The Atomic Structure and the Chemical Elements The Ideas of the Ancient Greeks on the Atom
Objective At the end of the lesson, lesson, you should be able able to describe describe the ideas of the Ancient Ancient Greeks on atoms. Can matter be infinitely divided into smaller particles?
Last Updated: 08.13.16
Learn about it! The Indivisible Atom
Democritus of Abdera (460 - 370 B.C.) and his teacher Leucippus Democritus of Leucippus of of Miletus (c.500 B.C.) were Greek scholars who believed that matter could be divided into tiny particles until such point where it can no longer be divided anymore. They became the first proponents of the atomic theory. Their early ideas on atoms are summarized below. 1. All matter is is made up of tiny, indivisible indivisible particles particles called atoms, atoms, which come come from the Greek word atomos meaning uncuttable. The atoms are indestructible, impenetrable, impenetra ble, and unchangeable. 2. The atoms make up the universe as they are continuously moving in a “void” that surrounds them, repelling each other when they collide, or combining into clusters. 3. Atoms are completely completely solid which means that there is no void void or empty space inside that will make them prone to disintegration or destruction. 4. Atoms are homogeneous homogeneous in nature. nature. They have no internal internal structures. structures. 5. Atoms come come in different shapes shapes and sizes. These proposed ideas about atoms were supported by some Greek philosophers but were strongly opposed by others others especially Aristotle. Aristotle.
Learn about it! Aristotle's Opposing View on Atoms Atoms
Aristotle, a Greek Greek philosopher, philosopher, had a different view on on atoms. He disregarded disregarded the existence of atoms proposed by Leucippus and Democritus. He did not believe that
matter is a collection of atoms. Instead, he believed that everything in the universe is made up of the four elements, air, fire, water, and earth. He stated that believing in atoms would mean putting restriction restriction on the gods, who have the power to divide elements smaller than the atom. Aristotle's beliefs beliefs greatly flourished flourished especially especially in the Middle Ages in Europe, where where Roman Catholics were strongly influenced by his ideas. They believed that ideas about the atoms equated to Godlessness. Thus, the whole concept of the atom was dismissed for centuries. However, the Greeks' concept of atoms and even Aristotle's arguments were rediscovered rediscovered in France France at the start of the Renaissanc Renaissancee period. The theory of Aristotle was proven incorrect, incorrect, and Democritus' Democritus' and Leucippus' Leucippus' theory on the existence existence of atoms was proven right.
Explore! Choose one digital photo and zoom in. Can you see a group of tiny squares? They are called pixels. How can you relate them to atoms?
Try it! Get a piece of paper. Divide it into half as many times as you can. What do you notice? How would you relate this activity to the concept of the atoms?
What do you think? Is the early idea of the existence of the atom true?
Key Points
Democritus and Leucippus, ancient Greek philosophers, first proposed the idea of the atom. Their theory states that all matter is made up of tiny, indivisible particles called atoms atoms.. They believed that the atoms are very small, have different shapes and sizes, are continuously moving, and can combine with other atoms. Aristotle did did not believe that matter matter is strictly strictly a collection collection of atoms, and that that matter can be made of air, fire, water, or earth. Democritus’ Democrit us’ idea on the existence of atoms was accepted, and Aristotle’s argument was proven incorrect.
The Atomic Structure and the Chemical Elements The Discovery of the Structure of the Atom and its Subatomic Particles
Objective At the end of this lesson, lesson, you should be be able to identify the main ideas in the discovery discovery of the structure of the atom and its subatomic particles. In the previous lessons, you have learned that matter is composed of atoms. What is the structure structure of the atom?
Last Updated: 08.13.16
Learn about it! Greek philosophers Democritus Democritus and Leucippus developed the idea that all matter is composed of tiny, indivisible particles called atoms atoms.. However, their atomic theory was based only on assumptions. assumptions. It was was not until the early 1800s that that experiments were performed to develop models for the structure of the atom. In 1803, John Dalton, Dalton , a British scientist, did experiments on mixtures of gases. He studied how the properties of individual gases affect the properties of the mixtures of these gases. He developed the hypothesis that the sizes of the particles making up different gases must be different. After several experiments, he concluded that all matter is composed of spherical atoms, which cannot be broken down into smaller pieces. He added that all atoms of one element are identical to each other but different from the atoms of another element.
Learn about it! Discovery of the Subatomic Particles
The Electrons in the Plum Pudding Model In 1897, Joseph John Thomson, Thomson , a British physicist physicist,, proposed an atomic model known as the plum pudding model. His model consisted of negatively charged particles (plum) spread evenly throughout the positively charged material (pudding). The small, negatively charged particles are called electrons electrons..
The Protons in the Planetary Model In the early 1900s, Ernest Rutherford, Rutherford, a New Zealand-born physicist, established the planetary model which described the atom as small, dense, and has a positively charged core called the nucleus nucleus.. Inside the nucleus are positively charged particles called the protons protons.. The nucleus is surrounded by negatively charged particles or electrons. The electrostatic attraction between electrons and nucleus mimics the gravitational force of attraction between planets and the sun. The Neutrons In 1923, James Chadwick proved proved the existence of the neutron neutron,, which is also situated in the nucleus together with the proton. It has the same mass as the proton but unlike the latter, it has no electric charge.
Learn about it!
Recent Atomic Models
Bohr’s Atomic Model Rutherford’s model showed that the electr electrons ons and nucleus have opposite charges which according to the laws of physics, will attract each other. Thus, Rutherford's model would have electrons collapsing into the nucleus, making the atom unstable. Niels Bohr solved Bohr solved this problem by proposing that the electrons orbit around the nucleus in set energy levels. An electron absorbs energy if it moves from lower to higher energy level, and it emits energy if it returns to the lower energy level. Quantum Mechanical Model The quantum mechanical model of the atom states that a nucleus is surrounded by a cloud of electrons called orbitals orbitals.. It explains that it is impossible to determine the exact location of the electron at a given time, but one can find its probable location. It incorporates incorporat es the concept of Bohr’s Bohr’s model model where the electrons move in one orbital to another by absorbing or emitting energy.
Explore! Look around your house. Try to think of Bohr’s discovery of energy levels. What things can you find in your house that use the same concept proposed by Bohr?
Try it! Create a timeline on the discovery of subatomic particles and development of the atomic theory. Who are the scientists who had important contributions contributions in atomic theory?
What do you think? Based on Bohr's atomic model, what causes an electron to move from one orbital to another?
Key Points
John Dalton described Dalton described the atom as spherical. Joseph John Thomson discovered Thomson discovered the electron. Ernest Rutherford proposed Rutherford proposed that the electrons orbit around the nucleus. He, together with his students, discovered the proton. Niels Bohr proposed Bohr proposed that electrons orbit around the nucleus in set energy levels.
James Chadwick discovered discovered the neutron. Niels Bohr proposed Bohr proposed that the electrons orbit around the nucleus in set energy levels. In the quantum mechanical model, the nucleus is surrounded by a cloud of electrons called orbitals orbitals..
The Atomic Structure and the Chemical Elements Understanding the Structure of Atom:The Contributions of J.J. Thomson, Ernest Rutherford, Henry Moseley, and Niel
Objective In this lesson, you will be able to cite the contributions of Joseph John Thomson, Ernest Rutherford, Henry Moseley, and Niels Bohr to the understandin understanding g of the structure of the atom. What are the the contributions contributions of Joseph John John (J.J.) Thomson, Thomson, Ernest Ernest Rutherford, Henry Moseley, and Niels Bohr to the understanding of the structure of the atom?
Last Updated: 08.13.16
Learn about it! J.J. Thomson’s Discovery of the Electron In 1897, J.J. Thomson discovered discovered the electrons by conducting a series of experiments using a high-vacuum cathode-ray tube that was composed of negatively charged particles 1000 times lighter than the hydrogen atom. He also proposed a sea of positive charge for the overall neutrality of the atom. He then proposed an atomic model known as the plum pudding model depicting a sphere of positive charge (pudding) with negatively charged particles (plums) embedded all throughout. Ernest Rutherford’s Discovery of the Nucleus In the early 1900s, Rutherford discovered the nucleus containing positively charged particles called protons protons.. He advised his students, Hans Geiger and Ernest Marsden, to bombard a thin sheet sheet of gold foil with alpha alpha particles. particles. He assumed that that the alpha particles would just pass straight through the foil, meaning an atom has a void space.
However, after the experiment, some particles passed right through it, and some were deflected. He arrived at these two conclusions: (1) The atom contained an empty space, as some particles went through the foil; and (2) The atom had a very dense center of positive charge. From these, Rutherford proposed the planetary model. He believed that the electrons moved around a nucleus.
Learn about it! Henry Moseley’s Atomic Number In 1913, Henry Moseley, a British chemist, developed the use of X-ray in studying the structure of the atom. During this time, a coherent structure of the atom was being developed, starting from J.J. Thomson’s discovery of the electron to Rutherford’s publication of his planetary model. He published results of his measurements of wavelengths of the X-ray emissions emissions of some elements elements that coincided coincided with the order of their atomic numbers. Moseley’s experimental data backed up Rutherford’s structure of the atom with a very dense center of positive charge. The data also justified that the atomic number of an element is the number of positive charges in its nucleus. Niels Bohr’s Atomic Model If you would recall in magnetism, unlike charges attract. In Rutherford’s model, since the electron and the nucleus have opposite charges, the electrons would collapse into the nucleus, making the atom unstable. Niels Bohr modified this model by proposing that the electrons move in fixed energy levels or orbits by absorbing or emitting energy.
Try it! Look at the periodic table. List down three elements with the lowest atomic number and three with the highest atomic number. What does the low and high atomic numbers signify?
What do you think? What is the importance importance of knowing knowing the atomic number number of elements? elements?
Key Points
J.J. Thomson discovered Thomson discovered the electron and proposed the plum pudding model. Ernest Rutherford discovered Rutherford discovered the nucleus and proposed the planetary model.
Henry Moseley used used the X-rays in studying the structure of the atom. The results of his experiments supported Rutherford's Rutherford's model. Niels Bohr proposed Bohr proposed that the electrons move in fixed energy levels or orbits.
The Atomic Structure and the Chemical Elements The Nuclear Model of the Atom
Objective In this lesson, you should be able to describe the nuclear model of the atom and the location of its major components (protons, neutrons, and electrons). If you have lived during the time when the atom was discovered, how would you describe its structure?
Last Updated: 08.13.16
Learn about it! As subatomic particles were were discovered, models models for their arrangement arrangement in the atom were developed. There was J. J. Thomson’s plum-pudding plum-pudding model, which he proposed after his discovery of the electrons. Then there was Ernest Rutherford’s planetary model, proposed after the discovery of the protons in the nucleus. The Nuclear Model
The nuclear model states that the nucleus is small, dense, and located at the center of the atom. It contains protons and neutrons. Overall, Overall, it is positively charged. It contains nearly all the mass of the atom. The electrons orbit around it.
The nuclear model has been deduced from the experiment done by Rutherford.
Learn about it! Geiger-Marsden Experiment
Under Rutherford's supervision, Hans Geiger, his assistant, and Ernest Marsden, an undergraduate student, student, shot a narrow beam of alpha particles particles at a very thin sheet of gold foil and measured the scattering pattern on a fluorescent screen. It was observed that some particles deflected, and others penetrated through the sheet of gold foil.
There was a force behind the deflections – the repulsion of the positively charged alpha particles by a positively charged material. If this charge existed in a sphere with the same size of the atom, the force should have been weak. This led to the assumption that the charge was concentrated in a small space or sphere. Rutherford proposed that the positive charge, and the mass of the atom were concentrated concentra ted in a small part of the total volume of the atom called the nucleus nucleus..
Learn about it! Issues Involved in the Nuclear Model
The main problem in Rutherford's model was how the electrons remained in their orbit without falling into into the nucleus. nucleus. Another problem problem is the electron's electron's continuous continuous centripetall acceleration and the energy lost through electromagnetic radiation. This centripeta energy would make it approach the nucleus while increasing the electrostatic force. force. The increase in force would increase the acceleration and the emission of energy. Thus, the nucleus and the electrons would collide with each other. This means that the atom would be unstable. unstable.
Explore! Imagine you're holding two magnets with the same magnetic poles (north to north poles or south to south poles). When you try to put the magnets together, there is a force acting upon them which makes it difficult to do so. The repulsion between the magnets represents the forces between the alpha particles and the positively charged material in the atom of the gold foil as presented in the Geiger-Marsden experiment of Rutherford and his colleagues. What is the importance of this scenario in studying the structure of the atom?
Try it! Research on the succeeding studies about the nucleus and its characteristics. characteristics.
What do you think? Rutherford postulated a neutral particle inside the nucleus. How did he come into this conclusion?
Key Points
The nuclear model states model states that the nucleus is small, dense, and located at the center of the atom. The nucleus is positively charged. It contains nearly all the mass of the atom. The electrons orbit around it. The nuclear model has been deduced from the experiment done by Rutherford. Ru therford.
