Atomic Structure

ATOMIC STRUCTURE
CHEMISTRY/PHYSICAL SCIENCE
MINI LESSON II

Vocabulary:

atom - metal - compound
nonmetal - bonding - ionic bond
nucleus - metalloid - covalent bond
proton - atomic number - chemical formula
neutron - atomic mass - subscript
electron cloud - family
periodic table - period
element - oxidation number

ATOM

The smallest unit of an element which still has the physical and chemical properties of that element.

Example: If you could divide a piece of iron into smaller and smaller pieces, you would reach a point where you could no longer divide it and still have the element iron. This would be an atom of iron.

This atom of iron will react with atoms of other elements in the same manner that
many grams of iron react with many grams of those same elements.

Parts of the Atom

An atom consists of a dense, central positive core called the nucleus
surrounded by
a cloud of negative charge (swirling electrons) called the electron cloud.

1. Nucleus - composed of protons and neutrons.

a. protons - positive charge
b. neutrons - no charge

2. Electron cloud (electrons) - negative charge in energy shells
(levels) surrounding the nucleus. Electrons move about the
nucleus much as bees swarm around the beehive waiting to get in,
and exist in energy levels that form concentric rings around the nucleus
similar to the rings in an onion.

NOTE: The cloud of negative charge (electrons) exactly balances the positive nucleus. Atoms are neutral.

Particle Mass Relationships:

1. protons & neutrons - are almost the same mass, each is 1 atomic mass unit (amu).
2. electron - despite a slight mass, it does not contribute toward the mass of the atom.

The mass of an atom is determined by the number of protons
and neutrons in its nucleus. The mass number of an atom
is the average atomic mass rounded off to the nearest
whole number and is equal to the sum of protons
and neutrons in the nucleus of an atom.
Therefore, if mass number is protons + neutrons; and,
if the atomic number is the number of protons only,
then, if you subtract the atomic number from
the mass number, you can always know
the number of neutrons in a nucleus.

MASS NUMBER - ATOMIC NUMBER = # OF NEUTRONS IN NUCLEUS

NOTE: The proton is about 1837 times more massive than the electron.

Periodic Table

A chart developed by the Russian chemist Dmitri Mendeleev showing the classification of elements according to their properties. Today, elements are arranged according to the number of protons in their nucleus referred to by the atomic number.

1. The periodic table has vertical columns called families or groups. Family members have similar properties.
2. The table has horizontal rows called periods or series. Period members have same number of energy levels.

Elements are classified as metals,nonmetals,or metalloids.

1. Metals - have 1,2,or 3 electrons in their outer energy levels.
Metals are usually good conductors, malleable, have a luster and many are ductile.

2. Nonmetals - have 5 or more electrons in their outer energy levels.
Nonmetals are poor conductors, brittle, lack luster and are not malleable.

3. Metalloids - elements that have properties of both metals and nonmetals.

The atomic numbers, atomic weight and atomic mass of elements in a period generally increase from left to right of the chart.

Groups (families) contain elements that have similar properties
and have a similar arrangement of electrons in their outer energy levels.

Atomic number - the number of protons in the nucleus and the number of electrons in the shells.

Atomic mass - the number of protons plus neutrons found in the nucleus of the atom.

NOTE: To find the number of neutrons in an atom - simply subtract the atomic number from the atomic mass.

Example:(for carbon) atomic mass 14
atomic number -_6__
8 neutrons

NOTE: The Roman numerals in the A columns indicate the number of electrons in the outer energy levels for the elements within that column (family).

Example: Group II A members all have 2 electrons in their outer level.

The most active metals are located in the lower left corner of the periodic table (Francium is most active).

The most active nonmetals are located in the upper right corner (Fluorine is most active).

The noble gases are group VIII A - these are practically inactive. They do not combine to form many compounds.

Element - a pure substance containing atoms of the same atomic number.

Compound - two or more elements chemically combined.

NOTE: Chemically combined means that they join (bond) together in a way that makes it impossible to separate them by physical means.

Common compounds: H₂O water, H₂O₂ hydrogen peroxide, NaCl table salt, C₁₂H₂₂O₁₁ cane sugar.

CHEMICAL BONDING

Bond - (to join) compounds are formed by bonding. Most atoms combine to complete an outer energy level of electrons. Atoms may share, gain or lose electrons in orded to become chemically stable.

NOTE: The number of electrons in the element's outer energy level (valence electrons) is important in the creation of chemical bonds.

BONDING TYPES:

1. Ionic bonding - the transfer of electrons resulting in the formation of oppositely charged ions. (Remember: opposite charges attract).

Sodium transfers 1 valence electron to an atom of Chlorine.
Sodium becomes a positive ion Na⁺,
Chlorine becomes a negative ion Cl^-.

Oppositely charged ions attract

Na⁺Cl^-

ion - an atom that either loses electrons to become positively charged
or gains electrons to become negatively charged.

2. Covalent bonding - the sharing of electrons between atoms to form molecules.

molecule - the smallest particle of a covalent compound.

Examples of covalent bonds: HCl, H₂O, O₂

CHEMICAL FORMULA

A way of representing a compound with symbols and subscripts.

1. reveals the different elements in the compound.
2. reveals the number of atoms of each element.

Example: CO₂ = 1 carbon atom for every 2 oxygen atoms, therefore, ratio is 1:2

Subscript - gives the ratio in which the atoms are combined in a compound,
but, we do not write 1's as subscripts.

Example: H₂O - the absence of a subscript next to a symbol means one atom.

Example: C₁₂H₂₂O₁₁ --- 12 carbon atoms,22 hydrogen atoms and 11 oxygen atoms.

Oxidation numbers - the name given to the combining ability of an atom. The number tells how many electrons an atom gains, loses or shares in forming a compound.

Some Common Oxidation Numbers

1+ 2+ 3+
copper(I) barium aluminum

lithium calcium
potassium

1- 2- 3-
bromine sulfur nitrogen

fluorine oxygen

EXAMPLE: Write the formula for copper (I) and sulfur.
Solution-- write the symbol of the element with the positive oxidation number first, then write the symbol of the negative ion.

Cu¹⁺S^2-

The sum of the oxidation numbers for the atoms in a compound must equal to zero. Here it does not.

In Cu¹⁺S^2- , we need another copper atom to balance the oxidation numbers.

Cu₂S is the correct formula!

PRACTICE
PROBLEMS: Check with Mr. Vincent or your Science Teacher for correct answers.......

Write the formulas for the following:
1. lithium and sulfur
2. barium and oxygen
3. lithium and nitrogen
4. potassium and sulfur
5. lithium and fluorine

CHEMICAL EQUATION

- A shorthand description of a chemical reaction containing symbols and formulas.

Example: 2H₂(g) + O₂(g) ------> 2H₂O(l)

Note the coefficients and state symbols...

The coefficients balance the equation and are located in front of the symbol.

The subscripts are located behind the element and are used to balance the compound.

Parts of an equation:

-the left side of the yield sign is called the reactants. -the right side of the yield sign is called the product.

2H₂(g) + O₂(g)------>2H₂O(l)

reactants product

Example: Balance the equation Mg + O₂-------------> MgO

Solution: If the atoms are not in balance, use coefficients to balance the equations. Never change the subscripts of the chemical formulas, this is not permitted once the correct formula is written.

2Mg(s) + O₂(g) ----------> 2MgO(s)

SYMBOLS USED IN EQUATIONS:

     l - LIQUID
    s - SOLID
    g - GAS
aq - AQUEOUS (in water solution)