Atoms, Elements and Compounds
- All elements are made up of atoms. When two or more atoms join together, a molecule is formed. The atoms in elements and compounds are held together by chemical bonds.
- An atom is made up of a central nucleus with electrons around it.
- All substances are made up of atoms.
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- A nucleus contains protons and neutrons. another particle called electrons orbits the nucleus.
- Number of electrons = Number of protons
- Protons- positive, Neutrons-neutral, Electrons-negative
- Protons and electrons balance out, so on any atom, the overall charge is zero.
- Atomic number = number of protons, the elements are arranged in this order.
- Mass number = protons+neutrons
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The arrangement of electrons in atoms
- Electrons are arranged around the nucleus in shells, each shell represents another energy level. The lowest energy level is closest to the nucleus. The shells hold the following amount of electrons in each- 18.104.22.168
- The electronic structure is the number of electrons in each energy level. the level below must be full before you go onto the next.
- The chemical properties of an element depend on how many electrons it has. This is deterrmined on how many electrons an element has in its outermost shell.
- All elements in each group have the same amount of electrons in their outermost shell so they all have the same reactivity. Group 0 (The noble gases) are very unreactive. This is because they have a complete noble gas configuration.
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- Sometimes atoms react by gaining or losing electrons to form chemical bonds (noble gas configurations) but sometimes they share the electrons with another atom.
- Ionic bonding is when they gain or lose electrons because the metal atoms give one or more electrons to the non-metal. Ionic bonding will only work with metals and non-metas. when this happens both atoms become charged particles called ions.
- Metal forms positive charges and non-metal forms negative so they attract eachother and form strong bonds called ionic bonds.
- Covalent bonding is when two non-metal atoms bond to eachother by their outermost shells overlapping and they share electrons. This forms a chemical bond between the atoms. They form molecules not ions.
- The chemical formula of an ionic compound tells us the ratio of each ion in the compound. Magnesium chloride- Magnesium forms Mg2+ ions and Chlorine forms Cl- ions, so their formulae would be MgCl2.
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- Chemical equations show the Reactants and the Products. In chemical reactions the atoms get rearranged.
- Using symbol equations is good because word equations are only useful if everyone spoke the same language. They also don't tell us the amount of each substance involved in a reaction. They get complicated quickly.
- Calcum carbonate decomposes on heating, we can show this like this: CaCO3 = CaO + CO2. This equation is balanced- there is the same number of each type of atom on each side of the equation. Remember- The total mass of the products formed in a reaction is equal to the total mass of the reactants.
- We can check if an equation is balanced by counting the number of each type of atom on each side of the equation (moles).
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