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1.9 understand that atoms consist of a central nucleus, composed of protons and neutrons, surrounded by electrons, orbiting in shells 1.10 recall the relative mass and relative charge of a proton, neutron and electron 1.11 understand the terms atomic number, mass number, isotopes and relative atomic mass (Ar) John Dalton had the first ideas about the existence of atoms over 200 years ago. However, it is only relatively recently that special microscopes (called electron microscopes) have been invented that can actually ‘see’ atoms. Electron microscopes produce images similar to this one. What could it be showing? The grey blobs are individual lead atoms. Atoms are very small – they are about 0.00000001 cm wide. Think about the thickness of a crisp. The number of atoms you would need to stack up to make the thickness of a crisp, is approximately the same number of crisps you would need to stack up to make the height of Mount Everest! That’s roughly 7 million crisps! For some time, people thought that atoms were the smallest particles and could not be broken into anything smaller. Scientists now know that atoms are actually made from even smaller particles. There are three types: proton neutron How are these particles arranged inside the atom? electron Protons, neutrons and electrons are not evenly distributed in an atom. The protons and neutrons exist in a dense core at the centre of the atom. This is called the nucleus. The electrons are spread out around the edge of the atom. They orbit the nucleus in layers called shells. There are two properties of protons, neutrons and electrons that are especially important: mass electrical charge. Particle Mass Charge proton 1 neutron 1 electron almost 0 +1 0 -1 The atoms of an element contain equal numbers of protons and electrons and so have no overall charge. • Protons (p+) – – – 1 + electrical charge mass = 1.672623 x 10-24 g relative mass = 1.007 atomic mass units (amu) but we can round to • Electrons (e-) – – negative electrical charge relative mass = 0.0005 amu but we can round to 0 • Neutrons (no) – no electrical charge – mass = 1.009 amu but we can round to 1 • Have a look at your Periodic table… • We will study the actual elements later BUT • Check out all the numbers!! All atoms of the same element have the same number of protons in the nucleus, Z 13 Al 26.981 Atomic number Atom symbol AVERAGE Atomic Mass • C atom with 6 protons and 6 neutrons is the mass standard • = 12 atomic mass units • Mass Number (A) = # protons + # neutrons • NOT on the periodic table…(it is the AVERAGE atomic mass on the table) • A boron atom can have 5 p + 5 n = 10 amu A = A 10 Z 5 B Show the name of the element, a hyphen, and the mass number in hyphen notation sodium-23 Show the mass number and atomic number in nuclear symbol form mass number 23 Na atomic number 11 • Protons: Atomic Number (from periodic table) • Neutrons: Mass Number minus the number of protons (mass number is protons and neutrons because the mass of electrons is negligible) • Electrons: – If it’s an atom, the protons and electrons must be the SAME so that it is has a net charge of zero (equal numbers of + and -) • Using your play-doh to make the molecular model for: • Boron P: 5 N: 6 E: 5 • Helium P: 2 N: 2 E: 2 3 N: 4 E: 3 • Lithium P: • Carbon • Oxygen