Transcript Chemistry 10 - Ms Dalby's Website

Chemistry 10
Ions (Cations & Anions)
Bohr Diagrams
Lewis Dot Diagrams
Old Textbook, pages 138147 for more review and
reading.
Review
Atoms are the basic building units of matter around us.
An atom consists of three basic particles: positively charged protons,
negatively charged electrons and neutral neurons.
Electrons revolve around the nucleus.
The nucleus is at the center of the atom and consists of protons and
neutrons.
The pathway of electrons around the nucleus is called orbit.
An atom is neutral due to equal number of electrons in the orbits and
number of protons in the nucleus.
All ions want to obtain an electron arrangement like that of the Nobel
gases, a full valence shell.
Words To Know
Ion – An atom, or group of atoms, that has a net positive or
negative charge.
Cation – An ion with a net positive charge, due to the loss of one or
more electrons from a neutral atom. Usually formed by metals.
Example: Sodium atom, Na, can readily lose an electron to
become a sodium cation, Na+
Anion – An ion whose net charge is negative due to an increase in
the number of electrons. Usually formed by non-metals.
Example: Chlorine atom, Cl, can gain an electron to become a
chloride ion, ClMonatomic Ion – Contains only one atom, such as Fe3+, Mg2+
Polyatomic/Molecular Ion - Two or more atoms can combine to form
an ion that has a net positive or negative charge, such as OH-, NH4+
Examples
A sodium atom, Na, whose atomic number is 11, has 1 electron in its
valence shell, surrounding inner shells with 2 and 8 electrons. A
sodium atom loses its extra becoming a cation.
Na –> Na+ + eA chlorine atom, Cl, whose atomic number is 17, has 7 electrons in its
valence shell. Thus a chlorine atom gains an electron to attain a
stable configuration with 8 electrons in valence shell. Chlorine forms
an anion in this process.
Cl + e- –> Cl-
Valence Shell: The outer shell of the atom. Electrons form a 2, 8, 18 &
32 arrangement around shells; the electrons that occupy the valence
shell are called valence electrons
Bohr Diagrams
A Bohr diagram is a simplified visual representation of an atom that
was developed by Danish physicist Niels Bohr in 1913. The diagram
depicts the atom as a positively charged nucleus surrounded by
electrons that travel in circular orbits about the nucleus in discrete
energy levels.
Rules For Drawing Bohr Diagrams
Consult the Periodic Table of the Elements for the type of atom you
are going to represent in a Bohr diagram. Write down its atomic
number and mass number. The atomic number is the number of
protons, and the mass number is the number of protons and neutrons.
The number of electrons is equal to the number of protons. See which
row of the Periodic Table your element is in. Elements in the first row
(hydrogen and helium) have one energy level, those in the second
row will have two energy levels and so on.
Draw a circle to represent the nucleus of the atom. Write the symbol
for the element, the number of protons and the number of neutrons
inside this circle. Draw one or more circles around the nucleus
depending on which row of the Periodic Table your element comes
from. Each ring represents a different energy level for the electrons.
Rules For Drawing Bohr Diagrams
Draw electrons as dots on the rings that represent the energy levels.
Each ring has a maximum number of electrons that it can hold. The
first (inner) ring can only hold 2 electrons, the second level can hold 8,
the third can hold 18 and the fourth holds 32.
Bohr Diagram
Which ion is this?
Electron Dot Diagrams
American chemist Gilbert Lewis invented electron dot diagrams to
help him understand and predict the bonding that occurs between
atoms. Also called Lewis diagrams, they represent the atom of an
element by showing only the outer valence electrons and the
chemical symbol.
Electron Dot Diagrams
Electron dots representing electrons are
usually placed around the element
symbol at the points of a compass
(North, South, East & West).
Electron dots are placed singly first until
the 5th electron is reached; at this point
the electrons are then paired.
Pairs of electrons in Electron Dot
Diagrams are called Lone Pairs.
Single electrons in Electron Dot
Diagrams are called Bonding Electrons,
because they are looking for another
electron to make a pair (bond) with.
Bonding
Ionic
Covalent
Ionic Bonding
Opposite ions attract each other
to form an ionic bond.
Anions and cations will form ionic
compounds – the attraction
between the positive and
negative ions will form ionic
bonds.
By using Electron Dot and Bohr
Diagrams, we can see how ionic
bonds are formed.
Covalent Bonding
Atoms that share a pair of electrons are joined by a covalent bond.
A neutral particle that is composed of atoms joined together by
covalent bonds is called a molecule; substances that are composed
of molecules are called molecular compounds.
Steps for drawing covalent bonds
1- Write the symbols for each
element.
2 - Use different colored circles
to create the Lewis structure for
each.
3 - Rearrange the electrons (or
circles) to pair up electrons from
each atom.
4 - Draw circles to show the
sharing of electrons.
5 - Draw the bond structure
using symbols and lines. Use one
line for each pair of electrons
that is shared.
6 - Write the chemical formula
for each molecule.