Atoms

Atoms and Elements The basic units of matter are called atoms Atoms cannot be broken down into smaller pieces by any common methods of separating matter Atoms are the building blocks of the universe

Models of the Atom All atoms share the same basic structure With new observations or experiments, the model had to be changed over the past 200 years

Democritus In 400 BC, a Greek philosopher named Democritus hypothesized that everything was made up of tiny particles He called these particles “atomos” “Atomos” was Greek for cannot be divided He said that these “atomos” were empty spheres

Dalton’s Model In the early 1808, John Dalton performed experiments with gases and found that matter was made up of tiny, indivisible particles Dalton observed that the same amounts of hydrogen and oxygen always combined to form a given amount of water He reasoned that each element must be made of its own unique kind of particle and that these particles combine in simple ways He called these particles atoms and pictured them as tiny, solid spheres

Dalton’s Atomic Theory All matter is composed of tiny indivisible particles called atoms Atoms of each element are exactly alike Atoms of different elements have different masses Atoms of different elements can join to form compounds

Thomson’s Model In the late 1897, J.J. Thomson discovered that atoms were not just simple, solid spheres Atoms contained even smaller, subatomic particles that were very small and were negatively charged

Thomson’s Experiment Thomson discovered these particles called electrons while experimenting with a glass vacuum tube containing metal electrodes When he connected the tube to a source of high-voltage electricity, glowing green rays appeared inside He discovered that electric charges bent the rays which made him infer that they were made of charged particles Thomson knew that atoms are electrically neutral so he reasoned that an atom must contain enough positive charge to balance the negative charge of the electrons

Rutherford’s Model By the early 1900s, scientists knew that the positive charge of an atom comes from subatomic particles called protons A proton is a positive particle with a mass much greater than that of an electron In 1911, Ernest Rutherford set out to test this theory

Rutherford’s Experiment Rutherford aimed a beam of positively charged particles at a sheet of gold foil only a few atoms thick Most of the particles went through the sheet but some bounced back The only explanation for this discovery was that the particles had struck objects larger than single protons Rutherford reasoned that the protons are concentrated in small area at the center of the atom called the nucleus

Bohr’s Model Niels Bohr modified Rutherford’s model in 1913 He proposed that each electron in an atom has a fixed amount of energy and this energy keeps an electron moving around the nucleus within a specific region called an energy level In Bohr’s model, energy levels surround the nucleus in rings or shells

Bohr’s Model

Energy Levels Electrons can move up and down energy levels and occupy space in each level but cannot occupy space in between levels In order for an electron to move up a level, it must gain energy In order for an electron to move down a level, it must lose energy

Orbitals Regions in an atom where electrons are found are called orbitals The s orbital is the simplest kind of orbital. It’s spherical shape enables it to surround the nucleus of an atom. The p orbital is dumbbell-shaped and can be orientated 3 different ways in space. There are 5 possible d orbitals and 7 possible f orbitals. They can each hold a maximum of 2 electrons

Sublevels 1st energy level: closest to the nucleus contains only the s sublevel 1 orbital and only 2 electrons 2nd energy level: contains both s and p sublevels 1 s orbital and 3 p orbitals can hold up to 8 electrons 3rd energy level: contains s, p, and d sublevels 1 s orbital, 3 p orbitals, and 5 d orbitals can hold up to 18 electrons 4th energy level: contains s, p, d, and f sublevels 1 s orbital, 3 p orbitals, 5 d orbitals, and 7 f orbitals can hold up to 32 electrons An electron in the outermost energy level of an atom is called a valence electron.

Electron Cloud Model The electron cloud model, 1920, electrons dart about within an energy level in an ever-changing path Most of this path falls into a region called an electron cloud

Chadwick’s Model In 1932, James Chadwick showed that most atomic nuclei contain a third kind of subatomic particle, called a neutron A neutron has about the same mass as a proton but has no electrical charge

Quarks In 1964, physicists Murray Gell-Mann and George Zweig discovered a new subatomic particle called a quark Quarks are subatomic particles that are found inside protons and neutrons There are six types, “flavors”, of quarks: Up, Down, Charm, Strange, Top, and Bottom