Atoms, Molecules and Ions

Postulates of Dalton’s atomic theory.

• Matter is composed of exceedingly small particles called atoms. An atom is the smallest unit of an element that can participate in a chemical change.
• An element consists of only one type of atom, which has a mass that is characteristic of the element and is the same for all atoms of that element (Figure 2.2). A macroscopic sample of an element contains an incredibly large number of atoms, all of which have identical chemical properties.
• Atoms of one element differ in properties from atoms of all other elements.
• A compound consists of atoms of two or more elements combined in a small, whole-number ratio. In a given compound, the numbers of atoms of each of its elements are always present in the same ratio (Figure 2.3).
• Atoms are neither created nor destroyed during a chemical change, but are instead rearranged to yield substances that are different from those present before the change.

Atomic Structure and Symbolism

• The number of protons in the nucleus of an atom is its atomic number (Z).
• The total number of protons and neutrons in an atom is called its mass number (A)
• The number of neutrons is therefore the difference between the mass number and the atomic number: A – Z = number of neutrons.
• Atoms are electrically neutral if they contain the same number of positively charged protons and negatively charged electrons. When the numbers of these subatomic particles are not equal, the atom is electrically charged and is called an ion.
• An atom that gains one or more electrons will exhibit a negative charge and is called an anion.
• Positively charged atoms called cations are formed when an atom loses one or more electrons.

Chemical Symbols

A chemical symbol is an abbreviation that we use to indicate an element or an atom of an element. For example, the symbol for mercury is Hg.

Isotopes

Atoms with the same number of protons but different numbers of neutrons are called isotopes. They share almost the same chemical properties, but differ in mass and therefore in physical properties.

You can use the below application to understand the Formation of Atoms.