Building Blocks of Matter
Everything around you - your desk, the air, your own body - is made of atoms. An atom is the smallest unit of an element that retains the chemical properties of that element.
Atoms are incredibly tiny. About 100 million atoms lined up would span just 1 centimeter!
Every atom is made of three types of particles:
| Particle | Symbol | Charge | Mass (amu) | Location |
|---|---|---|---|---|
| Proton | p+ | +1 | ~1 | Nucleus |
| Neutron | n0 | 0 (neutral) | ~1 | Nucleus |
| Electron | e- | -1 | ~0 (negligible) | Electron cloud |
Atomic number = number of protons
Mass number = protons + neutrons
\[\text{Atomic Number (Z)} = \text{Number of Protons}\]
\[\text{Mass Number (A)} = \text{Protons} + \text{Neutrons}\]
\[\text{Neutrons} = \text{Mass Number} - \text{Atomic Number}\]
For neutral atoms: Protons = Electrons
Problem: A carbon atom has an atomic number of 6 and a mass number of 12. How many protons, neutrons, and electrons does it have?
Solution:
Answer: 6 protons, 6 neutrons, 6 electrons
Problem: An atom of atomic number 53 and mass number 127 contains how many neutrons?
Solution:
\[\text{Neutrons} = \text{Mass Number} - \text{Atomic Number} = 127 - 53 = 74\]
Answer: 74 neutrons
Scientists use a special notation to represent atoms that shows both the atomic number and mass number.
\[{}^{A}_{Z}\text{X}\]
where:
Example: \({}^{127}_{53}\text{I}\) (Iodine-127)
Problem: For \({}^{139}_{56}\text{Ba}\), find the number of protons, neutrons, and electrons.
Solution:
Problem: Write the isotope notation for an atom with 24 protons, 31 neutrons, and 26 electrons.
Solution:
Answer: \({}^{55}_{24}\text{Cr}^{2-}\)
Isotopes are atoms of the SAME element (same number of protons) but with DIFFERENT numbers of neutrons.
All isotopes of an element:
Two atoms are isotopes if they have:
Same atomic number (Z)
Different mass number (A)
Problem: Which pair represents isotopes?
Solution:
Answer: b. \({}^{7}_{3}\text{Li}\) and \({}^{8}_{3}\text{Li}\) are isotopes
A neutral atom has equal numbers of protons and electrons. When an atom gains or loses electrons, it becomes an ion.
Lost electrons
Protons > Electrons
Positive charge
Metals typically form cations
Example: Na+, Ca2+, Al3+
Gained electrons
Electrons > Protons
Negative charge
Nonmetals typically form anions
Example: Cl-, O2-, N3-
\[\text{Charge} = \text{Protons} - \text{Electrons}\]
Or rearranged:
\[\text{Electrons in ion} = \text{Protons} - \text{Charge}\]
Problem: How many electrons are in \({}^{40}_{18}\text{Ar}\)?
Solution:
Problem: Substance X has 13 protons, 14 neutrons, and 10 electrons. Determine its identity and write its symbol.
Solution:
Answer: \({}^{27}_{13}\text{Al}^{3+}\) (Aluminum ion)
Problem: Complete the table for Silver ion: Ag+, mass number 108
Solution:
Look at the periodic table - why do atomic masses have decimal values like 35.45 for chlorine? It's because most elements exist as a mixture of isotopes in nature.
The average atomic mass is a weighted average of all naturally occurring isotopes, based on their relative abundance (how common each isotope is).
This is different from a simple average! More abundant isotopes contribute more to the average.
\[\text{Avg Atomic Mass} = \sum(\text{mass of isotope} \times \text{decimal abundance})\]
Or written out:
\[\text{Avg} = (m_1 \times \%_1) + (m_2 \times \%_2) + (m_3 \times \%_3) + ...\]
Important: Convert percentages to decimals first! (20% → 0.20)
Problem: Two naturally occurring isotopes of an element have masses and abundances as follows: 54.00 amu (20.00%) and 56.00 amu (80.00%). What is the average atomic mass?
Solution:
Step 1: Convert percentages to decimals
Step 2: Multiply each mass by its abundance and add
\[\text{Avg} = (54.00 \times 0.2000) + (56.00 \times 0.8000)\]
\[\text{Avg} = 10.80 + 44.80 = 55.60 \text{ amu}\]
Answer: 55.60 amu
Problem: Calculate the average atomic mass for an element with these isotopes:
Solution:
\[\text{Avg} = (85)(0.156) + (86)(0.521) + (88)(0.193) + (90)(0.130)\]
\[\text{Avg} = 13.26 + 44.81 + 16.98 + 11.70\]
\[\text{Avg} = 86.75 \text{ amu}\]
Answer: 86.75 amu
| Atomic Number (Z) | = Number of protons = Number of electrons (neutral atom) |
| Mass Number (A) | = Protons + Neutrons |
| Neutrons | = Mass Number - Atomic Number |
| Ion Charge | = Protons - Electrons |
| Isotopes | Same protons, different neutrons |
| Cation | Positive ion (lost electrons) |
| Anion | Negative ion (gained electrons) |
| Average Atomic Mass | = Σ(mass × decimal abundance) |