of -1 and is called a negatively charge ion. 

Let us summarized what we have just said. Proton’s charge equals +1. Neutron has no charge, and electron’s charge equals -1. Atomic number equals number of protons. Mass number equals number of protons and neutrons. Atomic weight, AMU, equals approximate average weight of an atom calculated from natural isotopes. Atomic weight does not equal mass number. 

Electrons exist in orbitals. Electrons are located outside the nucleus in electron orbitals. Each orbital can have two electrons. Orbitals are located in subshells, and the subshells are located in shells. To specify a particular electron of an atom, we indicate that electrons four quantum numbers. 

Shells are the first quantum number, which is symbolized by the letter N. When an electron is located in the first shell, the electron is said to have a first quantum number of one. 

Subshells are the second quantum number, which is symbolized by the letter L. An electron’s second quantum number specifies its subshell. There are four subshells called s, p, d, and f. The first subshell is called the s subshell. The second shell has two subshells; the s subshell and the p subshell. The third shell has three subshells; the s subshell, the p subshell, and the d subshell. The fourth, fifth, sixth and seventh shells have four subshells; the s subshell, the p subshell, the d subshell, and the f subshell. 

Orbitals are the third quantum number, which is symbolized by the letter M. To find the number of orbitals in a particular subshell, we take the quantum number that designates the subshell, multiply it by two and add one. Subshell s has quantum number zero, subshell p has quantum number one, subshell d has quantum number two, and subshell f has quantum number three. So, to find the number of orbitals that are in the f subshell, we multiply its quantum number, three, by two which will give us six and then we add one, which makes it seven. Knowing that each orbital can hold two electrons, we know that the f subshell can hold 14 electrons. 

Electrons have spin which is the fourth quantum number symbolized by Ms. An electron spin is either plus ½ for a positive spin or minus ½ for a negative spin. 

Electron configuration :
The electron configuration of an element tells us the order in which electrons fill the subshells from lower energy orbitals to higher ones. The following are the electron configurations of the first three elements. Hydrogen: 1s1; helium 1s2; lithium 1s2 2s1. So lithium, for example, has 1 electron in the 2s sub shell and 2 electrons in the 1s subshell.