Atoms are extremely small and mostly empty space.
| Particle | Charge | Mass (amu) | Key Note |
|---|---|---|---|
| Proton | +1 | 1.0073 | Determines Identity |
| Neutron | 0 | 1.0087 | Determines Isotope |
| Electron | -1 | 0.00055 | Mass is Negligible |
So, the majority of the mass of an atom comes only from the proton(s) and neutron(s) present in the nucleus. Electron mass is negligible.
The charge of an atom comes only from protons and electrons, so a neutral atom has #protons = #electrons.
Each element on the Periodic Table has a unique atomic number (Z), which tells the number of protons in the nucleus. The number tells us which element it is.
The total number of protons + neutrons in an atom is the mass number (A). Therefore, the number of neutrons in an atom is the difference: #neutrons = A – Z
An atom of a specific element always has the same number of protons, but the number of neutrons can differ between atoms of the same element – these are called isotopes.
If you look at the periodic table, Bromine has an atomic mass of approximately 80. However, if you could shrink down and weigh every bromine atom on Earth, you would almost never find one that actually weighs 80.
Bromine exists in nature as a nearly 50/50 mix of two isotopes:
Bromine-79: Contains 35 protons and 44 neutrons. (Approx. 50.7% of all Br)
Bromine-81: Contains 35 protons and 46 neutrons. (Approx. 49.3% of all Br)
In a chemical reaction, the number of protons and/or neutrons in an atom’s nucleus never change. However, the number of electrons outside the nucleus CAN change.
In a neutral atom, #protons = #electrons.
When an atom gains or loses one or more electrons, the charge balance is upset and an ion is formed.
Think of an atom as a mathematical ledger. The net charge is simply the balance between positive protons and negative electrons.
When you subtract a negative, the result is positive.
Clinical Example: Sodium (Na+) loses an electron to become a cation, essential for nerve impulses.
When you add a negative, the result is negative.
Clinical Example: Chloride (Cl-) gains an electron to become an anion, critical for fluid balance.
Each element on the Periodic Table has a chemical symbol, a one- or two-letter designation. The first letter is always capitalized, the second (if there is one) is always lower-case.
This is important to avoid confusion of chemical symbols with simple compounds or common acronyms and to avoid confusion when writing chemical formulas of more complex molecules.
While the number of protons defines an element, the number of neutrons can vary. These variations are called isotopes.
The mass shown on the Periodic Table is a weighted average of all naturally occurring isotopes. Here is how the math works for Carbon:
See example 2.4 pp 77-78
2.3 Atomic Structure and Symbolism
Review/try problems 11, 17, 19, and 23