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Atoms, orbitals and Bonds
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Atoms Orbitals and BondingElements of Human Body
Humans require 25 elements for healthy life- to create all blood, bone, muscle, and other tissue- ‘ESSESNTIAL
element’
4 elements- Hydrogen, Carbon, Oxygen, and Nitrogen – make up
almost 97% of total body mass
Atoms- Revision
Period- horizonal rows
Groups- vertical columns
Mendeleev- elements which fall in same groups tend to exhibit similar physical and chemical properties- these
properties follow regular pattern of change
Atom- smallest unit in which an element can exist
- smallest particle into which element can be divided while still retaining properties of element
Atoms make up molecules. There are three of types of subatomic particles- protons, neutrons and electrons
Protons carry single POSITIVE charge
Electrons carry single NEGATIVE charge
Neutrons DON’T CARRY charge
In biology, nervous system consists of complicated network of nerve cells- use
electrical charge to transmit nerve impulses throughout body.
Atomic Number- number of protons (equal to number of
electrons for an atom)
Mass Number- approximately sum of protons and neutrons
Atom is a neutral particle- contains exactly same number of
protons and electrons
Periodicity- Incremental changes in physical and chemical properties across period or group
Element- single, simple substance that can’t be split into any more separate substance by any chemical means
Atoms to ions – Revision
Atoms of element- uncharged- number of protons and electrons is equal
Ions- formed when number of electrons different from number of protons
Anion- formed when atom GAINS an electron- NEGATIVE CHARGE- contains more electrons than protons
Cation- formed when atom LOSES an electron- POSITIVE CHARGE- contains more protons than electrons
,Magnitude of charge will depend on number of electrons GAINED or LOST
- Losing single electron- CATION with charge of +1
e.g. Na+, Mg2+, Cl-, S2-
- Losing two electrons- CATION with charge of +2
Readiness with which atom GAINS/ LOSES electrons dictates its reactivity- elements whose atoms gains/ loses readily
tend to be very reactive
Number of protons is CONSTANT for any element
Number of neutrons can VARY- gives RISE to ISOTOPES- relevant to radioactive decay
Number of protons doesn’t VARY for given element- determines CHEMICAL IDENTITY of
element
Isotopes
Isotopes- Atoms that have same number of protons and electrons but different number of
neutrons
Atoms of hydrogen contain NO neutrons at all- Hydrogen exists as two isotopes
- 1 Proton, 1 Electron, 0 Neutron- Denoted 1H
- 1 Proton, 1 Electron, 1 Neutron- Deuterium- 2H
Element’s atomic weight (Relative Atomic Mass)- measure of the average mass of
an atom of that element
Element’s atomic mass- mass of single atom of single, specific isotope
Atomic Structure
Each atom has a NUCLEUS at its centre
- Diameter of nucleus is ~ 10000X smaller than diameter of atom
Nucleus contains protons and neutrons
Space around nucleus- much bigger- contains electrons
- Electrons move around nucleus continually- form ELECTRON ‘CLOUD’
- Electrons move within very specific regions of atom – ‘ORBITALS’- electrons move freely within an orbital
Protons and neutrons make up all the mass of atom- occupy only tiny fraction of its volume
Atomic orbitals
Electrons occupy different types of orbitals- s, p, d and f
These orbitals differ in their SHAPES and their proximity (nearness) to nucleus
Orbitals can have more than 1 lobe- Other than s-orbitals which always have 1 lobe
Orbitals are grouped into shells, identified by number:
- Shell 1 contains 1s orbital
- Shell 2 contains 1s orbital and 3 p orbitals
- Shell 3 contains 1s orbital, 3p orbitals and 5d orbitals
- Shell 4 contains 1s orbital, 3p orbitals, 5d orbitals and 7f
orbitals
Subshell- Group of equivalent orbitals occupying same shell ( 3
p orbitals in shell 3)
, Summary
Structure of atom is small nucleus surrounded by larger- diffuse area made of orbitals
Nucleus contains PROTONS and NEUTRONS (heavy)
Orbitals describe probability of finding electron in certain place (light)
Protons carry +1 charge, Electrons carry -1 charge, Neutrons are neutral
Atoms can GAIN or LOSE electrons- become ions which can carry
a charge
Number of protons in given element doesn’t change
There are 4 types of orbitals- arranged in shells and subshells
Orbital Occupancy
Any orbital can hold up a max of 2 electrons- Pauli Exclusion Principle
- Therefore, shell 1 can hold only 2 electrons- has 1s orbitals
As the electron number increases- electrons are forced to occupy orbitals of
progressively higher energy- orbitals lie further away from nucleus
Restriction in orbital occupancy leads to diffuse nature of ‘electron clouds’
Energies of all orbitals isn’t same- important consequences for reactivity
First approximation - orbitals that are close to nucleus have lower energy
- orbitals that are further away from nucleus in space
have higher energies
Filling up Orbitals
Energy of electron is equivalent to energy of orbital in which its located
- electron in 3s orbital has more energy than electron in 2p orbital
Electrons fill up orbitals according to their energy- starting with orbitals with lowest
energy – Aufbau Principle
Entities with lower energy are MORE STABLE than those with higher energy
Atom is most stable if its electrons have lowest energy possible
Electrons fill up lowest energy orbitals first- enter higher energy orbitals if lower energy orbitals are full.
Shells 1 to 4 posses progressively higher energies- shell 1- lowest energy
- shell 4- highest energy
s, p and d orbitals within same shell possess progressively higher energies- s orbitals are lowest energy
- d orbitals are highest energy
Orbitals in same subshell have same energy- DEGENERATE
- In shell 2, 3 x p orbitals will have same energy
Hund’s rule- equivalent orbitals are filled with one electron, before second electron enters.
4s orbital fills before than 3d orbital- 4s orbital is of slightly lower energy than 3d orbital
Atom is most stable if its electrons have the lowest energy possible- electrons locate themselves as close to the
nucleus
Summary of electrons in Orbitals
Aufbau Principle- Electrons fill orbitals of progressively higher energy
Pauli Exclusion Principle- Orbital contains no more than two electrons
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