Atoms, Elements, and Compounds

An Atom (a Greek word meaning indivisible) is the smallest constituent of an element that can get involved in a Chemical Reaction. What's an element? A substance that is purely made of one type of atom and cannot be decomposed into any purer substances. Each atom is distinct and what's inside an atom is what makes a difference between different elements. 

Every atom has a nucleus called atomic nucleus which contains Nucleons (Protons and Neutrons) and Electrons. Similarly, in cellular biology we have cell nucleus which contains the DNA and that is what makes a difference between living organisms. 

Since the mass of an atom is so small, a separate unit is defined to treat atoms and subatomic particles' masses more easily. One Atomic Mass Unit (amu) is 1/12 of the mass of Carbon-12 which is about 1.66 × 10-27 kg. So why is this? because the mass of a proton or neutron is equal to one amu. The other property of an atom is its weight called Atomic Weight (A) (mass number) which is equal to the number of its nucleons (protons + neutrons) and an Atomic Number (Z) is only the number of protons in the nucleus of an atom. An Electric Charge is a fundamental property of protons and electrons. Protons have positive charge, electrons have negative charge, and neutrons have no charge. In order for an atom to be neutral there should be no electric charge which means, that the number of electrons should be equal to the number of protons so their positive and negative charges neutralize each other and atom remains naturally neutral. 

The atomic number of an atom determines its chemical behavior and therefore, atoms with the same atomic number are essentially the same element. The atomic number is the number of protons, so what if the atoms with the same atomic number (having the same number of protons), contain different number of neutrons? Consequently, they will have different atomic weights too, because the atomic weight count for both neutrons and protons. Such atoms are called Isotopes. So it is fair to say all isotopes of the same atom behave the same way chemically because only the number of protons (which are the same among isotopes) determines chemical behavior. Now I know what it means to say Carbon-12 which is a carbon isotope: Carbon is the element name and 12 is the atomic wight. Now if we know that the atom of Carbon has only 6 protons, It means that Carbon-12 has 6 neutrons and Carbon-14 has 8 neutrons. Here is a good example of comparing the isotopes of Hydrogen and Carbon.

Here is a very nice video describing the Atom and its properties.

When different elements are combined together (through some chemical bonding) it is called a Compound. That is, in a compound material, we have several different atoms that are held together with Chemical Bonds. Binary compounds contain two elements; Ternary compounds contain three elements and so on. There are different types of chemical bonds: 

• Covalent Bonds create Molecular Compounds, 

• Ionic Bonds create Salts, 

• Metallic Bonds create Intermetallic Compounds, and 

• Coordinate Covalent Bonds create Complexes.

The smallest subdivision of a compound or an element that is electrically neutral (no electric charge), is called a Molecule. A chemical formula represents the relativenumber of each element in a compound. For example, H2O shows that in the molecule of water there are two atoms of Hydrogen (H) element and one atom of Oxygen (O). The chemical properties of elements are a periodic function of their atomic number and this is called Periodic Law in chemistry which is the reason elements can be grouped periodically in a table called Periodic Table. In a periodic table, each vertical column is known as a group which exhibit similar properties to each other because of the similar subatomic formation in their atoms. This means, elements in different columns, have different properties. That is, two elements that are adjacent to each other in the same row are different in their properties! 

Generally, all the elements are categorized as either Metal or Non-metal. The non-metals are very poor Electrical Conductors and very highly electronegative, mostly being in the form of gas or brittle solids except Bromine which is liquid. Noble Gases and Halogens are non-metal. All the other elements are metals which have low electron affinities, high electrical conductivity and high melting points. Metals are further categorized into Alkali Metals, Alkaline Earth Metals, Transition Metals, the Lanthanides, and the Actinides. Here is a good flowchart on how compounds are named.

Much of the chemical behavior of an atom is based on its power to attract electron. This property of an atom is called Electronegativity which is measured between 0-4; most of electronegative elements are on the right end side of the periodic table and those with low electronegativity are mostly on the left side. This property of the elements decreases within a group of elements in the table as we go down to elements with higher atomic weight, inclining toward more metallic properties. 

Electron affinity is the property to attract electrons. For example, Chlorine has 7 electrons in the outer shell of its atom and to become stable attracts one electron so it could have 8 electrons in its outer shell. In order to remove this one electron from another atom and own it Chlorine requires some energy that is called ionization energy. Although, we need to know the fact that Chlorine needs another electron to become stable does not mean that it is not already neutral. An atom being neutral is having the same number of electrons and protons and Chlorine is already neutral by having 17 electrons (2 on the inside shell, 8 in next shell, and 7 in the outer shell) as it also has 17 protons. It is just the electronegativity of Chlorine that makes it so eager to attract another electron and fulfill its outer shell to 8. When it does attract the one electron it needs, it will have a negative charge then (-1). An atomic nucleus with any charge is called an ion. The process of attracting the electron results in some kind of bond with another atom which will become positively charged after the electron donation. This donation of electron is called oxidation.  

The negatively charged ions are called anions and the amount of charge is equal to the number of electrons attracted (also known as valence). If an anion wants to be neutral it has to lose electron through oxidation.