Structure of Atoms – Structure of an Atom
* An atomic number is the number of protons in an atom.
* The arrangement of electrons in various energy levels of an atom is known as electronic configuration.
* The maximum number of electrons in any energy level of the atom is given by 2n2.
* The outermost shell in an atom is cannot more than 8 electrons.
* Combining capacity of the atom to form molecules either with the same or different elements is known as valency.
* Neutrons are a neutral particle present in the nucleus.
* The total number of protons and neutrons present in one atoms of an element is referred as atomic mass
* Isotopes are atoms with same atomic number but different mass numbers.
* Isotopes of certain elements are used in a nuclear reactor, and in the treatment of cancer and goitre.
* Atoms of different elements with different atomic numbers that have the same mass number are called Isobars.
Metals and Non – Metals
* Metals are elements that have a tendency to lose electrons and form positively charged ions or cations.
* Elements that have a tendency to accept electron to form negatively charged ions or anions are called non- metals.
* Metals are placed on the extreme left in the periodic table, while non-metals are placed on the extreme right in the periodic table.
* Metals are solids, malleable, ductile, and lustrous with high melting and boiling points.
* Non-metals are electro negative, bad conductors of heat and electricity, from acidic oxides which are covalent.
* Electrolysis is a process of decomposition of an ionic compound by passing electricity through it.
Carbon and its Compounds – Bonding in Carbon
* Covalent compounds have low melting and boiling points.
* Carbon achieves stability by sharing electrons.
* Bonds formation is represented using Lewis structures.
* A shared electrons pair is known as bond pair.
* A single covalent bond ifs formed by sharing a pair of electrons.
* A double covalent bond is formed by sharing two pair of electrons.
* A triple covalent bond is formed by sharing three pair of electrons.
* In a methane molecule, the carbon atom shares electrons with four hydrogen atoms.
Sub – Atomic Particles
* Matter is made up of atom that further consists of sub – atomic particles.
* Sub – atomic particles are electrons, protons and neutrons.
* Protons are positively charged, electrons are negatively charged and neutrons are neutral.
* Protons and neutrons have mass equivalent to Hydrogen whereas electrons has a negligible mass.
* Protons and neutrons, together called nucleons, are present in the centre of atom surrounded by revolving electrons and other uncommon sub – atomic particles.
* In 1808, John Dalton proposed the atomic theory of matter.
* According to Dalton, an atom is the smallest particle of matter.
* The research done by scientists like J.J Thomson, Goldstein, Rutherford,
* Chadwick and Bohr in the later part of the 19th century and 20th century proved that an atom is not the smallest indivisible particle.
Isotopes and Isobars
* Mass number is the sum of the number of protons and the number of neutrons.
* The atomic number is the number of protons.
* Isotopes have same number of protons but different number of neutrons.
* As the isotopes have the same number of electrons and protons, they have similar chemical properties.
* Isobars are atoms of different elements having different atomic numbers but same mass numbers.
* Isobars have different chemical properties because they have different atomic numbers.
Electronic Configuration
* The three main principles that guide us on the filling of orbitals in an atom are the Aufbau Principle, Pauli’s Exclusion Principle, and the Hund’s rule of Maximum Multiplicity.
* Electronic Configuration can be defined as the distribution of electrons into various orbitals of an atom.
We can represent the electronic configuration of atoms in two ways.
* Orbital diagram method
* (nlx) method
* We can also write the electronic configurations of elements whose atomic number are between 11 and 17, in terms of the electronic configuration of Neon.
* In some cases the actual configuration differs slightly from the expected ones – example, chromium and copper. This is because of the extra stability of half – filled and completely filled sub shell configuration.
* The cause of this extra stability has been attributed to the symmetry distribution and exchange energy.
Why do Atoms Combine?
* The atoms of the elements of the 18th group have minimum energy, and are highly stable among all the elements.
* The presence of two electrons in the first shell, called a duplet configuration, or eight electrons in the valence shell, called an octet configuration, is collectively referred to as inert gas configuration, which is the most stable electronics
configuration.
* In the formation of sodium chloride, sodium loses one electrons to form sodium ion, and attains the nearest inert gas configuration of neon. Chloride gains one electrons to form a chloride ion, and attains the nearest inert configuration of
argon.
* Atoms of elements such as chlorine and oxygen attain stability sharing electrons, and form molecules.
* The number of electrons lost, gained or shared by an atom of an atom of an element during a chemical reaction for attaining stable electronic configuration is called valency.