Fundamentals B: Elements and Atoms

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Ramesh D Arasasingham

Chemical Principles: The Quest for Insight by Atkins and Jones 5 Edition Study Guide by Gigi <3 Fundamentals B Elements and Atoms  element: a substance composed of only one type of atom  atom: the smallest part of an element that can exist  John Dalton developed atomic hypothesis in 1807: ◦ all the atoms of a given element are identical ◦ the atom of different elements have different masses ◦ a compound is a specific combination of more than one element ◦ in a chemical reaction, atoms are neither created nor destroyed; the exchange partners to produce new substances  nuclear model – current model of atom with four basic parts ◦ protons: positively charged particles ◦ electrons: negatively charged particles ◦ neutrons: particles with no charge ◦ nucleus: small, positively charged center made up of protons and neutrons; surrounded by a cloud of electrons ◦ neutrons and protons are jointly known as nucleons  atomic number: number of protons in the nucleus ◦ atomic number defines the element's identity ◦ denoted by letter Z  nucleus makes up most of the mass of an atom  Calculating number of atoms in a sample: ◦ N = (mass of sample)/(mass of one atom) ◦ Example: how many atoms are there in a 20.0 g sample of carbon? ▪ Mass C = 1.99 x 10^-26 kg ▪ (2.00 x 10^-1 kg)/(1.99 x 10^-26 kg) = 1.01 x 10^25 atoms Isotopes  atoms can be of the same element but have different masses due to neutrons  mass number: total number of protons and neutrons in an atom  mass in kilograms of atomic particles ◦ electron – 9.109 x 10^-31 ◦ proton – 1.673 x 10^-27 ◦ neutron – 1.675 x 10^-27  isotopes: atoms with the same atomic number but different mass numbers  mass number is written as a superscript to the left of the chemical symbol, atomic n
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