Download ATOMS AND ELEMENTS

ATOMS AND ELEMENTS NOTES
Atomic Structure
I. Atom – Smallest piece of an element that still has properties of that element. It is the
smallest particle of an element that can exist alone or in combination with other atoms.
It comes from the Greek word atomos, which means indivisible.
A. Scientific models are used to represent atoms
B. Atoms consist of a central positive core (nucleus) surrounded by a cloud of
negative charges (electron cloud). Most of the atom is empty space. If the atom
was the size of this room, then the nucleus would be the size of a grain of sand in
the center of the room. Most of the mass is of the atom is in the nucleus.
C. Atoms have no overall electric charge (they are neutral). The positive core
balances the negative cloud.
D. The atom consists of 3 main subatomic particles
1. electron – negatively charged particle (e-)
Found in the cloud surrounding the nucleus
Mass is negligible (next to nothing, or extremely small)
2. proton – positively charged particle (p+)
Found in the nucleus
It’s mass is about 2000 (1860) times that of an electron
3. neutron – no electric charge, they are neutral (n0)
Found in the nucleus
Has the same mass as a proton (about 2000 times that of an electron)
E. Atomic number – tells the number of protons in an atom. It is the smaller
number on the Periodic Table.
1. The number of protons determines an element. Most atoms are very stable
because it takes (or releases) huge amounts of energy to add or remove a
proton from an atom.
2. Periodic table is arranged according to increasing atomic numbers.
3. Atomic number also tells the number of electrons that element has (so
you have the same number of positive protons as you do of negative
electrons; hence atoms are considered neutral).
F. Mass number – the sum of the protons and neutrons in the nucleus. It is the
mass of the nucleus.
1. It is the bigger number on the Periodic Table. They usually round it to
the nearest whole number.
2. You use the mass number to figure out how many neutrons an element has.
Mass number
Ex: C mass # 12
- Atomic number
- atomic # 6
= Number or neutrons
= # neutrons 6
3. The electron mass is insignificant in determining the mass of an atom.
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G. Isotopes – atoms of the same element that have different numbers of neutrons.
So, they have the same number of protons, just a different number of neutrons.
1. They are the result of an element with different mass numbers.
2. Every element has at least 2 isotopes.
3. There are 2 ways to write isotopes:
a. write the element name and mass number
Ex: Carbon – 12
Carbon – 14
b. write the element symbol, mass #, atomic #
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Ex: 12 C
C
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H. Atomic mass -an average of all the masses of the all the known isotopes of an
element. It is the larger number that is not usually rounded on a Periodic Table.
I. Atomic Mass Unit (amu) – defined as the mass of 1/12 of a carbon – 12 atom
(6 protons and 6 neutrons in the nucleus plus 6 electrons outside the nucleus).
1. Each proton and neutron is assigned a mass of 1 amu.
2. Electrons have a mass of 1/1860 amu.
3. The electron mass is insignificant in determining the mass of an atom.
II. Energy Levels
A. Electrons travel around the nucleus in the electron cloud.
B. Electrons follow paths called energy levels or energy shells.
C. All elements have at least 1 energy level.
D. The period number (or the rows) on the Periodic Table tells you the number of
occupied energy shells that element has.
E. Electrons are arranged in pairs within the energy levels.
1. Each shell has 1 or more subshells, called orbitals. This is where the energy
levels can overlap.
2. When the outer subshell in each shell has 8 electrons, it is considered to be full
or stable. This becomes very important for bonding and is known as the octet
rule.
3. Valence electrons are those electrons in the outer energy level. They are the
only electrons involved in bonding.
4. Valence electrons are determined by the group number (Groups 1, 2, 13, 14,
15, 16, 17, 18 only, not the transition elements).
5. Practice: Write the number of valence electrons for:
C _______ K _______ Cl _______ Sn ________
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III. Periodic Table of the Elements
A. Dimitri Mendeleev – Russian chemist
1. First person to classify elements according to their properties (weight)
2. He put the elements in a chart called the Periodic Table (1860)
3. He put them in order of increasing atomic weights, not atomic numbers.
4. Henry Mosley (1913) figured out the elements needed to be arranged by
increasing atomic number, not weight.
B. Writing symbols
The elements are abbreviated by symbols and
they are written in a very specific way.
1. Capitalize the first letter of the element
name, like C for carbon.
2. Capitalize the first letter of the element
name and then make the second letter
of the element’s name small case,
like Ca for calcium.
3. Sometimes the element’s symbol does
not seem to fit the element’s name at all,
like Fe for iron. How come? Sometimes
the Latin word is used. Here is a chart
of the more commonly used element
names and their Latin word origins.
C. Groups or families are the vertical columns on the Periodic Table.
1. There are 18 groups on the Periodic Table.
2. Each group contains elements with similar chemical properties.
3. Most groups have special names.
4. The group number can tell you how many valence electrons an element has.
D. Periods are the horizontal rows on the Periodic Table.
1. There are 7 periods on the Periodic Table.
2. The period number tells you how many occupied energy levels (or shells) an
element has.
3. Each element in a period has different chemical properties.
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E. Metals are on the left side of the Periodic Table.
1. Physical properties of metals:
a. good conductors of heat and electricity
b. malleable
c. ductile
d. high density
e. shiny luster that reflects light when polished
f. usually silver-gray in color, except for copper and gold
g. solid at room temperature, except for mercury, which is a liquid
h. high melting points
i. usually show a reaction with acids
2. Chemical properties of metals
a. They form compounds by sharing or losing electrons.
F. Nonmetals are on the right side of the Periodic Table.
1. Physical properties of nonmetals
a. poor conductors of heat and electricity
b. brittle
c. nonductile
d. low density
e. dull in appearance and do not reflect light
f. solids or gases at room temperature, except for bromine, which is a liquid
g. low melting points
h. show little or no reaction with acids
2. Chemical properties of nonmetals
a. They form compounds by sharing or gaining electrons (but not 18)
G. Metalloids are between the metals and nonmetals.
1. They are the elements along the stair-step line, except for Aluminum
2. They have properties of both the metals and nonmetals
Generally, as you move left to right across a row on the periodic table (which is a period)
you have:
Strong metal→weak metal→metalloid→weak nonmetal→strong nonmetal→noble gas
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