What Is An Atom

DATE………………

TOPIC: ATOMIC STRUCTURE

CONTENT

1. Meaning of Atom; Concept of Electrons, Protons and Neutrons
2. Simple atomic model; Properties of atoms
3. Concept of Elements, Molecules, Compounds and Mixtures

Sub-Topic 1: Meaning of Atom; Concept of Electrons, Protons and Neutrons

MEANING OF ATOM

Matter (solid, liquid or gas) is made up of tiny particles called atoms.

An atom is the smallest particle of matter which can take part in a chemical reaction.

An English Chemist called John Dalton (1766 – 1844) put forward some statements about the atom in1803 and 1808 which are referred to as Dalton’s atomic theory. These helped to explain some observations about the structure and behavior of atoms. However with advancement in science and the contribution of more scientists like J.J Thomson in 1897, Robert Millikan in1909-1916 and Earnest Rutherford in 1911, the theory has been modified. Presently, scientists agree that:

1. An atom is made up of a number of small particles namely, the electron, the proton and the neutron. None of these can exist on its own naturally.
2. An atom is the smallest particle of matter that can take part in a chemical reaction.
3. In nuclear reactions atoms can be created or destroyed.
4. Some atoms of the same element have different masses for example in those elements that have isotopes.
5. Most inorganic compounds are formed by the joining of atoms of two or more elements in a definite whole number ratio, but for large organic molecules such as proteins, fats and starches which involve the joining of thousands of atoms, this theory does not hold.

CONCEPT OF ELECTRONS, PROTONS AND NEUTRONS

An atom is made up of three fundamental particles; the electrons, protons and neutrons. Structurally, an atom consists of a small dense, centrally placed nucleus which is surrounded by electrons. The nucleus is made up of the protons and the neutrons. The protons and neutrons are collectively called the nucleons. Most of the atom is empty space. Most of the mass is concentrated in the nucleus.

ELECTRONS: These are tiny particles situated at a comparatively great distance from the centre of the atom. They travel rapidly and continuously in orbits around the nucleus. An electron has a negative charge and a very negligible mass of about 9.1 × 10^-28g, this is about 1800 times lighter than the hydrogen atom.

PROTONS: These are tiny, positively charged particles found in the nucleus of an atom. The positive charge is equal in magnitude to the negative charge on the electron. Thus, an atom with the same number of electrons and protons is electrically neutral. The mass of the proton is 1 × 10^-24g, which is about 1800 times larger than that of the electron.

NEUTRONS: These are tiny particles found in the nucleus of an atom. They have no electric charge i.e. they are neutral. Neutrons have about the same mass as a proton.

EVALUATION

1. What is an atom?
2. Name the fundamental particles of an atom
3. State the charges on these particles and their relative masses.
4. Who was the first chemist to propound theories about atoms?