Chemistry: Atoms, Molecules, and Ions Study Notes

The periodic table is a fundamental tool used to organize chemical elements.
To date, 118 elements have been discovered.
Of these, 94 elements occur naturally on Earth, ranging from Hydrogen (atomic number 1) to Plutonium (atomic number 94).
Technetium (atomic number 43) and Promethium (atomic number 61) are exceptions among naturally occurring elements, as they are found only in trace amounts due to radioactive decay.
Elements with atomic numbers 95 and higher (beyond Plutonium) are synthetic, meaning they are man-made and do not occur naturally.

Groups (Columns):
- There are 18 vertical columns, known as groups.
- Elements within the same group share similar chemical properties because they have the same number of electrons in their outermost energy shell.
Periods (Rows):
- There are 7 horizontal rows, known as periods.
- Elements within the same period have the same number of electron energy shells.

Historically, two main systems for labeling groups existed:
- IUPAC (European) System: Used numbers 1-18.
- American System: Used "A" and "B" designations, where "A" groups represented main-group elements (taller columns) and "B" groups represented transition elements (shorter columns in the middle).
In 1988, the IUPAC officially adopted the modern system, which numbers groups 1 through 18 sequentially from left to right.

Elements with atomic numbers 57 to 71 (Lanthanides) and 89 to 103 (Actinides) are placed at the bottom of the table.
This placement prevents distortion of the main periodic table's shape, as these series contain more elements than can fit naturally into a single row within the main body.

A distinctive "staircase" line diagonally traverses the periodic table.
This line serves to separate elements into two primary classifications: metals (located to the left of the line) and nonmetals (located to the right of the line).

An element is fundamentally composed of atoms of the same type.
Atoms are considered the basic building blocks of all matter.
Scientists have elucidated that an atom consists of:
- A central nucleus, which contains protons and neutrons.
- Electrons, which move in distinct layers or shells, referred to as energy shells, around the nucleus.

Each element is uniquely identified by its atomic number.
The atomic number is defined as the number of protons found in an atom's nucleus.
Example: Carbon (C), with an atomic number of 6, invariably possesses 6 protons in its nucleus.

Location: Predominantly found on the left side of the periodic table.
Properties:
- Excellent conductors of both electricity and heat.
- Possess a characteristic shiny, metallic appearance (luster).
- Malleable: Capable of being hammered or pressed into shapes without breaking.
- Ductile: Capable of being drawn out into a thin wire.

Location: Found on the right side of the periodic table.
Properties: Generally exhibit characteristics opposite to those of metals.
- Poor conductors of heat and electricity.
- Lack metallic luster; are not shiny.
- Neither malleable nor ductile.
- Typically brittle in their solid form.

Location: Situated along the "staircase" line that separates metals from nonmetals.
Properties: Display an intriguing blend of properties that are intermediate between those of metals and nonmetals.
Exclusions: Certain elements near the staircase are specifically classified elsewhere:
- Aluminum (Al) and Polonium (Po) are classified as metals.
- Tennessine (Ts) and Oganesson (Og) are considered nonmetals.
Common Examples: Silicon (Si, atomic number 14) and Germanium (Ge).
Significance: Many metalloids are well-known for their use as semiconductors in electronic devices.

Definition: Isotopes are atoms of the same element (meaning they have the same atomic number and thus the same number of protons) but possess different mass numbers.
Key Characteristic: The difference in mass number stems from isotopes having varying numbers of neutrons.
Example: Carbon, for instance, exists in several isotopic forms, each with a different neutron count.

This mnemonic is a helpful tool for remembering properties of neutral atoms:

A P E
- Atomic Number (A) = # Protons (P) = # Electrons (E) (for a neutral atom).
M A N
- Mass Atomic Number (M) = # Protons (P) + # Neutrons (N).
- Alternatively, Mass Number (M) = Atomic Number (A) + # Neutrons (N).

The isotopic symbol provides a concise way to represent a specific isotope:
- ^{ \text{Mass Number} }_{ \text{Atomic Number} } \text{Chemical Symbol}
The atomic number (subscript) can often be omitted from the symbol, as it is uniquely determined by the chemical symbol itself (which can be found on the periodic table).
Alternative Notations:
- The mass number can be written as a superscript before the chemical symbol (e.g., ^{14} \text{N}).
- The mass number can be written after the chemical symbol, separated by a hyphen (e.g., N-14).
Reading an Isotopic Symbol: For ^{14} \text{N} or N-14, it is read as "Nitrogen 14".

To find the number of neutrons in an isotope, subtract the atomic number from the mass number:
- \text{# Neutrons} = \text{Mass Number} - \text{Atomic Number}
Example: Nitrogen-14 (^{14}_{7} \text{N})
- Mass Number = 14
- Atomic Number = 7
- \text{# Neutrons} = 14 - 7 = 7
- Therefore, Nitrogen-14 has 7 neutrons.

Problem: Identify element X whose atoms contain 26 protons. Write the symbol of the isotope of X that has 30 neutrons.
Solution:
- Since it has 26 protons, its atomic number is 26. Element number 26 on the periodic table is Iron (Fe). So, X = Fe.
- Mass Number (M) = \text{# Protons} + \text{# Neutrons} = 26 + 30 = 56.
- The isotopic symbol is ^{56}_{26} \text{Fe}.

Isotope / Element	# Protons	# Neutrons	# Electrons
^{15} \text{N} (Nitrogen)	7	15 - 7 = 8	7
^{60} \text{Co} (Cobalt)	27	60 - 27 = 33	27
^{131}_{53} \text{I} (Iodine)	53	131 - 53 = 78	53
^{148}_{58} \text{Ce} (Cerium)	58	148 - 58 = 90	58

Isotope / Element	Atomic Number (from PT)	# Protons	# Neutrons
^{27} \text{Al} (Aluminum)	13	13	27 - 13 = 14
^{32} \text{S} (Sulfur)	16	16	32 - 16 = 16
^{64} \text{Zn} (Zinc)	30	30	64 - 30 = 34
^{207} \text{Pb} (Lead)	82	82	207 - 82 = 125

Isotope	# Protons	# Neutrons	# Electrons
^{58} \text{Fe}	26	58 - 26 = 32	26
^{17} \text{O}	8	17 - 8 = 9	8
^{7} \text{Li}	3	7 - 3 = 4	3
^{235} \text{U}	92	235 - 92 = 143	92
^{33} \text{S}	16	33 - 16 = 17	16