This number is the atomic number given by the symbol Z.ДЄtomic number represents the number of protons in an atom.ДЄs atoms are electrically neutral, an atom contains as many electrons as it has protons. Since an atom is electrically neutral, the number of protons in the nucleus is exactly equal to the number of electrons. The electrons occupy the space outside the nucleus. These two components of the nucleus are referred to as nucleons. The nuclei of atoms are made up of protons and neutrons. Therefore when electrons start filling, they may go to a new outer shell even before the inner shell is filled to capacity. These sub shells can overlap, resulting in energies that may differ from that predicted purely on the basis of n=1, 2, 3 etc. The maximum number of electrons that can go into these are 2, 5, 10 and 14 respectively. The smaller sub shells are termed s, p, d and f. This abnormal behaviour can be explained as follows: Special case of potassium and calcium elementsДЄtomic number of potassium is 19 and its electronic configuration isДЄtomic number of calcium is 20 and its electronic configuration is Maximum number of electrons(2n 2)Д’ x (1) 2 Maximum number of electrons in the 4th energy level = 2n 2 Maximum number of electrons in the 3rd energy level = 2n 2 Maximum number of electrons in the 2nd energy level = 2n 2 Maximum number of electrons in 1 st energy level = 2n 2 Maximum number of electrons that can be accommodated in a shell is given by 2n 2 where n = shell number.Important Rules: Number of electrons in a shell The arrangement of electrons in the various shells/orbits/energy levels of an atom of the element is known as electronic configuration. The 19th and 20th electrons go into the 4th shell has a maximum of 8 electronsД¤th energy level is N shell and so on. It has a maximum of 8 electronsДЈrd energy level is M shell. It has a maximum of 2 electronsДўnd energy level is L shell. Notice that the number of electrons and that of electrons are always equal in neutral atoms.ДЎst energy level is K shell. Lithium has 3proton, 4 neutrons and 3 electrons as shown. The picture below represents an atom of lithium. The protons and neutrons are in the nucleus (centre) of the atom and the electrons orbit round the outside in shells (energy levels or layers). It is made up of sub-atomic particles like electrons, protons and neutrons.ДЄtoms of one type of element differ from those of the other due to different number of sub-atomic particles. Atoms cannot be created, divided or destroyed during any chemical or physical change.ДЄt present we know that the atom is the smallest particle of an element.Atoms combine with each other in simple whole number ratios to form compound atoms called molecules.Atom is the smallest unit that takes part in chemical combinations.Atoms of different elements are different in all respects. Each element is composed of its own kind of atoms.Atoms of the same element are identical in shape, size, mass and other properties.All substances are made up of tiny, indivisible particles called atoms.John Dalton provided a simple theory of matter to provide theoretical justification to the laws of chemical combinations in 1805. They discovered that the 'atom' maintains its chemical identity through all chemical and physical changes. The name 'atom' was derived from the Greek word 'atomos', meaning 'indivisibleвЂ™. They called the smallest particle an 'atomвЂ™.

When scientists started exploring matter, they realised that matter can be divided into smaller and still smaller particles. The Structure of the Atom and the Periodic Table

Chemistry Notes Form 2 - Chemistry Notes Form Two

Periodic table chemistry spm