The Structure of the Atom

The atom is the basic unit of matter, and it is made up of three subatomic particles: electrons, protons, and neutrons.

• Electrons were discovered by J.J. Thomson in 1897. They are negatively charged particles.
• Protons were discovered after E. Goldstein found new, positively charged radiations called canal rays in 1886. Protons have a positive charge.
• Neutrons were discovered in 1932 by James Chadwick. They have no charge.

Thomson's Atomic Model

• J.J. Thomson proposed that an atom is a positively charged sphere with negatively charged electrons embedded within it.
• This model is often compared to a Christmas pudding or a watermelon, where the positive charge is the pudding/edible part, and the electrons are like dry fruits or seeds.
• According to Thomson, the positive and negative charges in an atom are equal, making the atom as a whole electrically neutral.

Rutherford's Gold Foil Experiment

• In 1911, Rutherford conducted the alpha (α) scattering experiment by bombarding a thin sheet of gold foil (about 1000 atoms thick) with high-speed alpha-particles.
• Alpha-particles are positively charged helium nuclei (\( He^{2+} \)).

Observations:

• Most alpha-particles passed straight through the foil without deflection.
• Some alpha-particles were deflected by small angles.
• A very small number of alpha-particles bounced back.

Conclusions:

• Most of the atom is empty space.
• The atom has a small, positively charged nucleus.
• The nucleus is where most of the atom's mass is located.
• The nucleus is very small compared to the overall size of the atom.

Rutherford's Atomic Model

• An atom has a positively charged nucleus at its center.
• Electrons revolve around the nucleus in different orbits.

Bohr's Atomic Model

• An atom has a positively charged nucleus at its center, containing most of the atom's mass.
• Electrons revolve around the nucleus in specific, stable orbits called discrete orbits or shells.
• These shells are represented by the letters K, L, M, N or the numbers 1, 2, 3, 4, and so on.
• Electrons do not radiate energy while in these discrete orbits, which explains why atoms are stable.

• Valence Electrons: The electrons in an atom's outermost shell are called valence electrons.
• Octet Rule: An atom is stable if its outermost shell contains 8 electrons. Noble gases already have 8 electrons in their outer shell, so they are not reactive.
• Duplet Rule: The first shell (K shell) can hold a maximum of 2 electrons. A complete K shell also makes an atom stable, like in the case of Helium.
• Valency: The combining capacity of an element. It is the number of electrons an atom gives, accepts, or shares to form a bond.
  • For elements with 1, 2, or 3 valence electrons, the valency is equal to the number of valence electrons. These elements tend to lose electrons and become cations (positively charged ions).
  • For elements with 5, 6, or 7 valence electrons, the valency is calculated as 8 - (number of valence electrons). These elements tend to gain electrons and become anions (negatively charged ions).
  • Elements with 4 valence electrons tend to share them and have metalloid properties.

• Atomic Number (Z): This is the number of protons in an atom's nucleus. All atoms of the same element have the same atomic number. In a neutral atom, the atomic number is also equal to the number of electrons.
• Mass Number (A): This is the sum of the number of protons and neutrons (also called nucleons) in an atom's nucleus. The mass of an atom is primarily determined by its protons and neutrons.

• Isotopes: Atoms of the same element that have the same atomic number but different mass numbers. This means they have the same number of protons but a different number of neutrons. For example, Protium, Deuterium, and Tritium are isotopes of Hydrogen.
• Isobars: Atoms of different elements that have different atomic numbers but the same mass number. This means they have a different number of protons but the same total number of nucleons. For example, Argon-40, Potassium-40, and Calcium-40 are isobars.
• Radio-isotopes are isotopes that are radioactive and have various applications, such as Carbon-14 for measuring the age of fossils and Cobalt-60 for curing cancer and preserving food.

• Ions are formed when atoms lose or gain electrons to achieve a full outer shell (octet rule).
• Cations are formed when an atom loses one or more electrons, resulting in a positive charge. A metal that has lost electrons has the same name as the element, for example, Na\(^+\) is a sodium ion and Ca\(^{2+}\) is a calcium ion.
• Anions are formed when an atom gains one or more electrons, resulting in a negative charge. A nonmetal that has gained electrons changes its name to end in "-ide" (e.g., chloride ion, fluoride ion).
• A cation or an anion will often have the same number of electrons as its nearest noble gas, a state known as being isoelectronic.