Ch 4 studying atoms

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atomic structure

Chapter 17 The atomic nature of matterMatter is made up of atoms. Col 1:17 He is before all things, and in him all things hold together.

Matter & AtomsAtoms-building blocks of most matter.Most matter is made from only about 100 different kinds of atoms.Each kind of atom belongs to a different element.Atoms are too small to see with visible light but can be imaged with an electron microscope.

Atoms are recyclable. Atoms in your body have been around since long before the solar system came into existence. They cycle and recycle among innumerable forms, both living and non-living.

Atoms are small. There are about 10 23 atoms in a gram of water ( a thimbleful)10 23 is greater than the number of drops of water in all the lakes and rivers of the world. Atoms are so small that there are about as many atoms in the air in your lungs at any moment as there are breathfuls of air in the atmosphere of the whole world. Atoms are perpetually moving. They also migrate from one location to another.


Democritus developed the idea of atoms 460 BChe pounded up materials in his pestle and mortar until he had reduced them to smaller and smaller particles which he calledATOMA (greek for indivisible



Atomic ModelsEarly Spherical ModelsBecause atoms are too small to be seen, the first theoretical models were intellectual constructions based on the logical methods of inductive and deductive reasoning. The classical Greek philosopher Democritus was the first to propose the existence of atoms in 400 B.C. He reasoned that matter cannot be divided indefinitely and must consist of indivisible round particles called atoms. In 1800, John Dalton arrived at the same view of atomism by using the experimental method to study gases and compounds. His theory was called the solid sphere, or billiard ball, model.


suggested that all matter was made up of tiny spheres that were able to bounce around with perfect elasticity and called themATOMS



HISTORY OF THE ATOM1898Joseph John Thompsonfound that atoms could sometimes eject a far smaller negative particle which he called anELECTRON



HISTORY OF THE ATOMThompson developed the idea that an atom was made up of electrons scattered unevenly within an elastic sphere surrounded by a soup of positive charge to balance the electron's charge

1904like plums surrounded by pudding.PLUM PUDDINGMODEL



Plum Pudding ModelIn 1904 the British physicist J.J. Thompson posited the plum pudding, or raisin bun, model of atomism. It was based on knowledge of the recently discovered negatively charged subatomic particles called electrons. Thompson's experiments with cathode ray tubes prompted him to theorize the existence of tiny particles inside atoms that were fundamental parts of all atoms. His model envisioned the negative electrons, or plums, suspended inside a positively charged framework, or the pudding.




1910 Ernest Rutherford

oversaw Geiger and Marsden carrying out his famous experiment.

they fired Helium nuclei at a piece of gold foil which was only a few atoms thick.

they found that although most of them passed through. About 1 in 10,000 hit



atoms positive charge is concentrated in the nucleusproton (p) has opposite (+) charge of electronmass of p is 1840 x mass of e- (1.67 x 10-24 g) particle velocity ~ 1.4 x 107 m/s(~5% speed of light)(1908 Nobel Prize in Chemistry)2.213



Rutherfords new evidence allowed him to propose a more detailed model with a central nucleus.

He suggested that the positive charge was all in a central nucleus. With this holding the electrons in place by electrical attraction

However, this was not the end of the story.




Niels Bohr 1913

studied under Rutherford at the Victoria University in Manchester. Bohr refined Rutherford's idea by adding that the electrons were in orbits. Rather like planets orbiting the sun. With each orbit only able to contain a set number of electrons.



Planetary ModelsFrom 1910 to 1911, Ernest Rutherford proposed the planetary, or nuclear, model of the atom. He believed that atoms were composed mostly of empty space, with a dense nucleus. His experiments involved shooting alpha particles at gold foil. He concluded that the positive nucleus contains most of the atom's mass. With his orbit model, Niels Bohr refined the idea of the atom as a tiny solar system in 1913. Bohr's model had electrons orbiting the nucleus in shell-like layers.

Bohrs Atom

electrons in orbitsnucleus



Electron Cloud ModelLouis de Broglie and Erwin Schrodinger developed the electron cloud, or quantum mechanical, model. They based the model on the breakthroughs of the quantum mechanics branch of physics. Instead of electrons in fixed orbits, the cloud model has the orbits defined by a probability distribution around the nucleus. Depending on their observation and measurement, the electrons could be in many different places, sometimes simultaneously.

Evidence for atoms1. Brownian motion- pollen grains were in a constant state of agitation, according to Robert Brown; 1827; result from the motion of neighboring atoms and molecules2. 1970 first image of clearly distinguishable atoms taken with a very thin electron beam in a scanning electron microscope.3. mid 1980s- individual atoms can be seen in the handheld scanning tunneling microscope

Element-made up of atoms of the same kind115 elements are known to date, 90 occur in natureMore than 99% of the material on Earth is formed from only about a dozen of the elements. The majority of elements are relatively rare.

5 elements in living things: O, C, H, N and CaLightest element- hydrogen

Atoms combine to form larger particles calledMoleculesEx. Two atoms of hydrogen (H) combine with a single atom of oxygen(O) to form water molecule.

Compound-is a substance made of different elements combined in a fixed proportionSome compounds are made of molecules, which are particles made of atoms joined together. Other compounds are made of different kinds of atoms arranged in regular patternChemical formula - of the compound tells the proportions of each kind of atom.Ex carbon dioxide: CO2: For every carbon ( C0 atom there are two oxygen (O) atoms.

The Spacious AtomMicrocosms of our solar system, atoms are dominantly empty space:If an oxygen atom had a total radius of 100 km, the nucleus would be a ~1 m diameter sphere in the middle.

Electrons in OrbitIn a simplistic model, electrons float around the nucleus in orbits that are sometimes called shells. As the number of electrons increases, they start to fill orbits farther out from the nucleus. In most cases, electrons are lost or gained only from the outermost orbits.

The Stuff That Makes up AtomsAlthough one can subdivide atoms into numerous subatomic particles, we will be concerned only with protons, neutrons and electronsProtons and neutrons are together in the nucleus of an atom, whereas electrons are in motion in orbits around the central nucleus.

Protons- carry a positive electrical charge, electrons -carry a negative charge, and neutrons -carry no charge. Most atoms are electrically neutral, meaning that they have an equal number of protons and electrons.

The atom is mostly empty space. Its mass is almost entirely in its nucleus. The nucleus is made of protons and neutrons.The number of protons, the atomic number, determines the element to which the atom belongs.

An electrically neutral atom has electrons outside the nucleus equal in number to the protons inside the nucleus. The shell model of the atom-pictures electrons in spherical shells around the nucleus.

Atomic Weight: Its all in the NucleusSince electrons weigh virtually nothing, the mass of an atom is concentrated in its nucleus.

Each atom can be described by its atomic weight (or mass), which is the sum of the protons and neutrons.

lithium: atomic number = 3 3 protons 4 neutrons atomic weight = 3 + 4 = 7

Size of NucleiThe number of neutrons tends to closely follow the number of protons. Atoms with more of each are bigger and heavier.A uranium atom, with 92 protons and ~146 neutrons is gigantic compared to dinky helium (2 + 2).

Maintaining NeutralityMost atoms are electrically neutral, meaning that they have an equal number of protons and electrons. A schematic model of a lithium (Li) atom in the ground state. It has 3 protons in the nucleus, and 3 electrons in orbit.

Charged Atoms: IonsLeft to their own devices, atoms are electrically neutral. That means that they have an equal number of protons and electrons. During the course of most natural events, protons are not gained or lost, but electrons may be. Atoms with more or fewer electrons than protons are electrically charged. They are called ions:An atom that loses electrons takes on a positive charge (cation); An atom that gains electrons takes on a negative charge (anion).

Electronic and Nuclear PropertiesProperties of atoms reflect some combination of features related to electrons or to the nucleus.

The electronic properties are those related to how atoms connect to one another: bonding.

The nuclear properties include features like radioactivity.


Atomic NumberWe distinguish one element from another on the basis of the atomic number, which is the number of protons.

So, an atom of any element can have a variable number of electr