Atomic Structure and Electron Configuration
Translate atomic notation into particle counts, connect electron arrangements to the periodic table, and use evidence to predict ion behavior.
Atomic Structure and Electron Configuration
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The nucleus determines identity; the electrons determine charge and much of the chemistry
An atom’s atomic number (Z) is its number of protons. This fixes the element’s identity. Its mass number (A) is the total number of protons and neutrons.
Isotopes are atoms of the same element with different neutron counts. Ions form when electrons are gained or lost; the nucleus remains unchanged. Electron configuration shows how electrons occupy shells, subshells, and orbitals. For main-group elements, the outermost electrons help predict group, bonding, and common ion charge.
Do not mix up the numbers
Atomic number counts protons. Mass number counts protons plus neutrons. The decimal value on a periodic table is usually an isotope-weighted average, not one atom’s mass number.
Charge changes electron count
A positive ion has lost electrons; a negative ion has gained electrons. Changing electrons never changes which element it is.
Write P, N, and E before calculating anything
Problem: Find the particles in ⁵⁶₂₆Fe³⁺.
P: Atomic number 26 → 26 protons.
N: 56 − 26 → 30 neutrons.
E: A 3+ ion has lost three electrons → 26 − 3 → 23 electrons.
Do it fast: Positive charge means subtract electrons; negative charge means add electrons.
Why it works
Separating proton, neutron, and electron counts prevents the two most common errors: subtracting the charge from the mass number and treating a positive ion as if it gained electrons.
Five forms you should recognize
Problem: Interpret ³⁷₁₇Cl.
The atomic number gives 17 protons. The neutron count is 37 − 17 = 20. Because no charge is shown, it also has 17 electrons.
Problem: Find the particles in ²⁴₁₂Mg²⁺.
It has 12 protons, 24 − 12 = 12 neutrons, and 12 − 2 = 10 electrons.
Problem: Arrange the 13 electrons of aluminum.
Fill the first shell with 2 and the second with 8. The remaining 3 enter the third shell: 2–8–3.
Problem: An atom has arrangement 2–8–7.
Three occupied shells place it in Period 3. Seven valence electrons place this main-group element in Group 17.
Problem: An element is 75% X-10 and 25% X-11.
average mass = 0.75(10) + 0.25(11) = 10.25 uThe answer is closer to 10 because X-10 is more abundant.
Check before you commit
- Using mass number as the proton count
- Adding electrons for a positive ion
- Calling ions isotopes when only the electron count changed
- Treating periodic-table average atomic mass as the mass number of every atom
- Counting all electrons as valence electrons
- Assuming atoms form ions by changing their proton number
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Atomic Structure and Electron Configuration FAQ
Why is atomic mass often a decimal?
It is the weighted average of the naturally occurring isotopes of that element.
Do isotopes have different chemical properties?
They usually have very similar chemistry because they share the same electron arrangement, although mass-dependent physical or nuclear behavior can differ.
Why do atoms form ions?
Electron loss or gain can produce a lower-energy, more stable electron arrangement, often resembling a filled outer shell.
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