Chapter Three Summary: Atoms and atom's structure Ancient Greek Philosophers suggested that matter was composed of individual particles, which was proved by the theory of John Dalton. The main provisions of his theory are: 1. All matter is composed of tiny, indivisible particles called atoms. 2. All atoms of a particular element are identical but the atoms of one element differ from the atoms of other element. 3.

Atoms of different elements combine with each other in certain whole number proportions to form compounds. 4. In a chemical reaction, atoms are rearranged to form new compounds which are not created, destroyed or changed into atoms of any other element. Three fundamental laws give the idea to support the atomic theories which are: The law of conservation of mass, which says that in a chemical reaction, matter is neither created nor destroyed, was developed by French scientist Antone Lavoisier.

The law of definite proportions states that different samples of any pure compound contain the same elements in the same proportions by mass. This law was established by Proust. The law of multiple proportions state that the masses of one element can combine chemically with a fixed mass of another element in a ratio of small whole numbers. Cathode-ray tube experiments shows that atoms contain negative particles which are called electrons. Thomson and Milian established the idea that the electron has no mass.

Atoms contain positive particles named protons. Rutherford discovered that there is a minute, dense, positively charged nucleus at the center of atom. He was awarded the noble prize for his work. The nucleus of an atom is comprised of protons and neutrons. Protons have +1 charge and neutrons have no charge but it has the same mass as the protons. Electrons have -1 charge and its mass is very small which is easily negligible.

The positively charged protons in an element are balanced by the negatively charged electron, that's why the atom of that particular element has no charge. The atomic number is the number of protons present in the element. The sum of the protons and neutrons is termed the mass number of an element. Atoms of an element that has the same number of protons but different number of electrons are called the isotopes. The AMU (atomic mass unit) of a particular element expresses the mass of an atom of that element relative to the mass of the most common isotope of carbon which is 12. The average mass which takes into account the relative abundance of the different isotopes is called the atomic mass or atomic weight.

Periodic table was made by Russian scientist Mendeleev. He arranged the known elements in order of increasing atomic mass in horizontal rows and the elements with the similar properties in the vertical columns. The horizontal rows are called the periods and the vertical columns are called the groups. Each element in the periodic table is represented by its symbol along with its atomic number and atomic mass. The first 92 elements occur naturally and the rests were synthesized in the lab. Elements are classed into metal and non-metals.

Metals are good conductors of heat and electricity and they are ductile and malleable. At room temperature, metals are solid except mercury. Non-metals are not electric conductor and neither it is ductile nor malleable. At room temperature, many of the non metals are gases except the bromine.

Semi metals lie between the metals and the non metals. They conduct electricity well only at a high temperature. Semi-metals are also called the metalloid's.