Ch 4 studying atoms

Chapter 17 The atomic nature of matter Matter is made up of

atoms. Col 1:17

He is before all things, and in him all things

hold together.

Matter & Atoms Atoms -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.

HISTORY OF THE ATOM Democritus developed the idea

of atoms 460 BC he pounded up materials in his

pestle and mortar until he had reduced them to smaller and smaller particles which he called

ATOMA

(greek for indivisible

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Atomic Models

Early Spherical Models Because 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.

HISTORY OF THE ATOM

1808John Dalton

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

ATOMS

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HISTORY OF THE ATOM

1898Joseph John Thompson found that atoms could

sometimes eject a far smaller negative particle which he called an

ELECTRON

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HISTORY OF THE ATOM

Thompson 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

1904

like plums surrounded by pudding.

PLUM PUDDINGMODEL

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Plum Pudding Model In 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.

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HISTORY OF THE ATOM1910 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

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1. atoms positive charge is concentrated in the nucleus2. proton (p) has opposite (+) charge of electron3. mass of p is 1840 x mass of e- (1.67 x 10-24 g)

a particle velocity ~ 1.4 x 107 m/s(~5% speed of light)

(1908 Nobel Prize in Chemistry)

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HISTORY OF THE ATOM Rutherford’s 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.

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HISTORY OF THE ATOMNiels 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.

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Planetary Models From 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.

Bohr’s Atom

electrons in orbits

nucleus

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Electron Cloud Model Louis 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 atoms 1. Brownian motion- pollen grains were in a constant

state of agitation, according to Robert Brown; 1827; result from the motion of neighboring atoms and molecules

2. 1970 first image of clearly distinguishable atoms taken with a very thin electron beam in a scanning electron microscope.

3. mid 1980’s- individual atoms can be seen in the handheld scanning tunneling microscope

Element -made up of atoms of the same kind 115 elements are known to date, 90

occur in nature More 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 Ca

Lightest element- hydrogen

Atoms combine to form larger particles called

Molecules Ex. 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 proportion Some 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 pattern Chemical 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 Atom Microcosms 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 Orbit In 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 Atoms Although one can subdivide atoms into numerous subatomic particles, we will be

concerned only with protons, neutrons and electrons

Protons 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: It’s all in the Nucleus

Since 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 Nuclei The 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 Neutrality Most 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: Ions

Left 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 Properties

Properties 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 electrons and neutrons, but given the number of protons, the

fundamental properties of the element are unchanged.

This is the basis for Dmitri Mendeleev’s organization of the

Periodic Table of the Elements. The table is a way of organizing elements

on physical grounds, but also serves to group elements with

consistent chemical properties.

The periodic table is a chart of elements arranged

according to atomic structure and properties.

Elements in the same column have similar chemical properties, reacting with other elements in similar ways to form new compounds and materials because their outermost electrons are arranged in a similar fashion, they belong to the same

group of elements

The Periodic Table

The periodic table is read from top to bottom, left to right, as atomic number increases: 1=H, 2=He, 3=Li, 4=Be, 5=B, 6=C,

and so on.

Elements in columns (groups) have similar

outer-electron configurations, and so tend to behave similarly.

The Periodic Tableal

kal is

alkali earths

rare earths

halogens

noble gases

transition metals

actinides

The Periodic Table: the Bulk Earth

A small number of elements make up >99% of the solid Earth.

O = oxygenNa = sodiumMg = magnesiumAl = aluminumSi = silicon

S = sulfurCa = calciumFe = ironNi = nickel

The Periodic Table: the Crust

The crust is a little more elementally interesting (again, as a result of differentiation), but it is still mainly made of a

small number of elements.

C = carbonP = phosphorusK = potassiumTi = titaniumMn = manganese

BUT... although each

element has a defined

number of protons, the

number of neutrons is not

fixed.

Atoms of the same element, with the same

atomic number, having different numbers of neutrons are called

Isotopes of that element.

IsotopesCarbon (atomic # 6) has three natural isotopes

with atomic weights of 12, 13 and 14.

isotope #p #n====== == == C-12 6 6 C-13 6 7 C-14 6 8

Tin (Sn, atomic # 50) has ten natural isotopes with

atomic masses of 112, 114, 115, 116, 117, 118, 119,

120, 122 and 124. How many protons and neutrons

do these isotopes have?