Dalton’s Atomic Theory and the Structure of the Atom
Dalton’s Atomic Theory and Molecules
Dalton
Dalton’s theory aimed to explain the experimental laws of matter and served as a precursor to modern chemistry. He built upon the theories of Democritus and revived the concept of atoms.
Atom: The smallest part of a chemical element that retains the properties of that element.
Dalton’s Atomic Theory
- Matter is made of tiny, indivisible particles called atoms.
- Atoms of the same element are identical and have the same shape, size, and mass.
- Atoms of different elements are different and have different properties.
- Atoms cannot be created or destroyed.
- Chemical compounds are formed when atoms of different types combine in fixed proportions.
- Atoms of the same compound are identical and have the same properties.
Criticisms of Dalton’s Theory
- Atoms are not indivisible; they contain protons, neutrons, and electrons.
- Atoms of an element need not be identical in mass or properties; isotopes exist.
- Atoms can be created or destroyed in nuclear reactions, such as those in the Sun.
- Atoms of compounds are what we call molecules.
The Particles that Make Up an Atom
Discoveries of Subatomic Particles
- Experiments revealed limitations in Dalton’s model.
- Thompson discovered that atoms contain negatively charged particles called electrons.
- Milikan measured the mass and charge of the electron.
- Normally, matter has no electrical charge and is neutral.
- Rutherford discovered the proton.
- Chadwick discovered the neutron.
- Today, we know that there are even smaller subatomic particles called quarks.
Atomic Models
Atomic Models
Definition: Different ways to describe the interior of an atom.
Dalton
Dalton was the first to demonstrate the existence of atoms.
Thompson
- Atoms are spherical with a neutral charge.
- They have mixed positive and negative charges throughout their volume, with the total positive charge equal to the total negative charge.
- Atoms are solid.
Rutherford
Experiment:
- Alpha particles (positive) were fired at a thin gold foil (10-7 m).
- A fluorescent screen showed the results.
Results:
- Most alpha particles passed through without deviating.
- A few particles deviated slightly from the center.
- Very few deviated significantly, even bouncing back.
Conclusions:
- The atom has a central nucleus containing almost all the mass and positive charge.
- Negatively charged particles (electrons) orbit the nucleus.
- The negative particles describe circular orbits.
Bohr
- Bohr’s model built upon and improved Rutherford’s model using kinetic theory.
- Electrons can only rotate in certain stable circular orbits.
- While an electron is in one of these orbits, it neither gains nor loses energy.
- Bohr called these orbits stationary orbits.
The Atom
- The current model, introduced by Schrödinger, is the quantum-mechanical model.
- The size of the atom is 10-10 m in radius.
Parts:
- Nucleus
- Occupies the center of the atom and is 10,000 times smaller than the atom, with a radius of 10-14 m.
- Components:
- Protons: Subatomic particles in the nucleus that are positively charged.
- Neutrons: Subatomic particles in the nucleus that have a neutral charge.
- The mass of protons and neutrons are similar.
- Electron Cloud
- Surrounds the nucleus and is virtually empty.
- Component:
- Electrons: Subatomic particles in the electron cloud that are negatively charged.
- The mass of electrons is 2000 times smaller than that of protons and neutrons.
- Electrons orbit the nucleus in regions of space called orbitals.
Currently, the atom is understood to be composed of even more fundamental particles, such as quarks and leptons.
Atoms, Isotopes, and Ions
Atomic Number
Definition: A number that indicates the number of protons in an atom, symbolized by Z.
- All atoms with the same atomic number belong to the same element.
- Atoms with different atomic numbers belong to different elements.
Mass Number
Definition: A number that indicates the total number of protons and neutrons in an atom, symbolized by A.
- Also called atomic mass, it indicates the mass of an atom.
- Atoms with different mass numbers can be the same element or different elements.
Number of Neutrons: The number of neutrons (n0) is calculated as n0 = A – Z.
Isotopes
- Atoms of the same element (same Z) but with different mass numbers (different A).
- They have the same number of protons but different numbers of neutrons.
Mass of an Element:
Mass is measured in:
- Atomic mass units (amu or u).
- Equivalent to one-twelfth the mass of 12C (1.66 x 10-27 kg).
If an element has no isotopes:
- The mass of the element is equal to its mass number.
- Formula: Atomic Mass = A
If an element has isotopes:
- The mass of the element is the average of the masses of its isotopes.
- Formula:
Charged Atoms (Ions)
- Atoms are naturally neutral in charge.
Ion:
Definition: Atoms that have an electric charge, meaning they have different numbers of protons and electrons.
Types:
- Cations: Atoms with a positive electric charge, meaning they have more protons than electrons.
- Anions: Atoms with a negative electric charge, meaning they have more electrons than protons.
Ions are formed by changing the number of electrons:
- Adding electrons creates an anion.
- Removing electrons creates a cation.
Properties of Charges:
- Two like charges repel each other.
- Two opposite charges attract each other.
Electron Organization
- Electrons are organized in energy levels.
- There are seven energy levels, corresponding to the seven periods of the periodic table.
- Within each level, there are sublevels associated with each type of orbital: s, p, d, f.
- Each orbital can hold a specific number of electrons:
- s orbital: 2 electrons (s2)
- p orbital: 6 electrons (p6)
- d orbital: 10 electrons (d10)
- f orbital: 14 electrons (f14)
- Electron Configuration: The way electrons are arranged in the different orbitals of an atom in order of decreasing energy.
- Valence Electrons: Electrons in the outermost energy level, responsible for the chemical behavior of atoms.