T1 - The History of the Atom + The Atom

Dalton’s theory of Atomic Structure? (start of 19th century)

Dalton’s theory of Atomic Structure? (start of 19th century)

Described atoms as solid spheres, and said that different spheres made up the different elements

Thomson’s theory of Atomic Structure? (1897)

• plum pudding model

• showed the atom as a ball of positive charge with negatively charged electrons stuck in it

Plum Pudding Model

showed the atom as a ball of positive charge with electrons stuck in it

Rutherford’s theory of Atomic Structure? (1909)

• conducted gold foil experiment : fired positively charged alpha particles at an extremely thin sheet of gold

• expected particles to pass straight through because of plum pudding model, but some did go straight through, some deflected more than expected, and a small number deflected backwards. so plum pudding model could not be right

• rutherford came up with the nuclear model of the atom

Nuclear Model

• tiny, positively charged nucleus at centre, where most of the mass is concentrated

• a ‘cloud’ of negative electrons surrounds this nucleus - so most of the atom is empty space

Bohr’s theory of Atomic Structure?

• scientists realised that electrons in a ‘cloud’ around the nucleus of an atom, as Rutherford described, would be attracted to the nucleus, causing the atom to collapse

• Bohr proposed a new model of the atom where all electrons were contained in shells

• Bohr’s model is close to our currently accepted version of the atom

Bohr’s Model

• electrons orbit the nucleus in fixed shells and aren’t everywhere in between

• each shell is a fixed distance from the nucleus and has a fixed energy

Which three subatomic particles is the atom made of?

protons, neutrons and electrons

Proton

• heavy and positively charged

• relative mass of 1

• relative charge of +1

• in the nucleus

Neutron

• heavy and neutral

• relative mass of 1

• relative charge of 0

• in the nucleus

Electron

• hardly any mass, negatively charged

• relative mass of 0.0005

• relative charge of -1

• move around the nucleus in electron shells

• tiny, but their shells cover a lot of space

• size of their shell determines size of the atom

Nucleus

• in the middle of an atom

• contains protons and neutrons

• has a positive charge because of the protons

• almost the whole mass of the atom is concentrated in the nucleus

• compared to the overall size of the atom, the nucleus is tiny

Why are atoms neutral?

• same number of protons as electrons

• the charge on the electrons is the same size as the charge on protons, but opposite - so the charged don’t cancel out

Atomic (proton) number

• tells you how many protons an atom has

• every atom of an element has the same number of protons

• for a neutral atom, the number of protons = the number of electrons, so the number of electrons = the atomic number

Mass number

the total number of protons and neutrons in the atom