Atomic Structure and Subatomic Particles

In 1911, Rutherford's gold foil experiment established the structure of the atom.
Key Findings:
- At the center of an atom is a dense core called the nucleus.
- The nucleus contains protons ( $p^+$ ), which carry a positive charge.
- The nucleus also contains neutrons ( $n^0$ ), which have no charge. Neutrons are crucial for stabilizing the nucleus, as protons would otherwise repel each other due to their like positive charges.
- Outside the nucleus is largely "empty space" where electrons ( $e^-$ ) are found. Electrons carry a negative charge.
- Atoms are electrically neutral, meaning the number of protons equals the number of electrons (#p = #e^-).

The Bohr Model is a conceptual framework used to visualize the structure of atoms, depicting electrons orbiting the nucleus.

Particle	Symbol	Actual Charge (Coulombs)	Relative Charge	Actual Mass (kg)	Relative Mass (amu)
Electron	$e^-$	$-1.602 \times 10^{-19}$	$-1$	$9.11 \times 10^{-31}$	$0.0005$
Proton	$p^+$	$1.602 \times 10^{-19}$	$+1$	$1.67 \times 10^{-27}$	$1$
Neutron	$n^0$	$0$	$0$	$1.67 \times 10^{-27}$	$1$

Definition: The atomic number, denoted by the symbol $Z$ , is equivalent to the number of protons (#p) in the nucleus of an atom.
Uniqueness: Every element on the Periodic Table has a unique atomic number.
Formula: Z = #p
Value: Atomic numbers are always whole numbers.
Periodic Table: Atomic numbers start with $1$ (for hydrogen) and increase sequentially by $1$ across the Periodic Table.
Isotopes: Atoms of the same element (meaning they have the same atomic number and thus the same number of protons) are called isotopes. Isotopes share identical chemical properties but can have different physical properties.
- For example, all isotopes of hydrogen have a $Z$ value of $1$ .

Definition: The mass number, denoted by the symbol $A$ , is the sum of the number of protons and the number of neutrons in an atom's nucleus.
Formula: A = #p + #n^0
Isotopes and Mass Number: Isotopes of the same element have the same atomic number ( $Z$ ) but different mass numbers ( $A$ ). This difference in mass number arises from varying numbers of neutrons.
- Consequently, isotopes of the same element will have different masses and thus different physical properties, while maintaining the same chemical properties.

Purpose: Nuclide symbols provide a concise way to represent the number of protons, neutrons, and electrons in an isotope of an element.
Format: The general format for a nuclide symbol is $_{Z}^{A}X^{ ext{charge}}$ , where:
- $A$ is the mass number (protons + neutrons).
- $Z$ is the atomic number (number of protons).
- $X$ is the element symbol.
- $ext{charge}$ (optional) indicates the ionic charge.
Monatomic Representation: Nuclide symbols are written for a single atom (monatomic form) of an element, even if that element naturally exists in a polyatomic form (e.g., $O_2$ gas is represented as $O$ in nuclide symbols).
Example: The nuclide symbol for Carbon-12 is $_{6}^{12}C$ . Here, $A=12$ , $Z=6$ , which means $6$ protons, $6$ neutrons ( $12-6=6$ ), and $6$ electrons (for a neutral atom).

Charge Representation: The charge of an ion is indicated in the upper right-hand corner of the nuclide symbol.
Ion Formation: Ions are formed when atoms gain or lose electrons.
- Loss of Electrons: For each electron lost, the ion gains a charge of $+1$ . These positively charged ions are called cations.
- Gain of Electrons: For each electron gained, the ion takes on a charge of $-1$ . These negatively charged ions are called anions.
Calculating Number of Electrons: The number of electrons in an ion can be determined using the formula:
#e^- = Z - ext{charge}
- For a neutral atom, charge is $0$ , so #e^- = Z (number of protons).
Example: (Page 15) Sodium Ion ( $Na^+$ , Mass Number $23$ ):
- From the periodic table, Sodium (Na) has an atomic number, $Z = 11$ .
- Given mass number, $A = 23$ .
- Number of protons, #p = Z = 11.
- Number of neutrons, #n^0 = A - Z = 23 - 11 = 12.
- The charge is $+1$ . Using the formula, #e^- = Z - ext{charge} = 11 - (+1) = 10 electrons.
- This indicates that a neutral Sodium atom (with $11$ electrons) has lost $1$ electron to become $Na^+$ ( $10$ electrons).

Isotope	A	Z	#p	#e	#n
C-12	$12$	$6$	$6$	$6$	$12 - 6 = 6$
C-13	$13$	$6$	$6$	$6$	$13 - 6 = 7$
C-14	$14$	$6$	$6$	$6$	$14 - 6 = 8$

Element	#p	#n	#e	At # (Z)	Mass # (A)	Nuclide Symbol
K	$19$	$21$	$19$	$19$	$40$	$_{19}^{40}K$
Cl	$17$	$19$	$17$	$17$	$36$	$_{17}^{36}Cl$
S	$16$	$16$	$16$	$16$	$32$	$_{16}^{32}S$
Al	$13$	$14$	$13$	$13$	$27$	$_{13}^{27}Al$
O	$8$	$8$	$8$	$8$	$16$	$_{8}^{16}O$

Element	#p	#n	#e	At # (Z)	Mass # (A)	Charge	Nuclide Symbol
K	$19$	$21$	$18$	$19$	$40$	$+1$	$_{19}^{40}K^+$
Cl	$17$	$19$	$18$	$17$	$36$	$-1$	$_{17}^{36}Cl^-$
S	$16$	$16$	$18$	$16$	$32$	$-2$	$_{16}^{32}S^{2-}$
O	$8$	$8$	$10$	$8$	$16$	$-2$	$_{8}^{16}O^{2-}$